U.S. patent application number 14/703840 was filed with the patent office on 2015-12-17 for apparatus for controlling a solid fuel forced hot air furnace.
The applicant listed for this patent is Charles Holland Dresser. Invention is credited to Charles Holland Dresser.
Application Number | 20150362217 14/703840 |
Document ID | / |
Family ID | 54835858 |
Filed Date | 2015-12-17 |
United States Patent
Application |
20150362217 |
Kind Code |
A1 |
Dresser; Charles Holland |
December 17, 2015 |
Apparatus for Controlling a Solid Fuel Forced Hot Air Furnace
Abstract
In a wood pellet fueled forced hot air heating system, a burner
is regulated in response to an ambient air, or supply plenum,
temperature. The burner regulating means allows for increased
burner cycle times. The air handler is regulated in response to a
space to be heated, or room, temperature, typically through the use
of a mechanical thermostat. The air handler regulating means when
used in conjunction with the burner regulating means allows for
immediate heat response during a heat call.
Inventors: |
Dresser; Charles Holland;
(Bethel, ME) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dresser; Charles Holland |
Bethel |
ME |
US |
|
|
Family ID: |
54835858 |
Appl. No.: |
14/703840 |
Filed: |
May 4, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61988324 |
May 5, 2014 |
|
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Current U.S.
Class: |
237/2A ;
237/53 |
Current CPC
Class: |
F23N 2225/12 20200101;
F23N 2241/02 20200101; F23N 2225/13 20200101; F23N 2223/36
20200101; F24D 19/1084 20130101; F24D 2200/07 20130101; F24D 5/06
20130101; F23N 2239/02 20200101; F23N 2223/08 20200101; F23N 3/082
20130101 |
International
Class: |
F24H 9/20 20060101
F24H009/20; F24H 3/08 20060101 F24H003/08; F24H 9/18 20060101
F24H009/18; F24H 3/00 20060101 F24H003/00 |
Claims
1. A solid fuel, forced hot air heating system, the solid fuel
forced hot air heating system comprising: a burner means for
combusting a solid fuel and providing exhaust gases in response to
a burner power signal, a heat exchanger means for transferring
thermal energy from said exhaust gases to an ambient air, a space
thermosensitive means for providing a space temperature feedback in
response to the temperature of a space to be heated, an air handler
means for transporting the ambient air to the space to be heated in
response to an air handler power signal; and an air handler
regulating means for providing the air handler power signal in
response to the space temperature feedback.
2. The system of claim 1, further comprising: a means for providing
a space temperature setpoint; and wherein said air handler
regulating means, further comprises: a means for relating the space
temperature feedback and the space temperature setpoint.
3. The system of claim 1, wherein said air handler regulating
means, further comprises: a thermostat.
4. The system of claim 1, further comprising a thermal compliance
means for storing said thermal energy.
5. A solid fuel, forced hot air heating system, the solid fuel
forced hot air heating system comprising: a burner means for
combusting a solid fuel and providing exhaust gases in response to
a burner power signal, a heat exchanger means for transferring
thermal energy from said exhaust gases to an ambient air, an
ambient thermosensitive means for providing an ambient temperature
feedback in response to the temperature of the ambient air, a space
thermosensitive means for providing a space temperature feedback in
response to the temperature of a space to be heated, an air handler
means for transporting the ambient air to the space to be heated in
response to an air handler power signal, an air handler regulating
means for providing the air handler power signal in response to the
space temperature feedback; and a burner regulating means for
providing the burner power signal in response to the ambient
temperature feedback.
6. The system of claim 5, wherein the ambient thermosensitive
means, further comprises: sensing the temperature of a substance
that is in thermally communication with the ambient air.
7. The system of claim 5, further comprising: a means for providing
an ambient temperature setpoint; and wherein the burner regulating
means, further comprises: a means for relating the ambient
temperature feedback and the ambient temperature setpoint.
8. The system of claim 7, wherein the burner regulating means
further comprises a PID controller means for relating the ambient
temperature feedback and the ambient temperature setpoint over
time.
9. The system of claim 5, further comprising: a means for providing
a low ambient setpoint; and wherein the air handler regulating
means, further comprises: a means relating the ambient temperature
feedback and the low ambient setpoint.
10. The system of claim 9, wherein the means for providing the low
ambient setpoint further comprises a thermostatic switch.
11. A solid fuel, forced hot air heating system, the solid fuel
forced hot air heating system comprising: a burner means for
combusting a solid fuel and providing exhaust gases in response to
a burner power signal, a heat exchanger means for transferring
thermal energy from said exhaust gases to an ambient air, an
ambient thermosensitive means for providing an ambient temperature
feedback in response to the temperature of the ambient air, a space
thermosensitive means for providing a space temperature feedback in
response to the temperature of a space to be heated, an air handler
means for transporting the ambient air to the space to be heated in
response to an air handler power signal; and a furnace regulator
switching means for selectively granting control of the burner
power signal and the air handler power signal between a continuous
operation furnace regulating means and a periodic operation furnace
regulating means; said continuous operation furnace regulating
means, comprising: a means for providing the burner power signal in
response to the ambient temperature feedback; and a means for
providing the air handler power signal in response to the space
temperature feedback; said periodic operation furnace regulating
means, comprising: a means for providing the burner power signal in
response to the space temperature feedback; and a means for
providing the air handler power signal in response to the ambient
temperature feedback.
12. The system of claim 11, wherein the furnace regulator switching
means further comprises a means for calculating a heat load.
13. The system of claim 11, wherein the furnace regulator switching
means, further comprises: a means for measuring a duration the
burner power signal is LOW; and a means for relating the duration
the burner power signal is LOW to a burner low timeout value.
14. The system of claim 11, wherein the furnace regulator switching
means, further comprises: a means for measuring a duration the
burner power signal is HIGH; and a means for relating the duration
the burner power signal is HIGH to a burner high timeout value.
15. The system of claim 11, wherein the furnace regulator switching
means, further comprises: a means for determining an overheat
status.
16. The system of claim 11, wherein the furnace regulator switching
means, further comprises: an outdoor thermosensitive means for
providing an outdoor temperature feedback in response to the
temperature of the outdoor air.
17. The system of claim 11, further comprising: a means for
providing an ambient temperature setpoint, a means relating the
ambient temperature feedback and the ambient temperature setpoint,
a means for providing a space temperature setpoint; and a means for
relating the space temperature feedback and the space temperature
setpoint.
18. The system of claim 5, wherein the burner means further
comprises: a means regulating the speed of a combustion fan in
response to the burner power signal.
19. The system of claim 5, wherein the burner means further
comprises: a means for regulating a flow of the solid fuel in
response to the burner power signal.
20. The system of claim 5, wherein the solid fuels is wood pellets.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to and claims the benefit of
priority to U.S. Provisional Patent Application Ser. No. 61/988,324
filed on May 5, 2014, the entirety of which is hereby incorporated
by reference.
BACKGROUND
[0002] Various embodiments of the present invention generally
relate to systems and methods for improving the heating efficiency
and comfort of a heated space; in particular, to heating system
component regulating, in order to improve heating system efficiency
and effectiveness.
[0003] Heating in colder climates is primarily performed by central
heating systems. Typically these central heating systems burn
liquid fossil fuels, for example: heating oil, propane and natural
gas. As the high economic and environmental costs of fossil fuel
consumption have become realized, heating with alternative fuels
has become more attractive.
[0004] Solid biomass fueled central heating systems are a promising
alternative to fossil fuel central heating systems. Solid biomass
fuel is renewable, less costly than most fossil fuels, local to
North America and does not release long sequestered carbon into the
atmosphere when burned. The use of solid biomass hydronic heating
systems has become commonplace throughout Western Europe, and
notably in Upper Austria and Scandinavia. Wood pellet fueled
boilers are regularly used in commercial, municipal and residential
heating applications in these regions. These European wood pellet
boilers have varying levels of sophistication and amount of
intervention required by the operator. The most sophisticated wood
pellet boilers, exemplified well by the OkoFEN brand of boilers
from OkoFEN Forschungs- und Entwicklungs G.m.b.H. of Niederkappel,
Austria, require little more operator intervention than a fossil
fuel boiler, and have efficiencies and features that rival those of
the most technologically advanced fossil fuel boilers.
[0005] Some solid fuel burners, such as the burner incorporated on
OkoFEN brand boilers, allow for modulated operation. Modulated
operation allows the thermal power of the burner to vary,
decreasing from the burner's maximum power output. Cord wood
burners typically achieve modulated operation by choking a flow of
air to the burner, limiting the combustion rate of the cord wood.
Granular solid fuel burners, such as wood pellet and wood chip
burners, typically limit the flow of air as well as a flow of fuel
in order to modulate the thermal power output of the burner.
Modulating wood pellet burners may vary the flow of air and fuel by
decreasing the speed of a combustion air fan and reducing the speed
of a fuel dosing auger. An example of a modulating burner is
Janfire NH pellet burner, from Janfire AB of .ANG.mal, Sweden. The
Janfire NH has 7 modulation levels in a modulation range between a
maximum heat output of 78,000 btu/hr and a minimum heat output of
10,000 btu/hr and a LOW modulation level of 2,000 btu/hr for
maintaining operation of the burner when no additional heat is
needed. The LOW modulation level on the Janfire NH keeps the burner
operating longer, decreasing the number of ignition sequences
required by the burner. The modulation level of the Janfire NH may
be controlled by a signal voltage of 0-10V. Other solid fuel
burners do not modulate and operate at a single thermal power at
all times, like most fluid fueled heater burners. An example of a
non-modulating burner is the PB-1525 from Pellergy LLC of
Montpelier, Vt. The maximum thermal power of the PB-1525 is 120,000
BTU/hr, although it may be manually set a single thermal power
anywhere in the range of about 60,000 BTU/hr-120,000 BTU/hr. The
operation of the PB-1525 burner is controlled by a normally open
connection, which when closed causes the PB-1525 to operate at the
single power output. The normally open connection of the PB-1525 is
designed to work with a thermostat, such that the burner operates
when the thermostat senses a demand for heat.
[0006] Unlike fluid burning devices, solid fuel burners may take
about 10 minutes to initiate operation. During a startup, a small
fire is ignited, usually by a blast of hot air, above the
auto-ignite temperature of the fuel, and stoked to a combustion
level of the desired power. The blast of hot air is usually
provided for by a glow plug and the combustion air fan of the
burner. The startup usually results in incomplete combustion, and
therefore results in inefficient consumption of fuel, and exhaust
gases having high carbon monoxide levels and high particulate
matter.
[0007] Forced hot air heating systems generally perform a large
number of startups and cycles. This is because; current forced hot
air heating system control methods operate, such that the burner on
a forced hot air system only produces heat when a thermostat call
is made. The thermostat call existing only when the space to be
heated is below a thermostat setpoint. With the thermostat call the
burner ignites and begins producing heat, heating air within a
plenum. Once the air within the plenum is above a temperature such
that it is warm enough to heat a space, a blower blows the air to
the space. When the thermostat in the space is satisfied, meaning
no longer providing a call for heat, the forced hot air burner
stops generating heat. The lack of heat generation causes the air
temperature within the plenum to drop. Once the air in the plenum
drops below a certain temperature, typically just above room
temperature, the blower stops blowing. This cycle repeats many
times throughout a day during a heating season. Because of the
solid fuel burner's poor performance during startup and long
startup time, solid fuel burners have been poorly suited for use as
a component in a forced hot air heating system.
[0008] The use of solid fuel burners in forced hot air heating
systems may result in uncomfortable heating as solid fuel burner's
typically have long startup times. As the burner ignites after a
thermostat call the temperature of the space to be heated continues
to drop further below the setpoint temperature. Even after ignition
is achieved it typically takes solid fuel burners, such as wood
pellet burners, longer to raise the plenum temperature than an oil
or gas burner. The result is that the temperature of the space to
be heated is much colder than the setpoint temperature before the
air within the plenum reaches the desired temperature and heat is
delivered. Often this drop in temperature is noticeable to
occupants in the space.
[0009] U.S. Pat. No. 4,842,190 describes a wood pellet furnace
control system that attempts to shorten the length of time for a
plenum to heat. This reference fails to solve the problem of slow
warming of the air within the plenum with the use of a solid fuel
burner, as it only affects the speed of the heat transfer once the
burner has ignited and fails to address the time taken during
startup by the burner to ignite. U.S. Pat. No. 4,842,190 also does
not address the poor efficiencies and emissions during the startup
of a typical solid fuel burner.
[0010] The United States is heated primarily with forced hot air
central heating systems. This is unlike Europe, which heats almost
exclusively with hydronic central heating systems. The stated
incompatibilities between current forced hot air heating systems
and methods and solid fuel burning is a problem that prevents the
benefits of solid fuel heating from being realized in the United
States on a large scale. A solid fuel forced hot air heating system
and method that allows for prolonged burner on-time and fewer
startups is needed.
[0011] Additionally, the current methods of heating with forced hot
air are criticized as being less comfortable than hydronic heating
systems. Because of the ON-OFF heating of the forced hot air system
it is often the case that the space being heated becomes too hot,
when the forced hot air system is blowing hot air into the space
and too cold before the forced hot air system begins to heat again.
Oscillation about a desired temperature is made even more
noticeable by a roaring noise that is present during the operation
of the forced hot air system. When a fluid fueled forced hot system
is running, the roaring noise may be created by the combustion of
the fluid fuel. This roaring noise is transmitted throughout the
space to be heated by ducting and the blowing of the air. A more
comfortable means of heating with forced hot air is desired.
Therefore, a forced hot air heating system that is quieter and more
comfortable than what is currently achievable is desired.
[0012] For the foregoing reasons, there is a need for forced hot
air heating system and method for the North American market that
cleanly, efficiently, and comfortably heats using solid fuel.
SUMMARY OF THE INVENTION
[0013] In various embodiments, a system in accordance with the
present invention facilitates forced hot air heating with solid
fuel, such as wood pellets, with near immediate response to demands
for heat in a space to be heated. This is achieved, in part, by
sensing the temperature of a space to be heated, providing a space
temperature feedback, and transporting an ambient air to the space
to be heated in response to an air handler power signal. The air
handler power signal is provided in response to the space
temperature feedback, allowing the transportation of the ambient
air to be immediately responsive to the temperature of the space to
be heated. The ambient air having thermal energy transferred to it
from exhaust gases resulting from the combustion of a solid fuel.
The combustion of the solid fuel is provided for by a burner
operating in response to a burner power signal. The transportation
of the ambient air occurs immediately once a need for heat in the
space to be heated is determined and ceases immediately once the
need for heat is satisfied. Heating of the space to be heated is
thus not directly dependent on the burner, instead it is only
required that the ambient air being transported to the space to be
heated is of an elevated temperature, such that it warms the space
to be heated.
[0014] In order to provide immediate response to the need for heat,
the ambient air must either be maintained at an elevated
temperature or immediately heated by a thermal store. Thus in some
embodiments, the temperature of the ambient air is sensed,
providing an ambient temperature feedback and the burner power
signal is provided in response to the ambient temperature feedback.
Thus combustion, the resulting exhaust gases and thermal energy are
provided in response to the temperature of the ambient air, rather
than the temperature of space to be heated, which is the
conventional method. The thermal energy being either stored in a
thermal mass, for immediate transfer to the ambient air, or
transferred directly to the ambient air for the exhaust gases
allows the ambient air to be of an elevated temperature when
transported to the space to be heated. Thus the generation of
thermal energy is decoupled from the temperature of the space to be
heated. With appropriate burner heat output sizing and the optional
use of thermal compliance, maintaining an elevated temperature of
the ambient air is efficient during periods of consistent heating
needs, such as Maine in February. However, during shoulder seasons
that have a periodic need for heat constantly maintaining an
elevated ambient air temperature may be inefficient or result in
overheating of the heating system.
[0015] In order to allow for safe and efficient, automatic,
year-round heating some embodiments include a furnace regulator
switching means. The furnace regulator switching means selectively
grants control of the burner and air handler power signal to either
a continuous operation furnace regulating means or a periodic
operation furnace regulating means. Where the continuous operation
furnace regulating means provides the burner power signal in
response to the ambient temperature feedback, maintain the ambient
air at an elevated temperature, such that the air handler power
signal may be immediately responsive to the space temperature
feedback. And, the periodic operation furnace regulating means
provides the burner power signal in response to the space
temperature feedback as is the case with conventional forced hot
air heating systems. Control of the burner and air handler power
signals may be selectively granted according to a number of
variables, including: The presence of an overheat status in which
the exhaust gases, ambient air or any other system component is
found to be over a high limit setpoint temperature. A duration of
time that the power signal has remained at a specific state, such
as HIGH, ON, LOW, or OFF, exceeding a timeout value. A calculated
heat load, representing the amount of thermal energy being provided
over a determinable time exceeding a heat load threshold value. Or
the outdoor temperature, as provided by an outdoor temperature
feedback, being cold enough to warrant the granting of control to
the continuous operation furnace regulating means or warm enough to
grant control to the periodic operation furnace regulating
means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows a block diagram of components of a forced hot
air solid fuel heating system;
[0017] FIG. 2A shows examples of thermosensitive means;
[0018] FIG. 2B shows a block diagram of a PID controller;
[0019] FIG. 3 shows a block diagram of components of the forced hot
air solid fuel heating system;
[0020] FIG. 4 shows thermal energy storage;
[0021] FIG. 5A shows a flow chart for a burner regulating
means;
[0022] FIG. 5B shows a flow chart for an air handler regulating
means;
[0023] FIG. 6A shows a flow chart for a furnace regulating
means;
[0024] FIG. 6B shows a flow chart for a furnace regulator switching
means;
[0025] FIG. 6C shows a flow chart for a burner regulating means for
a continuous operation regulating means;
[0026] FIG. 6D shows a flow chart for an air handler regulating
means for a continuous operation regulating means;
[0027] FIG. 6E shows a flow chart for a burner regulating means for
a periodic operation regulating means;
[0028] FIG. 6F shows a flow chart for an air handler regulating
means for a periodic operation regulating means;
[0029] FIG. 7 shows an axiomatic design decomposition of the solid
fuel heating system;
[0030] FIG. 8 shows an example electrical circuit for amplification
and smoothing; and
[0031] FIG. 9 shows the burner regulating means controlling a
dosing auger and a combustion fan.
[0032] FIG. 10A shows a Temperature vs. Time plot for a prototype
heating system comprising a periodic operation regulating
means;
[0033] FIG. 10B shows a Temperature vs. Time plot of a heating
system comprising a burner regulating means and an air handler
regulating means;
DETAILED DESCRIPTION
[0034] FIG. 1 shows a burner that burns solid fuels to provide
combustion resulting in exhaust gases. The burner may be fueled by
wood pellets, wood chips, cord wood or any other type of solid
fuel. The amount of thermal energy output by a non-modulating
burner, such as the Pellergy 1525, is determined only by the amount
of time the non-modulating burner is operating. The amount of
thermal energy delivered by a modulating burner, such as the
Janfire NH, is determined by the amount of time the modulating
burner is operating as well as a modulation level of the modulating
burner at any given time. The operation of the burner is controlled
by a burner power signal. The burner power signal for the
modulating burner provides the modulation level for the modulating
burner to operate at. The burner power signal for the
non-modulating burner provides an operational state for the
non-modulating burner, either ON or OFF.
[0035] The burner is located proximate to a heat exchanger, so that
thermal energy from the exhaust gases may be transferred by the
heat exchanger. The heat exchanger is an air to air type heat
exchanger having two or more sides, all sides being sealed from one
another. The exhaust gases are contained adjacent to a hot side of
the heat exchanger and an ambient air is contained adjacent to a
warm side of the heat exchanger. The heat exchanger may be a shell
and tube type heat exchanger, wherein one fluid, either exhaust
gases or ambient air, is contained within one or more tubes and the
other fluid is contained within a shell which surrounds the
tubes.
[0036] An air handler is located in fluidic communication with the
heat exchanger, such that operation of the air handler moves the
ambient air toward a space to be heated. Examples of spaces to be
heated include: residences, municipal buildings and business.
Examples of air handlers include blowers, and dampeners. Air
handlers may be variable or discrete in operation. A variable speed
blower for example, allows for the ambient air to be transported to
the space to be heated at various rates within a speed range. The
amount of air delivered to the space to be heated by the variable
speed blower is determined by the amount of time the variable speed
blower is operating as well as the flow rate of the variable speed
blower at any given time. The variable speed blower may be a
multi-speed blower, such as a 3-speed furnace blower model
#6BLR12FAP from Fantech, Inc. of Lenexa, Kans. The 6BLR12FAP may be
run at three speeds LOW, MEDIUM, and HIGH. The 6BLR12FAP comprises
a permanent split capacitor (PSC) motor. Variable speed blowers
that comprise PSC motors and shaded pole motors may be controlled
by a variable speed blower controller. Such variable speed blower
controllers take as input a signal voltage and drive the variable
speed blower at a speed proportional to the signal voltage. The
variable speed blower controller allows the variable speed blower
to be operated within a range of speeds rather than three discrete
speeds. For example, the pairing of the 6BLR12FAP with a Nimbus--AC
Fan Control, Model No. 240B7T00-F from Control Resources, Inc. of
Littleton, Mass., allows for the blower to be operated variably
over a range of speeds not just 3 discrete speeds. One of the
benefits of the variable speed blower controllers, such as the
Nimbus, is that it provides for the gradual ramping up and ramping
down of air flow. Gradually increasing (and/or decreasing) the flow
rate of air into an occupied space makes the blowers operation less
noticeable to the occupants of the space. The variable speed blower
controller may comprise TRIAC, variable frequency or AC to DC
inverter technology in order to control the speed of the variable
speed blower.
[0037] Alternatively, the air handler may be comprised by a single
speed blower. The flow rate for the single speed blower is
constant, and the amount of ambient air delivered to the space to
be heated is controlled only by the amount of time the single speed
blower is operating. The G-12 blower mated with a 3/4 horsepower
motor from Delhi Industries, Inc. of Brockville, Ontario Canada is
an example of a single speed blower that may provide 100,000 BTU/hr
to the space to be heated, given a typical ambient air temperature
rise. The ambient air is typically directed toward the space to be
heated through one or more supply ducts, which are in fluidic
communication with the air handler, and from the supply ducts into
the space to be heated through one or more supply outlets. The
ambient air is typically returned to the heat exchanger by one or
more return ducts.
[0038] An ambient thermosensitive means senses an ambient air
temperature and provides an ambient temperature feedback. A space
thermosensitive means senses a space to be heated temperature and
provides a space temperature feedback. An ambient temperature
setpoint and a space temperature setpoint are provided relative the
ambient temperature feedback and the space temperature
feedback.
[0039] The ambient temperature feedback and the space temperature
feedback may be provided continuously. Examples of thermosensitive
means that provide temperature feedback continuously are shown in
FIG. 2A. The ambient temperature feedback and space temperature
feedback may be provided continuously by a resistance temperature
detector (RTD), a thermistor or a thermocouple. The thermocouple
provides an output potential that correlates with a change in
temperature. The RTD and the thermistor provide an output
resistance that varies with temperature. The model# ATH100K1R25T70
from Analog Technologies, Inc. of Santa Clara, Calif. is an
exemplary thermistor for measuring temperatures associated with the
ambient air. By applying an input potential of a known value, for
example a signal voltage such as 5V, to the thermistor or the RTD
and measuring the resulting flow or potential over the sensor the
output resistance of the thermistor or RTD and the correlating
temperature may be found. Typically the thermistor or the RTD is a
part of a thermistor circuit or an RTD circuit that comprises: a
potential divider, a Wheatstone bridge, or an equivalent circuit.
The thermistor circuit or the RTD circuit provides an output
resistance through the use of one or more reference resistances, of
known value, and the input potential. The output potential of the
thermistor circuit or the RTD circuit varies according to the
output resistance provided by the thermistor or RTD. The thermistor
circuit, the RTD circuit or a thermocouple circuit may further
comprise an analog to digital converter (ADC), providing for
measurement of the output potential. The ADC converts the output
potential to an output code, which is a digital number
representative of the output potential. The precision of the ADC is
measured by a resolution, which is the maximum number of the
possible output codes over the measurable range of the output
potentials. For example, the resolution of an 8-bit ADC allows for
2 8 or 256 discrete values. An 8-bit ADC measuring over a 0-5V
range is 5/(256-1), or roughly 20 mV. The resolution is equal to
the smallest change in the output potential that is required to
produce a change in the output code. In the case of a continuous
thermosensitive means, such as those described above and as well as
eqivalents such as a Silicon diode bandgap sensor circuit, the
corresponding temperature setpoint will typically be an
electronically or digitally coded value.
[0040] The ambient temperature feedback and space temperature
feedback may alternatively function discretely. The thermosensitive
means functioning discretely provide a temperature feedback that is
relative to a setpoint temperature. The thermosensitive means
functioning discretely provide the ambient temperature feedback or
the space temperature feedback discretely, such that the ambient
temperature feedback or the spacer temperature feedback is either
TRUE or FALSE. The ambient temperature feedback or the space
temperature feedback functioning discretely gages if the sensed
temperature is higher or lower than the ambient temperature
setpoint or the space temperature setpoint. Examples of
thermosensitive means that function discretely include: a
mechanical thermostat and a thermostatic switch. Generally the
mechanical thermostat and the thermostatic switch employ the use of
a bimetallic strip to sense the temperature. The bimetallic strip
is comprised of two metal strips with differing rates of thermal
expansion, fused together. As the temperature of the bimetallic
strip changes, a first metal deforms more than a second metal
causing the bimetallic strip to bend proportionally with the change
of temperature. For example, the mechanical thermostat generally
makes use of a bimetallic coil. Depending on the orientation of the
bimetallic coil, the bimetallic coil may grow tighter or looser
with increasing temperature. The change in the bimetallic coil
results in an outside end of the bimetallic coil moving. A moving
contact is attached to the moving end of the bimetallic coil. An
electrical connection is made within the mechanical thermostat,
between the moving contact and a static contact at a specified
temperature. The electrical connection is often made when the
temperature falls below the specified temperature. Typically the
specified temperature of the mechanical thermostat is set by
adjusting the location of the static contact. The thermostatic
switch usually incorporates a bimetallic disk. The thermostatic
switch is typically not adjustable, although thermostatic switches
that allow for adjustment are available. The electrical connection
is made within the thermostatic switch between a first contact at
the center of the bimetallic disk and a second contact at the
circumference of the bimetallic disk. At a specified transition
temperature the bimetallic disk pops, inverting itself. The
bimetallic disk once inverted creates (or breaks) the electrical
connection with the first and second contact of the thermostatic
switch. Other types of thermostatic switches may include a bellows
with a fluid or a wax, of a known rate of thermal expansion. The
bellows expands under increasing temperature causing a mechanical
switching mechanism, such as a micro-switch, to create (or break)
the electrical connection. In the case of discretely functioning
thermosensitive means the corresponding temperature setpoint will
typically be a mechanical arrangement that allows for electrical
connection or a change in electrical continuity, at the transition
temperature.
[0041] A burner regulating means is shown in FIG. 1. The burner
regulating means shown in FIG. 1 provides a burner power signal in
response to the ambient temperature feedback and the ambient
temperature setpoint. The ambient temperature feedback is provided
continuously as the coded output from the RTD circuit. The ambient
temperature feedback may be provided as a direct measure of the
temperature of the ambient air by sensing the temperature of the
ambient air directly. It may also be practical, in certain
situations, to provide the ambient temperature feedback through
indirect measurement. For example, the ambient temperature feedback
may be indirectly measured through the sensing of the temperature
of the warm side of the heat exchanger, or another substance that
is in or near thermodynamic equilibrium with the ambient air. The
temperature of the substance in or near thermodynamic equilibrium
with the ambient air may provide an indirect measure of the ambient
air and the ambient air temperature feedback. The ambient
temperature feedback may be a coded ambient setpoint. The coded
ambient setpoint may be stored within a digital storage memory,
such as FLASH, random access memory (RAM), or read only memory
(ROM). The burner regulating means may comprise a
proportional-integral-derivative (PID) burner controller, shown in
FIG. 2B. The PID burner controller may be implemented through the
use of a microcontroller (MCU) or a field programmable gate array
(FPGA). The PID burner controller calculates an ambient temperature
error, e(t), which is the difference between the ambient
temperature setpoint and the ambient temperature feedback. The
ambient temperature error may be calculated at regular intervals
defined by a time constant. The current ambient temperature error
is weighted by a proportional (P) factor creating a proportional
variable. The sum of the ambient temperature errors over time is
weighted by an integral (I) factor creating an integral variable.
The difference between the previous ambient temperature error and
the current ambient temperature error is divided by the time
constant and weighted by a derivative (D) factor creating a
derivative variable. The proportional variable, integral variable,
and derivative variable are summed to create a control variable.
The control variable is then used as the burner power signal for
the modulating burner. Once tuned the PID burner controller will
minimize the ambient temperature error, or the difference between
the ambient temperature feedback and the ambient temperature
setpoint, by adjusting the burner power signal.
[0042] An air handler regulating means provides the air handler
power signal. The air handler regulating means shown in FIG. 1
comprises the mechanical thermostat located in the space to be
heated. The static contact of the mechanical thermostat provides
the space temperature setpoint. The moving contact of the
mechanical thermostat provides the space temperature feedback. As
the temperature of the space to be heated drops below the space
temperature setpoint, the moving contact and the static contact of
the mechanical thermostat form the electrical connection. The
electrical connection may be used directly, to deliver the air
handler power signal, as with an inline thermostat. Alternatively,
the electrical connection of the mechanical thermostat may carry
only a smaller control current that closes a relay, which delivers
the air handler power signal. The signal speed blower shown in FIG.
1 operates when the mechanical thermostat determines that the
temperature of the space to be heated is below the space
temperature setpoint.
[0043] The ambient air setpoint in FIG. 1 may be in a range 40-95
degrees Celsius. This range is selected as it is an air temperature
range that is sufficiently above room temperature so that air at
this temperature would act to heat a space to be heated. The burner
regulating means in FIG. 1 maintains the ambient air temperature in
this range. When the temperature of the space to be heated is below
the space temperature setpoint the air handler regulating means,
which comprises the mechanical thermostat, operates the single
speed blower and the ambient air is delivered into the space to be
heated, warming it. When the air handler is operating and moving
the ambient air past the heat exchanger, the temperature of the
ambient air decreases and the burner regulating means increases the
modulation level of the burner. As the space to be heated is heated
to the space temperature setpoint, the air handler regulating means
stops the single speed blower. The ambient air flow decreases,
causing the ambient temperature feedback to increase, and the
burner regulating means decreases the modulation level of the
burner.
[0044] A similar embodiment of the present invention is shown in
FIG. 3. The thermostatic switch is shown as the ambient
thermosensitive means for providing the ambient temperature
feedback. The thermostatic switch also provides the ambient
temperature setpoint as the specified transition temperature of the
thermostatic switch. The thermostatic switch may directly provide
the burner power signal to the non-modulating burner, shown in FIG.
3. In the shown embodiment the thermostatic switch regulates the
operation of the non-modulating burner in order to keep the
temperature of the ambient air at or above the ambient temperature
setpoint. Typically the ambient temperature setpoint is about 40
degrees Celsius, so that the non-modulating burner operates when
the ambient air temperature falls below 40 degrees Celsius. A
hysteriesis of the thermosensitive means defines an operation range
proximate the setpoint temperature according to the current
operational state. The hysteriesis of the thermostatic switch will
cause the non-modulating burner, which is currently operating, to
stop once the ambient temperature feedback is outside of the
operation range. The operation range typically has an upper limit
that is about 60 degrees Celsius. This operation range may
alternatively be defined by the use of more than one thermostatic
switch. Additionally, one or more high limit temperature sensors
may be used to cease the operation of the burner through the burner
power signal. These high limit temperature sensors may be located
on the hot side or the warm side of the heat exchanger, in the
exhaust gas or in the ambient air.
[0045] A digital thermostat shown in FIG. 3 employs the thermistor
circuit to provide the space temperature feedback. The digital
thermostat also has an input for the space temperature setpoint,
allowing the selection of a desired temperature in the space to be
heated. The microcontroller takes as input the space temperature
feedback and the space temperature setpoint and provides the air
handler power signal. The flow rate of the variable speed blower is
controlled by the air handler power signal. The microcontroller
shown in FIG. 3 may use PID controls or any other closed feedback
control algorithm to provide the air handler power signal that
determines the flow rate of the variable speed blower. As the
burner will operate at times when the space to be heated is at or
above the space to be heated temperature feedback, the heat
exchanger or exhaust gases could potentially become dangerously hot
before the ambient temperature feedback is greater than the ambient
temperature setpoint. In order to prevent the heat exchanger or
exhaust gases from overheating the variable speed blower may
operate at a low flow rate. The low flow rate is generally defined
as the flow rate produced when the variable speed blower is
operating at 50% or less than the variable speed blower's maximum
speed. The low flow rate moves enough ambient air away from the
heat exchanger to allow more thermal energy to be transferred away
from the exhaust gases, but does not move enough ambient air to
raise the temperature of the space to be heated more than about
2.degree. C. Also, the air handler means may include a dampener
that allows the flow of ambient air past the heat exchanger, but
not into the space to be heated at times when the temperature of
the space to be heated is above the space temperature setpoint to
prevent over heating of the heat exchanger or exhaust gases. As the
air handler regulating means determines an increased need for heat
in the space to be heated the flow rate of the variable speed
blower is increased up to 100% speed.
[0046] Independent regulation of the ambient air temperature and
the space to be heated temperature may require the incorporation of
a thermal storage means for providing a thermal mass. The thermal
mass may act as an energy buffer during times when the heat
provided by the burner is not balanced by the heat provided to the
space to be heated. Many solid fuel burners, such as the Janfire NH
and the Pellergy 1525 must cease operation and purge the ash, which
has accumulated in the burn chamber, periodically. An ash scrape
mechanism related to the Janfire NH is the subject of U.S. Pat. No.
7,739,966. During this time the burner is not operating and no heat
is being generated, however the space to be heated may still
require heat. The thermal mass allows for the heat stored in it to
be transferred to the ambient air and transported to the space to
be heated when the burner is not operating. The thermal mass should
be sized to provide for delivery of heat to the space to be heated
throughout the off-time of a typical ash scrape cycle. Additionally
at times when the space to be heated is not in need of heat, the
thermal mass may accumulate the heat that is generated by the
burner in order to prevent heat from being vented, and subsequently
fuel from being unnecessarily consumed. FIG. 4 shows some examples
of thermal storage means. The thermal mass may be provided through
a wall thickness, or mass of the heat exchanger. The wall thickness
when increased creates the thermal mass. The ambient air itself may
also act as the thermal storage means. Recirculating the ambient
air past the heat exchanger allows for a greater volume of ambient
air for thermal energy storage. Although the heat capacity of air
at standard room conditions is low, during recirculation, all
surfaces that come into contact with the ambient air as it
recirculates additionally act as the thermal storage means.
Recirculation of the ambient air may be achieved through the use of
the air handler and a recirculation dampener, which when closed
directs the ambient air back to the air handler and when open
directs the ambient air to the space to be heated.
[0047] FIG. 5A depicts the burner regulating means for providing
the burner power signal configured for use with the non-modulating
burner, such as the Pellergy 1525, and comprising thermosensitive
means that function discretely. The ambient thermosensitive means
shown in figure FIG. 5A is the thermostatic switch. The
thermostatic switch may be a Klixon, open on rise, thermostat part
number: M223L140050521 from Sensata Technologies, Inc. of
Attleboro, Mass. The Klixon, open on rise, thermostat may open at
140.degree. F. and close about 135.degree. F. Optionally, the
burner regulating means may include an exhaust gas high limit
switch, such as a thermal safety switch part No. WMO-1 200 from
Field Controls, LLC of Kinston, N.C. and/or a heat exchanger high
limit switch. The heat exchanger high limit switch may be, for
example, a SHL250 Thermostat Limit Control from Sealed Unit Parts
Co., Inc. of Manasquani, N.J. The SHL250 may be automatically
resetting. The SHL250 cuts out at 250.degree. F. and cuts in at
210.degree. F. If the exhaust gases are sensed by the exhaust gas
high limit switch to be at a higher temperature than an exhaust gas
high limit setpoint, then the burner power signal is interrupted
and the exhaust gas high limit switch will need to be manually
reset before the burner power signal will be uninterrupted. If the
heat exchanger is sensed by the heat exchanger high limit switch to
be at a temperature in excess of a heat exchanger high limit
setpoint the burner power signal is interrupted. As the ambient air
temperature feedback falls below the ambient air setpoint as sensed
by the thermostatic switch, the burner power signal operates the
non-modulating burner. The non-modulating burner once operating
warms the ambient air until the ambient air is outside of the
operational range causing the burner regulating means to interrupt
the burner power signal and cease the non-modulating burner from
operating. It is shown in FIG. 5A that the burner regulating means
provides the burner power signal, generally in response to the
ambient temperature feedback, as it is in response to the ambient
temperature feedback that the burner power signal initiates the
burner. The burner power signal may be interrupted or changed to an
OFF signal in response to other feedbacks, such as with the exhaust
gas high limit switch above, however the ON signal provided by the
burner power signal requires the ambient temperature feedback be
within the operational range, defined by the burner regulating
means.
[0048] FIG. 5B shows the air handler regulating means for providing
the air handler power signal, configured for use with the single
speed blower and implemented through discrete mechanisms. The air
handler power signal causes the operation of the single speed
blower when the space temperature feedback is below the space
temperature setpoint. The space temperature feedback may be
provided for by the mechanical thermostat, for example a Honeywell
Model # CT8K from Honeywell International, Inc. of Belmont, N.C.,
and a relay circuit. Whereby, the air handler regulating means
provides the air handler power signal powering the single speed
blower. Once the space to be heated has been sufficiently warm to
overcome the hysteresis of the mechanical thermostat the air
handler power signal will be interrupted and the single speed
blower will cease to operate. Optionally, a minimum ambient
temperature switch and a maximum ambient temperature switch may be
located proximate the warm side of the heat exchanger to sense the
temperature of the ambient air. If included in the air handler
regulating means the minimum ambient temperature switch will
interrupt the air handler power signal if the ambient air has a
temperature lower than a minimum ambient temperature setpoint. The
minimum ambient temperature switch may be a close on rise
thermostat that closes at 90.degree. F. such as SHF90 Thermostat
Fan Control from Sealed Unit Parts Co., Inc. The purpose for the
minimum ambient temperature switch is to prevent ambient air from
being transported to the space to be heated, if the ambient air is
too cool. If included in the air handler regulating means the
maximum ambient temperature switch will interrupt the air handler
power signal, and optionally the burner power signal if the ambient
air has a temperature that is above a maximum ambient air setpoint.
The maximum ambient air switch may be, for example a SHL200
Thermostat Limit Control from Sealed Unit Parts Co., Inc. The
SHL200 is an open on rise type limit switch and cuts out at
200.degree. F. The function of the maximum ambient temperature
switch is to prevent ambient air from being transported to the
space to be heated, if the ambient air may be unsafe to deliver to
the space to be heated. The State of Maine requires that solid fuel
burning furnaces have the functionality provided by the maximum
ambient temperature switch. Specifically the Maine Solid Fuel Board
requires: "Furnaces shall have a 250 degree Fahrenheit limit
control installed in the supply plenum not more than ten (10)
inches above the top surface of the heat exchanger. The limit
control shall extend at least twelve (12) inches into the supply
plenum."
[0049] FIG. 6A depicts a furnace regulating means for selectively
providing the burner power signal and the air handler power signal.
The furnace regulating means may be software based and operate on
the microcontroller. The furnace regulating means is comprised of
three sub-functions: a continuous operation means and a periodic
operation means, each of which provides the burner power signal and
the air handler power signal and a furnace regulator switching
means for selectively granting control of the burner power signal
and the air handler power signal to either the continuous operation
means or the periodic operating means. The continuous operation
means for providing the burner power signal is provided in response
to the ambient air temperature, and independent of the space
temperature feedback. This allows for the burner to operate more
continuously reducing the number of times the burner starts up.
Thus, the continuous operation means is well suited when demand for
heat is high, greater than 50% maximum heat output of the heating
system. Alternatively, the periodic operation means for provides
the burner power signal only during a thermostat call, which is
provided for by the space to be heated temperature feedback. Thus,
the periodic operation means will allow for heat to be generated by
the burner only at times when the space to be heated requires
heating and not operate the burner when the space to be heated does
not require heat. The periodic operation means allows for reduced
fuel consumption, and is well suited when demand for heat is
moderate or low, or less than 50% of the maximum heat output of the
heating system. The furnace regulator switching means,
deterministically grants control based upon a perceived need for
continuous operation, to either the continuous operation means or
the periodic operation means.
[0050] An embodiment of the furnace regulator switching means for
selectively granting control of the burner power signal and the air
handler power signal is shown in FIG. 6B. The perceived need for
continuous operation may be based on an outdoor temperature
feedback. The outdoor temperature is sensed as input as the outdoor
temperature feedback. A heat load is calculated as a total energy,
the summation of the burner heat output beginning at an energy
start time, divided by the difference between the current time and
the energy start time. If the burner power signal is currently at a
LOW power level and a previous burner power signal, defined as the
last burner power signal, is not LOW, the current time is recorded
as a low fire start time. If the burner power signal is currently
at a HIGH power level, defined as 100% heat output for the burner,
and the previous burner power signal is not HIGH, the current time
is recorded as a high fire start time.
[0051] If the continuous operating means currently has control of
the burner power signal and air handler power signal, the furnace
regulating switching means decides based on one or more criteria if
the continuous operating means should continue to have control
according to the embodiment of the furnace regulator switching
means shown in FIG. 6B. The continuous operation means loses
control with the presence of one or more of the following
conditions, which may or may not be determining factors in other
embodiments of the invention: If the burner is currently operating
at LOW and has been operating at LOW for longer than a burner low
timeout value, if the outdoor temperature feedback is found to be
above a maximum continuous outdoor temperature, if an overheat
status is present, in which case the overheat status may be
cleared, if the heat load is below a minimum continuous heat load
value and the time the heat load is measured over is greater than a
minimum heat load time. If conditions that cause loss of control by
the continuous operation means are not present the furnace
regulator switching means continues granting control to the
continuous operation means. If conditions that cause loss of
control by the continuous operation means are present, the total
energy is reset to zero and the current time is recorded as the
energy start time.
[0052] If the continuous operation means currently does not have
control, the furnace regulator switching means shown in FIG. 6B
decides based on one or more criteria if the continuous operating
means should be granted control of the burner power signal and the
air handler power signal. According to the embodiment of the
furnace regulator switching means shown in FIG. 6B the continuous
operation means are granted control under one or more of the
following conditions, which may or may not be determining factors
in other embodiments of the present invention: If the burner is
currently operating at HIGH and has been operating at HIGH for
longer than a burner high timeout value, if the outdoor temperature
feedback is found to be below a minimum periodic outdoor
temperature, if the heat load is above a maximum periodic heat load
value and the time the heat load is measured over is greater than
the minimum heat load time. If conditions that cause the grant of
control to the continuous operation means are present, the total
energy is reset to zero and the current time is recorded as the
energy start time.
[0053] An embodiment of the continuous operation means provides the
burner power signal and the air handler power signal according to
the flow chart shown in FIG. 6C and FIG. 6D. The continuous
operation means may optionally, as a safety precaution sense the
temperature of the exhaust gases and the heat exchanger and compare
them against the exhaust gas high limit temperature and the heat
exchanger high limit temperature. If either the exhaust gas
temperature or the heat exchanger temperature is found to be higher
than its corresponding high limit temperature, the burner power
signal sends an OFF signal to the burner and an overheat error is
flagged. If there is no overheating of the exhaust gases or heat
exchanger, or if those safety features are not present the burner
power signal is provided in response to the ambient air temperature
feedback. For example, the burner power signal provided for the
modulating burner may be provided by the PID controller. Once the
burner power signal has been provided the continuous operation
means performs the logic to provide the air handler power signal.
Optionally, the continuous operation means may check the
temperature of the ambient air in order to ensure that it is within
a range defined by the minimum ambient air temperature and the
maximum ambient air temperature. If the temperature of the ambient
air is found to be outside of the range defined by the minimum
ambient air temperature and the maximum ambient air temperature the
air handler signal sends an OFF signal to the air handler. If the
temperature of the ambient air is found to be above the maximum
ambient air temperature then the burner power signal is provided as
an OFF signal to the burner. If the temperature of the ambient air
is found to be with the range defined by the minimum ambient air
temperature and the maximum ambient air temperature, the continuous
operation means provides the air handler power signal in response
to the space temperature feedback. The space temperature feedback
may be provided by the mechanical thermostat. Such that, the air
handler power signal is provided a HIGH signal when there is a
thermostat call and a LOW signal when there is no thermostat call.
The air handler power signal may be used to operate the variable
speed blower, such that a LOW signal results in a small ambient air
flow, typically less than <50% speed of the variable speed
blower and the HIGH signal results in a large ambient air flow
typically between 90% and 100% speed of the variable speed blower.
The small ambient air flow may be defined as being large enough to
remove heat from the heat exchanger, and prevent overheating while
allowing the burner to continue to operate. The small ambient air
flow may also be defined as being small enough to not raise the
temperature of the space to be heated during an average heat load
during a heating season.
[0054] An embodiment of the periodic control means provides the
burner power signal and the air handler power signal according to
the flow chart shown in FIG. 6E and FIG. 6F. The periodic control
means may optionally, as a safety precaution sense the temperature
of the exhaust gases and the heat exchanger and compare them
against the exhaust gas high limit temperature and the heat
exchanger high limit temperature. If either the exhaust gas
temperature or the heat exchanger temperature is found to be higher
than its corresponding high limit temperature, the burner power
signal sends an OFF signal to the burner and an overheat error is
flagged. If there is no overheating of the exhaust gases or heat
exchanger, or if those safety features are not present the burner
power signal is provided. The periodic control means provides the
burner power signal operating the burner only when the space
temperature feedback is lower than the space temperature setpoint.
For example, the space temperature thermosensitive means may be
provided for by the mechanical thermostat and the burner may be the
modulating burner. In this case the periodic control means may
provide the burner power signal based upon the ambient temperature
feedback only once the mechanical thermostat is calling for heat.
The periodic control means may provide the air handler power signal
according to the ambient air temperature. If the temperature of the
ambient air is found to be outside of the range defined by the
minimum ambient air temperature and the maximum ambient air
temperature the air handler signal sends an OFF signal to the air
handler. If the temperature of the ambient air is found to be above
the maximum ambient air temperature then the burner power signal is
provided to send an OFF signal to the burner. If the temperature of
the ambient air is found to be within the range defined by the
minimum ambient air temperature and the maximum ambient air
temperature, the periodic operation means provides a HIGH air
handler power signal to the air handler. The air handler may be the
variable speed blower in which case the speed of the blower may be
ramped up and/or ramped down in order to improve comfort.
[0055] FIG. 7 is an axiomatic design decomposition of the
invention. The decomposition shows a list of Functional
Requirements (FR) to the left and a list of corresponding Design
Parameters (DP) to the right. It can be seen from FIG. 7 that FR1
"Control the generation of heat" is an independent function from
FR4 "Control the flow of ambient air toward the space to be
heated". Characterizing the present invention, the generation of
heat is done as needed to maintain enough available heat, such that
the ambient air may be warmed FR3. The generated heat may be stored
FR2, by either the ambient air or other means prior to its delivery
to the space to be heated. Delivery of the ambient air occurs as
needed to maintain a set temperature in the space to be heated.
Although procedural coupling does exists, for example heat may not
be stored until FR2 or transfer to the ambient air FR3 before it
has been generated FR1, the design is decoupled. Current solid fuel
forced hot air systems are not decoupled as the heat is generated
in response to the temperature of the space to be heated,
preventing the generation of heat from occurring independently from
DP4.1, which "sense(s) the temperature of the space to be heating."
Because of the decoupled design shown in FIG. 7 it is possible for
the designer of the forced hot air system to setup a design
equation that defines the heating of the space to be heated in
terms defined in the design decomposition. The solving of this
design equation allows for optimal performance:
[0056] Decoupling these functional requirements allows for a solid
fuel hot air furnace that operates with longer burner ON times,
more efficient combustion, and results in more comfortable heating.
The space to be heated is maintained at the setpoint temperature
without excessive oscillation about the setpoint. The heating of
the space is often unnoticeable by the occupants, as the
temperature of the space remains steady and the roar that often
accompanies a fluid fueled burner is absent, with solid fuel
burning appliances.
[0057] In an embodiment of the present invention, the
microcontroller used to operate the regulating means described
above may be an Arduino Mega 2560, which is an Open Source
microcontroller for development. Controlling the modulation level
of the Janfire NH burner may be achieved through the burner power
signal provided for by one of a number of PWM Analog 0-5V analog
outputs of the ATMega2560. An operational amplifier or equivalent
means may be used to double the potential of the PWM 0-5V analog
output, as shown in FIG. 8. An RC filter may be used to smooth the
PWM 0-10V analog signal. The resolution of the analog outputs of
the ATMega2560 is 8-bit. Therefore the 0-10V signal to the Janfire
NH burner must be mapped to 0-255 digital bits.
[0058] The ATMega2560 may control a single speed blower through the
use of a digital output and solid state relay, such as Crydom, Inc.
model No. DC60S3 from Crydom of San Diego, Calif. The ATMega2560
may control a variable speed blower through the use of an AC fan
controller, such as the Nimbus--AC Fan Control Model No. 240B7T00-F
from Control Resources, Inc. of Littleton, Mass. The ATMega2560 may
communicate to the Nimbus via a control signal, which has a
variable potential of 0-10V. The Nimbus provides the air handler
power signal to the variable speed blower proportionally to the
control signal. The control signal may be output by via an analog
output of the ATMega2560 through the schematic shown in FIG. 8 and
equivalents thereof.
[0059] The ATMega2560 may control the operation of the burner
through the PWM analog outputs, as shown in FIG. 9. The ATMega2560
may control the speed of the combustion fan, which is suited to
deliver air for the burner. The ATMega2560 may control the amount
of wood pellet fuel delivered to the burner through the use of the
dosing auger. The dosing auger may include an auger position
feedback that measures the position of the dosing auger allowing
for a known amount of wood pellet fuel to be delivered. In the case
that the dosing auger and combustion fan are directly controlled by
the burner regulating means, the burner power signal comprises the
signals operating both the dosing auger as well as the combustion
fan.
[0060] The ATMega2560 may take as digital inputs: the mechanical
thermostat and thermostatic switches and may take as analog inputs
thermocouple circuits and RTD or thermistor circuits. The digital
inputs detect a change in the continuity of the circuit attached
(mechanical thermostat or thermoswitch) and change the value of a
boolean variable to match the switches state. The analog inputs of
the ATMega2560 are 8-bit and allow for mapping of the output
potential to a 0-255 output code.
[0061] For functions that require timing the ATMega2560 may use the
millis ( ) function to return the current time from an external 16
MHz oscillator. Digital storage on the ATMega2560 is provided for
by 256 Kb of FLASH storage.
[0062] Software code for regulating a wood pellet forced hot air
furnace is shown in Appendix A. The code in Appendix A may be run
on an Arduino, such as ATMega2560, in order to provide the burner
power signal and the air handler power signal according to an
exemplary embodiment of the invention.
[0063] The Temperature vs. Time plots shown FIG. 10A-C illustrate
the operation of a solid fuel forced hot-air heating system
operating in accordance with various embodiments of the present
invention. The system comprising a shell-tube type heat exchanger,
a Janfire NH wood pellet burner, a blower, and an Arduino
ATMega2560 running software illustrated in Appendix A. The system
is configured to heat a room, the interior space of The God-Damned
Jewel and Ideal Woman Tavern of Bethel, Me. The room is of
approximately 150 cubic meters. Hot air is provided through a
supply duct and return air is return to the system through a return
duct. The system employs a thermostat in the room to provide a
space temperature feedback and a space temperature setpoint. The
system employs thermistors in thermal communication with the
ambient air and the heat exchanger to provide an ambient
temperature feedback. An ambient temperature setpoint is included
within the code running on the Arduino.
[0064] FIG. 10A illustrates the initial cold-start start of the
system. The initial start of the system demonstrates the periodic
operation means outlined in FIG. 6D as well as the current control
methods employed by common forced hot-air heating systems. It can
be seen that it takes a substantial amount of time, .about.750
seconds, after the thermostat call for the ambient air within the
heat exchanger to warm sufficiently for the blower to begin to
transport the ambient air to the room. During this amount of time
the room will continue to lose heat and drop lower below the space
temperature setpoint set on the thermostat. The long delay between
the thermostat call and the initial delivery of heat is due to the
time required for the burner to initialize combustion and heat the
heat exchanger and ambient air. It is shown in FIG. 10A to take the
heating system nearly 3000 seconds for the room to reach the space
temperature setpoint set on the thermostat and for the thermostat
to be satisfied. It then takes another .about.750 seconds for the
ambient air within the heat exchanger to cool causing the blower to
stop transporting the ambient air (and heat) to the room. During
this amount of time the room is continuing to be heated and the
temperature of the room is exceeding the space temperature setpoint
set on the room thermostat. The system operating in the periodic
operation regulating means illustrated in FIG. 10A results in
temperature swings within the room in excess of 10.degree. F. above
and below the temperature set at the thermostat when operated with
outside temperatures approaching freezing. Temperature swings of
this magnitude are noticeably uncomfortable for those occupying the
room. Fortunately for the frequenters of the tavern, noticeable
temperature swings, even at outdoor temperatures of -20.degree. F.,
are eliminated with the employment of system regulating means in
accordance with the present invention.
[0065] FIG. 10B illustrates the same system as FIG. 10A, but after
the initial start and operating according to the burner and air
handler regulating means detailed in FIG. 5A and FIG. 5B. At Zero
seconds the ambient air is shown to have an elevated temperature of
about the ambient air setpoint coded in the software running on the
Arduino. For the system an air temperature above 100.degree. F. and
below 250.degree. F. is coded in the software as effectively being
about the ambient temperature setpoint. After .about.100 seconds
the thermostat calls for heat, evidencing the space temperature
feedback being significantly below the space temperature setpoint.
This triggers the air handler power signal to immediately change
state to an ON signal. The system may optionally vary the rate of
the blower via the air handler power signal additionally base upon
the temperature of the ambient air, such that full speed ambient
air transport occurs at ambient air temperatures in excess of
150.degree. F. and slower speed transport of the ambient air occurs
when it is at lower temperatures. The function of the air handler
is principally responsive to the space to be heated temperature
through the thermostat. Before 500 seconds it can be seen that the
ambient air temperature falls out of a range that is effectively
about the ambient air setpoint. At this point the burner power
signal responds by changing the burner to an ON state. At
.about.750 seconds the room has reached the hysteresis temperature
of the thermostat and the space temperature feedback changes
reflecting no need for heat in the room. This triggers the blower
to cease transporting ambient air to the room. The burner power
signal does not change the state of the burner until the ambient
air temperature exceeds the acceptable range that is effectively
about the ambient temperature setpoint at .about.900 seconds. The
heating system is shown in FIG. 10B to deliver heat immediately in
response to a thermostat call and stop delivery of heat immediately
when the thermostat is no longer calling. Additionally, the burner
operates without direct dependence upon the temperature of the
room. With sufficient thermal mass and the correct burner heat
output, the burner may provide thermal energy with fewer stops and
starts and more efficiently.
[0066] Although the present invention has been described in
considerable detail with reference to certain preferred versions
thereof, other versions are possible. For example, the use of
software coded control algorithms, which do not perform PID
calculations to provide the burner power signal or the air handler
power signal. Also the use of the ambient air feedback and one or
more other feedback signals, such as: an exhaust gas oxygen sensor
feedback, a flame presence illumination sensor feedback, or an
exhaust gas pressure sensor feedback. Therefore, the spirit and
scope of the appended claims should not be limited to the
description of the preferred versions contained herein.
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