Apparatus for Controlling a Solid Fuel Forced Hot Air Furnace
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.
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.
BACKGROUNDVarious 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.
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.
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 ÖkoFEN brand of boilers from ÖkoFEN 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.
Some solid fuel burners, such as the burner incorporated on ÖkoFEN 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 Åmål, 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.
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.
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.
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.
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.
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.
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.
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 INVENTIONIn 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.
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.
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.
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.
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.
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 ¾ 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.
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.
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
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.
A burner regulating means is shown in
An air handler regulating means provides the air handler power signal. The air handler regulating means shown in
The ambient air setpoint in
A similar embodiment of the present invention is shown in
A digital thermostat shown in
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.
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
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
If the continuous operation means currently does not have control, the furnace regulator switching means shown in
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
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
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.
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
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
The ATMega2560 may control the operation of the burner through the PWM analog outputs, as shown in
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.
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.
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.
The Temperature vs. Time plots shown
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.
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.
Type: Application
Filed: May 4, 2015
Publication Date: Dec 17, 2015
Inventor: Charles Holland Dresser (Bethel, ME)
Application Number: 14/703,840