US20210190365A1

US20210190365A1 - Method, System and Temperature Control of a Heating, Ventilation and Air Conditioning Unit

Info

Publication number: US20210190365A1
Application number: US16/949,665
Authority: US
Inventors: David R. King; Robert Shaw
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2019-12-18
Filing date: 2020-11-10
Publication date: 2021-06-24

Abstract

A control assembly for controlling an HVAC unit, and method of controlling an HVAC unit, the HVAC unit having a burner assembly and at least one of a blower, an inducer fan, an air/fuel control system, the method including: measuring a temperature of the burner assembly; comparing the measured temperature of the burner assembly with a predetermined temperature based on firing rate; controlling the HVAC unit to cause the temperature of the burner assembly to come within predetermined limits; wherein the controlling step includes at least one of the steps of: initiating, terminating or modifying, the operation of one or more of the blower, the inducer fan, or modifying an air/fuel mixture of the air/fuel control system, reducing the temperature of the burner assembly to within predetermined limits.

Description

CROSS REFERENCE TO A RELATED APPLICATION

The application claims the benefit of U.S. Provisional Application No. 62/949,681 filed Dec. 18, 2019, the contents of which are hereby incorporated in their entirety.

BACKGROUND

The subject matter disclosed herein generally relates to heating, ventilation, and air conditioning (HVAC) systems, and more particularly to a method, system and temperature control of an HVAC unit using a temperature sensor.
Heating, ventilation and air conditioning systems are generally operable in response to changes in temperature which may be measured using a bi-metal temperature switch, such as a burner thermal switch (BTS) typically internal or external to an HVAC unit, to turn the unit either on or off based on pre-set temperature limits. However, as an increasing number of HVAC units move away from single stage units, and toward higher efficiency systems such as multi-stage HVAC units (including furnaces), the use of control systems using a BTS is impractical because components such as the burner assembly can be hotter during operation at stages less than 100% capacity (i.e., low or modulating). This operating condition makes it difficult to pass mandatory product safety and performance requirements (e.g., shutting the HVAC system off before it produces unsafe levels of carbon monoxide), without causing the HVAC unit to experience an undesirable nuisance condition and shut the HVAC unit off unexpectedly.
Another drawback of using a BTS is that a BTS will typically activate during a power brownout or blackout because the heat generated in the area of the burner that is normally dissipated by the movement of air is not cooled because the electrically-powered inducer fan that generates airflow, also powers off. This condition typically leaves excessive heat in the area around the BTS which may cause a BTS pre-set temperature limit to be reached even when a safety issue is not present, resulting in shut down of an HVAC unit. In addition, a BTS may shut down a single stage HVAC unit if an excess temperature limit is reached. In this example, a fault condition typically shuts down a single stage HVAC unit preventing restart until the switch is reset, generally by a trained professional.
What is needed then, is an improved method, system and control of an HVAC unit using a closed-loop temperature feedback device, such as a thermistor. A thermistor controlled device and system avoids system shutdown due to blackout or brownout because a system using a thermistor can be programmed to monitor the thermistor response during discrete operating stages or capacities (i.e., high, low or modulating). Further, a thermistor controlled system provides greater overall flexibility because a multistage HVAC unit is not limited to operating at either full capacity or off. Rather, a multistage HVAC unit is operable at various levels, generally described as “high,” “low” and “modulating” typically in reference to the speed of a blower or inducer fan, and the air/fuel mixture, as further described below.
In addition, a thermistor controlled system allows for the detection, monitoring and control of various aspects of an HVAC system generally, but more specifically allows for detection and monitoring of a burner assembly, thereby providing greater control over aspects of an HVAC unit. Operational control may include operating the HVAC unit at less than full capacity, detection of burner assembly temperature, including burner firing rates as a function of temperature, characteristics of a furnace air/fuel mixture which may affect burner assembly temperature, and carbon monoxide or nitrogen oxide emissions. Examples of a control device, system and method for controlling an HVAC unit relevant to the needs existing in the field are discussed below.

BRIEF DESCRIPTION OF THE INVENTION

According to one non-limiting embodiment, a control assembly for controlling an HVAC unit, including: a temperature sensing device capable of measuring at least one of a surface temperature or an internal temperature of a burner assembly of an HVAC unit and providing a signal corresponding to the measured temperature; a control device operatively connected to the temperature sensing device, the control device configured to: receive the signals from the temperature sensing device; compare the measured temperature to at least one predetermined temperature, and operate in a plurality of operational modes based at least in part on the difference between the measured temperature and the at least one predetermined temperature.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly operable in a first operational mode to send at least one signal to operate at full capacity at least one of a blower, inducer fan or air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly operable in a second operational mode to send at least one signal to operate at less than full capacity at least one of a blower, inducer fan or air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly operable in a third operational mode to send at least one signal to operate at a variable capacity or rate, at least one of a blower, inducer fan or air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly operable in a fourth operational mode to send at least one signal to operate at varying time intervals at least one of a blower, inducer fan or air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly operable in a fifth operational mode to send at least one signal to prohibit the operation for a predetermined interval at least one of a blower, inducer fan or air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly wherein one or more sensors are operably coupled to at least one of a burner box, the one or more components of a burner assembly, and/or the one or more components of an air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a control assembly wherein one or more sensors are thermistors.
According to one non-limiting embodiment, a method of controlling an HVAC unit, the HVAC unit having a burner assembly and at least one of a blower, an inducer fan, an air/fuel control system, the method including: measuring a temperature of the burner assembly; comparing the measured temperature of the burner assembly with a predetermined temperature based on firing rate; controlling the HVAC unit to cause the temperature of the burner assembly to come within predetermined limits; wherein the controlling step includes at least one of the steps of: initiating, terminating or modifying, the operation of one or more of the blower, the inducer fan, or modifying an air/fuel mixture of the air/fuel control system, reducing the temperature of the burner assembly to within predetermined limits.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a method wherein the controlling step includes measuring at least one of a surface temperature, or an internal temperature, of a burner assembly.
In addition to one or more of the features described above, or as an alternative, in further embodiments a method wherein the controlling step further includes the steps of: setting at least one desired temperature; setting at least one predetermined temperature and choosing at least one predetermined temperature limit.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a method wherein the controlling step further includes the steps of: determining safe operation of the HVAC unit based on one or more of: pulse-width modulation duty cycles, revolutions per minute (RPMs), increases over steady-state RPMs, predetermined temperature limits.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a method wherein the controlling step further includes the steps of: using temperature data based on a steady-state temperature of the burner assembly wherein an operating temperature range at the steady-state is determined for each firing rate.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a method wherein the controlling step further includes the steps of: determining temperatures at various air/fuel mixture conditions.
According to one non-limiting embodiment, a system for controlling the temperature of an HVAC unit including: an HVAC unit having a burner assembly; at least one of a blower, an inducer fan or an air/fuel control system; a temperature sensing device operably coupled to the burner assembly; a control device in communication with the temperature sensing device, the control device configured to: receive at least one signal from the temperature sensing device, the at least one signal indicative of the burner assembly temperature; compare the at least one signal indicative of the burner assembly temperature to a predetermined temperature; transmit a signal to operate the HVAC unit in at least one operational mode when the temperature of the burner assembly exceeds the predetermined temperature limit.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a system wherein the control device is further configured to operate in a first operational mode to send at least one signal to operate at full capacity at least one of a blower, an inducer fan or an air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a system wherein the control device is further configured to operate in a second operational mode to send at least one signal to operate at less than full capacity, at least one of a blower, an inducer fan or an air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a system wherein the control device is further configured to operate in a third operational mode to send at least one signal to operate at a variable capacity or rate at least one of the blower, the inducer fan or the air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a system wherein the control device is further operative in a fourth operational mode to send at least one signal to operate at varying time intervals at least one of the blower, the inducer fan or the air/fuel control system.
In addition to one or more of the features described above, or as an alternative, in further embodiments, a system wherein the control device is further operative in a fifth operational mode to prohibit the operation for a predetermined time interval at least one of the blower, the inducer fan or the air/fuel control system.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings form a part of the specification. Throughout the drawings, like reference numbers identify like elements.

FIG. 1 is a graphical representation of the temperature of a burner assembly in accordance with embodiments of the disclosure.

FIG. 2 is a graphical representation of the temperature of a burner assembly in accordance with embodiments of the disclosure.

FIG. 3 is a perspective view of a portion of a burner assembly in accordance with embodiments of the disclosure.

FIG. 4 is an expanded perspective view of a burner assembly in accordance with embodiments of the disclosure.

FIG. 5 is perspective view of an HVAC unit in accordance with embodiments of the disclosure.

FIG. 6 illustrates a method of controlling the temperature of an HVAC unit in accordance with embodiments of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

As will be described in greater detail below the present disclosure provides for controlling the temperature of an HVAC burner assembly. It should be evident however to one skilled in the art that the present disclosure is not limited to the specific examples given and could be used in other systems where heat removal from a burner assembly (FIG. 3, 300, and in an alternate embodiment, FIG. 4, 400) or an HVAC unit (FIG. 5, 500) is required.
Overheating of an HVAC unit (FIG. 5, 500), can result in damage to the HVAC unit overall, and if unchecked, may create a safety risk such as increasing carbon dioxide or nitrogen oxide emissions. For example, temperatures that exceed such limits may indicate an underlying problem such as improper combustion (e.g., too little or too much air in the fuel/air mixture), or a clogged burner inlet (FIG. 3, 312 or 316). Temperature sensing and monitoring of a burner assembly (FIG. 3, 300; FIG. 4, 400) is used to control an HVAC unit in a variety of operational modes using predetermined temperatures and predetermined temperature ranges for initiating, terminating or modifying a variety of control actions. Control actions may include, but are not limited to initiating, terminating or modifying any one or more of the following HVAC components: blower motor and fan (FIG. 5, 514), inducer motor and fan (FIG. 5, 518), and/or an air/fuel control system (FIG. 5, 512).
Referring to FIG. 1 there is illustrated a graphical representation of the temperature of a burner assembly in accordance with embodiments of the disclosure. A time-based method of control uses temperature data at a given air/fuel firing rate to establish a base (normal) increase in temperature over time when a burner is first initiated. Algorithms relating firing rate vs. time over varying operating conditions establish expected performance in a generally non-linear manner, from which predetermined limits are established. For example, the method may include use of one or more algorithms in combination with the control device, for determining safe operation of the HVAC unit based on one or more of: pulse-width modulation duty cycles, revolutions per minute (RPMs), increases over steady-state RPMs, predetermined temperature limits. In another non-limiting embodiment, the method includes the use of temperature data based on steady-state temperature of the burner assembly wherein an operating temperature range at steady-state is determined for each firing rate.
In one non-limiting embodiment, the method further includes determining temperatures at various air/fuel mixture conditions. Generally, a lean air/fuel mixture occurs when more air (oxygen) is present than is necessary to combust fuel (resulting in a cooler flame). Similarly, a rich air/fuel mixture occurs when less oxygen is present than is necessary to combust fuel (resulting in unburned fuel). In some cases, a lean air/fuel mixture may include operating with an air/fuel mixture having more than 50% excess air, and a rich air/fuel mixture may include operating with an air/fuel mixture having less than 50% excess air. Operating a furnace with a richer air/fuel mixture can result in higher than normal operating temperatures, which over time, may lead to decreased operating efficiency, greater carbon dioxide or nitrogen oxide emissions, or damage to an HVAC unit. In a non-limiting embodiment, the method includes determining, based on fuel delivery rates, whether the air/fuel mixture is rich or lean compared to a desired nominal firing rate, and relating expected temperature of a thermistor for nominal combustion conditions as well as boundary conditions to determine ranges at which an operational mode is or is not initiated, terminated or modified.
An operational mode for controlling an HVAC unit, includes initiating, terminating or modifying one or more control actions, when a predetermined temperature range is exceeded. For example, in FIG. 1, “high fire” includes a range of normal operating temperatures over time, represented by the space between the dotted lines on either side of the solid line denoted as “High Fire”. Similarly, in FIG. 1, “low fire” includes a range of normal operating temperatures over time, represented by the space between the dotted lines on either side of the solid line denoted as “Low Fire”. An operational mode may include initiating, terminating or modifying a control action to bring the temperature of a burner assembly (FIG. 3, 300; FIG. 4, 400) to within desired operational ranges, when a furnace is operating in a high fire or low fire mode. By way of example and not limitation, the control system may initiate a first operational mode, activating or modifying the operational speed of an inducer motor and fan (FIG. 5, 518) to draw more air (FIG. 3, 316) through burner assembly (FIG. 3, 300; FIG. 4, 400). As the temperature of the burner assembly (FIG. 3, 300; FIG. 4, 400) is reduced to within predetermined limits, but before normal is achieved, a second operational mode may reduce the speed of the inducer motor and fan (FIG. 5, 518) until normal is reached. Once normal limits are reached, a third operational mode, may terminate all control actions (i.e., turning off inducer motor and fan (FIG. 5, 518)). In another non-limiting embodiment, a first operational mode may include increasing or decreasing the speed of an inducer motor and fan (FIG. 5, 518), and a second operational mode may include increasing or decreasing the speed of a blower motor and fan (FIG. 5, 514).
Referring to FIG. 2, a graphical representation of the temperature of a burner assembly in accordance with embodiments of the disclosure is shown. A steady-state method of control is disclosed. Steady-state temperature, namely burner firing rate vs. temperature of the burner assembly (FIG. 3, 300; FIG. 4, 400), is described. Operating temperature ranges of a burner assembly (FIG. 3,300; FIG. 4,400) are determined for each firing rate. The temperature of leaner and richer combustion conditions may also be considered in determining whether the fuel rate is in an over- or under-firing condition compared to a desired nominal rate or range.
In a non-limiting embodiment, an operational mode may be initiated, modified or terminated when the temperature and firing rate is within a predetermined range, but over normal as indicated by the solid line between the dotted lines denoted as “First Level Region.” When an HVAC unit is operating over normal limits, but within an acceptable range, a first operational mode may be initiated, until temperature reaches normal. In the area represented outside the solid dotted lines to the solid line denoted as a LIMIT, at least one operational mode may be initiated, modified or terminated until normal is reached. By way of example and not limitation, a control system initiates a first operational mode, modifying the inducer speed until the burner assembly (FIG. 3, 300; FIG. 4, 400) is within the First Level Region. If the limit is further exceeded, or a new limit (e.g., Second Level Region) is reach (e.g., excess emissions are sensed), a second operational mode may initiate by activating or modifying a blower motor and fan (FIG. 5, 514) to reduce emissions and expel fumes through a vent (not shown).
In another non-limiting embodiment, one or more operational modes may be initiated when the HVAC system is outside normal operating conditions and within the area denoted as “Second Level Region.” Real-time operating data from the air/fuel control system (FIG. 5, 512) as compared to burner assembly (FIG. 3, 300; FIG. 4, 400) temperature and preset temperature limits may activate the control system when normal operating parameters are exceeded. For example, when an HVAC unit is operating in the Second Level Region, the speed of an inducer motor and fan (FIG. 5, 518) may be modified (e.g., increasing or decreasing combustion air). If one or more additional control actions need to be initiated to return the burner assembly (FIG. 3, 300; FIG. 4, 400) to within normal operational conditions, such actions will be initiated, terminated or modified. For example, a second operational mode may include the control action of initiating or modifying the operational speed of a blower (FIG. 5, 514) when predetermined limits reach or remain at a second boundary as a further means for reducing burner assembly (FIG. 3, 300; FIG. 4, 400) temperature.
Referring to FIG. 3, a portion of a burner assembly 300 is shown. A burner box 302 having at least one temperature sensor, preferably a thermistor 304, is operably coupled to an exterior surface of a burner box 302. Thermistor 304 is operably coupled to a harness assembly 306 which is in fluid communication with a control device 308 such as an integrated furnace control (IFC). Thermistor 304 measures the temperature of burner box 302. Thermistor 304 may be of any type, for example, an NTC (negative coefficient) or PTC (positive temperature coefficient), providing real-time feedback in a closed loop system. The thermistor 304 as shown, is on the bottom exterior surface of burner box 302, however, it may be placed anywhere along any exterior surface of burner box 302. It can be appreciated that one or more thermistors 304 or other sensors may be utilized as part of the control assembly for controlling an HVAC unit, said sensors operatively coupled to any one or more of a heat exchanger (FIG. 5, 506), exhaust vent(s) (not shown), or furnace housing (FIG. 5, 521).
To generate flame and hot combustion gases (not shown) in a direction as shown by arrows 310, a mixture of fuel and air is formed and then provided to the interior of burner box 302. Air 316 necessary for combustion is introduced into the burner assembly 300 and directed into the interior of burner box 302. Flame sensor 324 and harness assembly 322, and igniter 318 and harness assembly 320, may each also be in fluid communication with control device 308.
In one non-limiting embodiment an operational mode may initiate, terminate or modify a control action related to the burner assembly 300. For example, a first operational mode may initiate a control action such as activating a switch to initiate a blower (FIG. 5, 514) reducing burner assembly 300 temperature to within predetermined limits. If the temperature of burner assembly 300 does not reach a predetermined temperature or temperature range within a predetermined period of time, a second operational mode may be initiated, terminated or modified. It can be appreciated that a first level operational mode and a second level operational mode and related control actions may be initiated, terminated or modified in any combination, at varying predetermined time intervals or periods, or operation of a control action at full or less than full capacity. By way of example and not limitation, a third operational mode may be initiated wherein an inducer motor and fan (FIG. 5, 518) operates at less than full capacity and for a predetermined period based on predetermined temperature ranges.
Turning to FIG. 4, an expanded perspective view of an alternate embodiment of a burner assembly 400 is shown. An air/fuel control system (FIG. 5, 512), associated with burner assembly 400 and may include a premix burner or in shot burner 402, burner box 302; flame sensor 322 to sense the extent of the flame across burner 402; an igniter 318 to light the air/fuel mixture; and one or more brackets, gaskets, plates or panels, which operatively couple burner assembly 400 to a heat exchanger (FIG. 5, 506). Fuel (typically natural gas or propane) is introduced to an inlet 312 of a mixing tube 314 and to burner box 302, and air is introduced using a motorized induction motor and fan (FIG. 5, 518). An igniter 318 and optionally, a sensor (not shown), lights the air/fuel mixture within the interior of burner box 302. In one non-limiting embodiment an operational mode may initiate, terminate or modify a control action related to the operation of an air/fuel control system (FIG. 5, 512). For example, a first operational mode may initiate a control action by activating a switch to shut off a gas valve (not shown) terminating the flow of gas into the burner assembly 400.
In another non-limiting embodiment, an operational mode may start an inducer motor and fan (FIG. 5, 518) for ejecting carbon monoxide from heat exchanger (FIG. 5, 506), and/or exhaust vent(s) (not shown) to reduce the temperature of burner assembly 400 and/or reduce noxious gas emissions. For example, inducer motor and fan (FIG. 5, 518) draws air 316 into the burner assembly 400, reducing the heat of combustion within burner box 302 and thus reducing the temperature of the burner assembly to within predetermined limits.
In another non-limiting embodiment, the control system may initiate, terminate or modify any one or more operational modes based on burner assembly 400 temperature in combination with one or more sensors. For example, burner assembly may have an igniter 318 and optionally a sensor (not shown), downstream from burner 402 relative to a direction of air flow (FIG. 3, 310) through burner 402. Flame sensor 324 may be configured to determine if ignition has carried over across burner 402 and may also be in fluid communication with control device 308. The flame sensor 406 may be coupled to burner 402 at any location or anywhere within the burner assembly 400. In yet another non-limiting embodiment, at least one sensor (not shown) is operatively coupled to a cell panel (not shown). In yet another non-limiting embodiment, at least one sensor (not shown) is operatively coupled to refractory panel 410. It can be appreciated that any of the foregoing sensors may be in fluid communication with control device 308. Depending on preset limits for the flame sensor 324, a cell panel sensor or other optional sensors, one or more operational modes may initiate, terminate or modify a control action to bring the burner assembly 400 to within predetermined limits.
Referring to FIG. 5, a representation of an HVAC unit 500 in accordance with embodiments of the disclosure is shown. Included in the system is a control device 308 having a microprocessor and a memory. Control device 308 is operatively connected to HVAC unit 500 and in communication with at least one sensor, for example, a thermistor (FIG. 3, 304). It will be appreciated that memory of control device 308, may be in whole or in part, external of control device 308, for example on an external device or hosted on a cloud server (not shown). The control device 308 may initiate, terminate or adjust one or more operational modes dependent upon a comparison of predetermined limits or ranges stored in memory versus real-time data obtained by a sensor, for example, thermistor (FIG. 3, 304).
In one non-limiting embodiment, an HVAC unit 500 may include a burner assembly 300, an air/fuel control system 512, a blower motor and fan 514, an inducer motor and fan 518, a vent (not shown) for expelling noxious gases such as flue gas and carbon monoxide, a heat exchanger 506, and furnace casing 521. In a non-limiting embodiment any one or more of the foregoing have at least one sensor in fluid communication with control device 308.
Referring to FIG. 6, there is illustrated a flow chart of a method of controlling an HVAC unit using a temperature sensor, preferably a thermistor, the method including operating a control device to sense the temperature of a burner assembly of an HVAC unit, comparing said temperature of a burner assembly with predetermined limits or desired temperature(s), controlling the HVAC unit to cause the temperature of a burner assembly to come within predetermined limits, reducing the temperature of a burner assembly by initiating an operational mode to terminate a control action (e.g., turn off inducer motor) when a burner assembly is within predetermined limits.
While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.

Claims

We claim:

1. A control assembly for controlling an HVAC unit, comprising:

a temperature sensing device capable of measuring at least one of a surface temperature or an internal temperature of a burner assembly of an HVAC unit and providing a signal corresponding to the measured temperature;

a control device operatively connected to the temperature sensing device, the control device configured to:

receive the signals from the temperature sensing device;

compare the measured temperature to at least one predetermined temperature, and operate in a plurality of operational modes based at least in part on the difference between the measured temperature and the at least one predetermined temperature.

2. The control assembly according to claim 1 operable in a first operational mode to send at least one signal to operate at full capacity at least one of a blower, an inducer fan or an air/fuel control system.

3. The control assembly according to claim 1 operable in a second operational mode to send at least one signal to operate at less than full capacity at least one of the blower, the inducer fan or the air/fuel control system.

4. The control assembly according to claim 1 operable in a third operational mode to send at least one signal to operate at a variable capacity or rate, at least one of the blower, the inducer fan or the air/fuel control system.

5. The control assembly according to claim 1 operable in a fourth operational mode to send at least one signal to operate at varying time intervals at least one of the blower, the inducer fan or the air/fuel control system.

6. The control assembly according to claim 1 operable in a fifth operational mode to send at least one signal to prohibit the operation for a predetermined interval at least one of the blower, the inducer fan or the air/fuel control system.

7. The control assembly according to claim 1 wherein one or more sensors are operably coupled to at least one of a burner box, the one or more components of the burner assembly, the one or more components of the air/fuel control system.

8. The control assembly according to claim 1 wherein one or more sensors are thermistors.

9. A method of controlling an HVAC unit, the HVAC unit having a burner assembly and at least one of a blower, an inducer fan, an air/fuel control system, the method comprising:

measuring a temperature of the burner assembly;

comparing the measured temperature of the burner assembly with a predetermined temperature based on firing rate;

controlling the HVAC unit to cause the temperature of the burner assembly to come within predetermined limits;

wherein the controlling step includes at least one of the steps of: initiating, terminating or modifying, the operation of one or more of the blower, the inducer fan, or modifying the air/fuel mixture of the air/fuel control system, reducing the temperature of the burner assembly to within predetermined limits.

10. The method according to claim 9 wherein the controlling step includes measuring at least one of a surface temperature, or an internal temperature, of the burner assembly.

11. The method according to claim 9 wherein the controlling step further includes the steps of:

setting at least one desired temperature;

setting at least one predetermined temperature and choosing at least one predetermined temperature limit.

12. The method according to claim 9 wherein the controlling step further includes the steps of: determining safe operation of the HVAC unit based on one or more of: pulse-width modulation duty cycles, revolutions per minute (RPMs) increases over steady-state RPMs, predetermined temperature limits.

13. The method according to claim 9 wherein the controlling step further includes the steps of: using temperature data based on a steady-state temperature of the burner assembly wherein an operating temperature range at the steady-state is determined for each firing rate.

14. The method according to claim 9 wherein the controlling step further includes the steps of: determining temperatures at various air/fuel mixture conditions.

15. A system for controlling the temperature of an HVAC unit comprising:

an HVAC unit having a burner assembly;

at least one of a blower, an inducer fan or an air/fuel control system;

a temperature sensing device operably coupled to the burner assembly;

a control device in communication with the temperature sensing device, the control device configured to:

receive at least one signal from the temperature sensing device, the at least one signal indicative of the burner assembly temperature;

compare the at least one signal indicative of the burner assembly temperature to a predetermined temperature;

transmit a signal to operate the HVAC unit in at least one operational mode when the temperature of the burner assembly exceeds the predetermined temperature limit.

16. The system according to claim 15 wherein the control device is further configured to operate in a first operational mode to send at least one signal to operate at full capacity at least one of the blower, the inducer fan or the air/fuel control system.

17. The system according to claim 15 wherein the control device is further configured to operate in a second operational mode to send at least one signal to operate at less than full capacity, at least one of the blower, the inducer fan or the air/fuel control system.

18. The system according to claim 15 wherein the control device is further configured to operate in a third operational mode to send at least one signal to operate at a variable capacity or rate at least one of the blower, the inducer fan or the air/fuel control system.

19. The system according to claim 15 wherein the control device is further operative in a fourth operational mode to send at least one signal to operate at varying time intervals at least one of the blower, the inducer fan or the air/fuel control system.

20. The system according to claim 15 wherein the control device is further operative in a fifth operational mode to prohibit the operation for a predetermined time interval at least one of the blower, the inducer fan or the air/fuel control system.