Multistage warm air furnace with single stage thermostat and return air sensor and method of operating same

A multistage furnace for heating residential or commercial spaces is operated by a single stage thermostat and a return air temperature sensor providing sensed temperature information to a controller. The furnace is started at a first, low firing rate and after a predetermined time is increased to a second firing rate if the rate of change of temperature of air returning to the furnace from the space is less than a predetermined amount. The method of operation is carried further to increase the firing rate to second and third rates based on predetermined rates of temperature change of return air from the enclosed space as sensed by the return air sensor.

BACKGROUND

Multistage warm air furnaces are known for use in systems for heating residential dwellings and commercial buildings. In prior art multistage warm air furnace systems a multistage thermostat has been used to call for respective lower and higher firing rates to satisfy the demand of the space being heated. The cost of multistage thermostats and the installation of same has been a consideration when installing multistage furnaces. Efforts to provide lower cost single stage thermostats for controlling multistage furnaces have not been entirely successful.

One technique for operating a multistage furnace with a single stage thermostat has been the provision of a control algorithm which is time based. The furnace is operated in an “on—off” mode with a single stage thermostat and a time based algorithm determines when the furnace is to be transitioned from a lower firing rate to a higher firing rate. This approach has certain disadvantages including noise generated by the furnace and excessive temperature swings in the controlled space. However, a single stage thermostat operating with a multistage furnace is desirable in instances where multistage furnaces are retrofitted into a building in which one or more single stage thermostats already exist. Moreover, as mentioned above, single stage thermostats are less expensive than multistage thermostats which also favors using a single stage thermostat with a multistage furnace.

Accordingly, there has been a desire to provide an improved system and method for controlling a warm air furnace, particularly a multistage furnace with a single stage thermostat, and it is to these ends that the present invention has been developed.

SUMMARY OF THE INVENTION

The present invention provides a warm air furnace control system including a single stage thermostat and a return air sensor operating in conjunction with a controller or control circuit to provide improved performance of warm air furnaces and to facilitate retrofitting multistage furnaces into facilities which include single stage thermostats.

In accordance with one aspect of the present invention, a multistage furnace may be operated with a single stage thermostat and a return air temperature sensor for measuring the temperature of air returning to the furnace over time and utilizing the measurements to determine whether the furnace is achieving the recovery or demand of the controlled space at a desired rate.

The present invention also includes a method embodied in a control algorithm for the system to control the furnace to operate at the lowest firing rate commensurate with the demand of the space being warmed by the furnace. The control system operates the furnace at higher or lower firing rates as determined by the conditions defined in the control algorithm.

In accordance with the invention, the controlled space is allowed to recover to the desired temperature in a manner that is consistent with predefined space comfort parameters. For example, lower furnace firing rates have proven to provide greater satisfaction of the furnace user due to reduced noise emitted by the furnace and reduced temperature variations in the controlled space.

In accordance with another aspect of the present invention, a method for operating a multistage furnace with a single stage thermostat is provided wherein the thermostat can terminate furnace operation by ending the call for heat in a conventional manner. However, while the furnace operates at respective firing rates, the return air temperature is sampled periodically at a predefined rate during furnace operation, each measurement is saved and used for a comparison with the next measurement so that a determination can be made as to whether or not the return air temperature is increasing or decreasing. The rate of return air temperature change is also determined. For example, if the temperature is decreasing or not increasing at a predefined rate, the method of the invention commands the furnace to the next higher firing rate and after a predetermined period of time, if the return air temperature is increasing, but not at a sufficient rate of recovery, the furnace is commanded to the next higher firing rate. However, if the return air temperature is increasing at a sufficient rate, the furnace continues to operate at the lowest firing rate permissible.

In accordance with a further aspect of the present invention, a control system and control method for a warm air furnace is provided wherein it is determined if the furnace should be operating at a higher or lower firing rate or remain at its present rate. Predetermined rates of temperature change are established and the furnace is commanded to operate at a particular firing rate depending on a particular rate of change of temperature sensed by a return air sensor. Accordingly, an improved furnace control system and method of operation are provided by the invention which takes advantage of single stage thermostats and multistage furnaces operating together.

Those skilled in the art will further appreciate the above mentioned advantages and superior features of the invention together with other important aspects thereof upon reading the detailed description which follows in conjunction with the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the description which follows, like elements are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale and flow diagrams or flowcharts may show only essential steps of the improvements of the present invention while conventional or ancillary operating steps may be omitted in the interest of clarity and conciseness. The letters Y and N in a flowchart mean “yes” and “no”.

Referring toFIG. 1, there is illustrated a forced air multistage gas furnace of a type which may be operated in accordance with the present invention and is generally designated by the numeral10. Furnace10includes a generally rectangular cabinet12in which is disposed a burner assembly16, a gas valve assembly18operably connected to a source of gas, not shown, and a controller20. Controller20is preferably a programmable microcontroller of a type commercially available and which may be programmed to carry out the method of the invention by one of ordinary skill in the art. A heat exchanger assembly22includes plural heat exchangers24and an induced draft blower26. A circulating air blower28is disposed in cabinet12and draws air from a return air duct29in which is disposed a temperature sensor29a. Sensor29ais operably connected to controller20for transmitting signals thereto indicating the temperature of air returned from an enclosed space31which is to be heated by the furnace10. Although an upflow type furnace is illustrated inFIG. 1, those skilled in the art will recognize that other conventional gas furnaces or other types of forced airflow furnaces may be operated in accordance with the present invention.

Enclosed space31includes a temperature sensor or thermostat33which is also in signal transmitting communication with the controller20. In multistage furnaces operating with multistage thermostats there are typically multiple thermostat signal conductors connected to corresponding plural terminals of a controller, such as controller20. In accordance with the present invention a single signal transmitting conductor from thermostat33may be connected to each of the thermostat signal receiving terminals of controller20and operation of the controller is carried out in accordance with the methods set forth herein.

Referring further toFIG. 1, burner assembly16includes plural inshot burners30manifolded to a supply of fuel supplied from gas valve assembly18which includes a variable position gas control valve32, so that an appropriate fuel-air mixture is provided to the burners30. Combustion air may enter the cabinet12through a combustion air inlet34. Suitable ignitors36are arranged to ignite a fuel-air mixture and the hot gases produced thereby circulate through serpentine passages38. Induced draft blower26may be controlled by pressure switches, not shown, also operable to provide signals to the controller20.

The temperature sensor or thermostat33may be characterized as a single stage thermostat as mentioned above but the furnace10may be operable in multiple stages at various firing rates. For example, the furnace10may be called upon to provide heat at a so-called low firing or heating rate in a first stage call from a multistage thermostat and then called upon to provide heat at a mid range firing or heating rate, and then a high firing or heating rate if the demand for heat is not satisfied at the lower rates. However, if a single stage type thermostat is used with a multistage furnace controller, such as used in prior art systems, the multistage furnace may not be capable of taking advantage of the efficiencies and comfort sensed by occupants of the heated space by generating heat at various selected rates.

In accordance with the present invention, temperature sensor29, which may be disposed in various positions, such as in return air duct29, within the cabinet12upstream of blower28, or elsewhere, measures the temperature of air returning from space31to the furnace10. Periodic temperature measurements taken by sensor29aare used to make a determination as to whether or not the space31is recovering, that is, being heated at a predefined rate. By providing a control method in accordance with the invention, the controller20may operate the furnace10at a higher or lower firing rate by controlling gas flow with a variable position control valve, such as the valve32, and as determined by the conditions defined in the method. Of course, in the interest of savings of fuel and the cost of heating, the method is adapted to operate the furnace10at the lowest firing rate consistent with obtaining recovery of the desired temperature within the space31. In other words, the space31is allowed to recover temperature in a manner that is consistent with comfort norms that have been defined for persons occupying heated spaces, for example. Moreover, lower firing rates of gas furnaces and the like have proven to provide greater comfort due to reduced noise emitted from the furnace and better temperature control.

In accordance with a preferred embodiment of the invention, three stages of furnace operation are provided by way of example. The invention contemplates that furnaces operable at various numbers of firing stages or rates may be operated in accordance with the invention and a three stage furnace and single stage thermostat are described as one preferred example. At any time during the process of firing the furnace10to satisfy the call for heat within the space31, the thermostat33may end the call for heat by terminating furnace operation, that is by closing gas valve32under control of the controller20. When operating at the lowest firing rate, the return air temperature sensed by sensor29is sampled at a selected frequency during furnace operation. Each successive measurement is saved and used for comparison with the next following measurement. Accordingly, the first temperature measurement establishes a starting point temperature for the method and subsequent temperature measurements are taken at predetermined periods, such as every fifteen seconds to every five minutes, for example. In this way a determination is made by the controller20as to whether or not temperature in the space31is increasing or decreasing. If the temperature in space31is increasing, as sensed by the temperature of return air flowing through duct29, a rate of change in temperature sensed is calculated. If the temperature of return air in duct29is decreasing, the method contemplates commanding the furnace10to operate at the next higher firing rate. Once this command is given, after a predetermined period of operation, if the temperature of return air is increasing, but not at a predetermined sufficient rate of recovery, the furnace10is commanded to operate at the next higher firing rate. On the other hand, if the temperature of the return air is increasing at a sufficient rate the furnace continues to operate at the same or a lower firing rate and temperature sampling is continued.

If the furnace10is operating at a mid range or middle firing rate, the method contemplates providing the ability to determine if the furnace should be operating at a higher or a lower firing rate or stay at the middle rate. For example, if the return air temperature sensed by sensor29continues to decrease, the firing rate is increased to the high firing rate after a predetermined period of operation at the middle firing rate, such as five minutes, for example. After a predetermined time of middle firing rate, that is, for example, ten minutes, the rate of temperature recovery in space31is again calculated. If the rate of recovery is greater than a predetermined amount, the furnace is transitioned back to the low firing rate. However, if the rate of recovery is below a predetermined designated rate the firing rate is increased to the high firing rate. Still further, if the rate of recovery of the temperature in the space31is at a predetermined target rate then the furnace is commanded to continue to operate in the middle range or middle rate and temperature sampling is continued.

Still further, in accordance with the invention, when the furnace10is operating at the high firing rate the method contemplates providing the ability to determine if the furnace should continue to operate at that rate or operate at the middle firing rate. This may also be carried out by sensing return air temperature with the sensor29and, if the temperature continues to decrease or if the rate of recovery of temperature in the space31is at the predetermined target rate or less, the furnace continues to operate at the high rate while temperature sampling with the sensor29and the controller20is continued. If the rate of temperature recovery is too high under these operating conditions the furnace10is commanded to return to operation at the middle firing rate while temperature sensing is continued. The process described hereinabove is, of course, continued until the call for heat by the sensor or thermostat33is satisfied at which time the furnace shuts off until the temperature in the space31decreases below the thermostat setpoint a predetermined amount in accordance with conventional thermostat operation.

Referring now toFIGS. 2A through 2C, there is illustrated a flow chart of an exemplary and preferred method of controlling a multistage furnace, such as the furnace10, with a single stage thermostat by incorporating a routine in the controller20in accordance with the method of the invention. Referring toFIG. 2A, in particular, when furnace10including the controller20, is enabled at the step labeled “Start” and the thermostat33calls for heat at step50inFIG. 2A, the controller20opens gas valve32and energizes ignitors36to light flame in the burners30, as indicated at step52. Normally, the furnace flame is lit and established at a furnace firing rate greater than the lowest firing rate in order to assure that the flame actually becomes established and stabilized. After a short time delay, typically about forty-five seconds, the furnace firing rate then transitions to the “low” or first stage rate. Accordingly, once the furnace10is energized and flame is established, controller20adjusts gas valve32and blower26to operate at a “low” or first stage firing or heating rate, as indicated at step54in FIG.2A. With the gas valve32and the inducer blower26operating at the low or first stage firing rate a time delay process is simultaneously commenced by starting a fifteen minute timer at step56.

Upon energizing the furnace10at the low firing rate the return air temperature sensed by sensor29is sampled every fifteen seconds after the first five minutes of operation, as indicated at step58. Each sampling of return air temperature is compared with a previous sampling to determine the rate of temperature change in degrees Fahrenheit (F) per hour (HR) as indicated at step60. After five minutes of operation, if the rate of temperature change (increase), as sensed by sensor29, is equal to or greater than two degrees Fahrenheit (F) per hour, as indicated at step62, the furnace continues to operate at the low firing rate, as indicated at step64. Of course, in this routine the return air temperature is continued to be sampled every fifteen seconds after a five minute interval, the temperature rate of change in degrees F per hour is calculated, and the rate of temperature change is monitored.

Referring further toFIG. 2A, and alsoFIG. 2B, if the rate of temperature change is negative after five minutes elapsed time from step58, but less than fifteen minutes, step63, the furnace10is commanded to start the middle firing or heating rate as indicated by step66. The controller20also opens the gas valve32further and increases the speed of the inducer blower26to begin the “middle” or second stage firing, rate. If the rate of temperature change is positive but less than two degrees F per hour after fifteen minutes, step65, the middle or second stage firing rate begins also. With initiation of step66a fifteen minute timing cycle is begun at step68. The return air temperature, as sensed by sensor29, is again sampled every fifteen seconds after five minutes of operation at the middle or second stage firing rate as indicated at step70in FIG.2B. The rate of temperature change is calculated at step72, as in step60, and, after five minutes, if the rate of temperature increase at step74is equal to or greater than four degrees F per hour, for example, the controller20will reset the gas valve32to the low firing rate and reduce the speed of the inducer blower26, as indicated at step76. The process flow is entered at the encircled A inFIG. 2Awhich starts the low firing rate all over again, including initiation of the fifteen minute timer, step56.

Referring further toFIG. 2B, if the rate of temperature increase at step74is less than four degrees F per hour but greater than two degrees F per hour and fifteen minutes has elapsed, as indicated at step77, the furnace continues to operate at the middle or second stage firing rate, as indicated at step78, wherein the routine is continued from the point of entry at the encircled B. In this mode, the fifteen minute time interval is not initiated but the process continues to sample the return air temperature in the duct29every fifteen seconds and, after five minutes, if the rate of temperature increase is equal to or greater than four degrees F per hour, the system will return to the low or first stage firing rate, as indicated by step76.

If the rate of temperature increase is less than two degrees F per hour at the middle firing rate, as indicated at step79, the controller20will cause the gas valve32and the inducer blower26to begin operating at the “high” or third stage firing or heating rate, as indicated at step80in FIG.2B. Upon initiation of the third stage firing rate, a fifteen minute timer is initiated, also at step82. The return air temperature is sensed by the sensor29every fifteen seconds after five minutes, as indicated at step84, and the rate of temperature change is calculated at step86.

Referring toFIG. 2C, if the furnace10is producing a rate of temperature increase of return air in duct29at greater than six degrees F per hour when operating in the high firing rate, and the rate of temperature increase has exceeded six degrees F per hour before the fifteen minute timing cycle has elapsed, as indicated at step90, the controller20initiates operation of furnace10again at the middle or second stage firing rate and a fifteen minute timing cycle is initiated, as indicated by entry in the routine at the encircled letter C in FIG.2A. On the other hand if the rate of temperature increase is greater than six degrees Fahrenheit per hour after the first fifteen minutes of operation at the high firing rate, the controller20will return the furnace mode of operation to that of operating at the middle firing rate as indicated at step91, but entry into the routine is at the point indicated by the encircled letter B in FIG.2B. This is the same routine that is effective if, when the system is operating at the middle firing rate, the rate of temperature increase is less than four degrees F per hour but greater than two degrees F per hour.

Referring further toFIG. 2C, if the rate of temperature increase is less than six degrees F per hour at the high firing or heating rate, as indicated at step88the furnace continues operating at the high rate, step92, by running the routine indicated inFIG. 2Cwherein the method steps are entered at the encircled letter D.

As mentioned previously, the controller20may be fitted with or modified to include a processor which will accept programming to carry out the method steps set forth above and utilizing a single stage thermostat which, basically, is suited for signaling the controller20when a temperature within a particular range above and below a setpoint is reached in accordance with conventional thermostat operation. Moreover, by providing a controller and a return air temperature sensor arranged with respect to the furnace generally upstream of the main circulating blower, and preferably in a return air duct, multistage, furnaces may be retrofitted into facilities having single stage thermostats already in place while providing for multistage furnace operation.

One preferred embodiment of the invention has been described in detail herein wherein a multistage furnace, typically a furnace capable of operating at three firing rates or heating rates, is operated in conjunction with a thermostat capable of providing only a single stage signal to the furnace controller. However, the system and method of the invention are operable in conjunction with other furnaces having multiple firing or heating rates whose stage numbers are greater than that of the signal generating capability of the thermostat. For example, the invention may be adapted for operation of a five stage furnace, that is a furnace having five firing rates or heating rates and operable in conjunction with a two stage thermostat, or a thermostat capable of operating at three stages. Also, for example, if a two stage thermostat was operating with a three stage furnace, the thermostat would be operable to control operation of the furnace at its first or lowest firing rate, for example, and then in the second stage of thermostat operation or signal transmission to the furnace controller, the method of the invention would be implemented to operate the furnace at its second and third stages of operation. When multistage thermostats are operated with multistage furnaces in accordance with the invention transition from one stage or heating rate called for by the thermostat to the next higher rate called for by the thermostat is based on elapsed time. In other words if heating demand is not satisfied at a lower rate the thermostat calls for a higher rate at the termination of a preset period of time at the lower rate. However, when the thermostat sends its second or last stage signal to the controller, the process of the present invention is implemented.

Another example of how the invention might be utilized in a situation where a thermostat having fewer numbers of furnace stage operating signals than the furnace was capable of would be where the furnace was capable of operating at five heating rates or firing rates and the thermostat was only operable to provide signals at two stages of operation of the furnace. In this situation the thermostat would provide a signal to operate the furnace at its first stage rate and the thermostat, when calling for heat at its second stage operating condition would then control the furnace for operation at the second, third, fourth and fifth stages of operation of the furnace generally in accordance with the present invention. Thus, the invention basically contemplates a method of operating a multiple stage furnace with a thermostat operable to provide a signal or signals to the furnace controller to call for heat at furnace heating rates less than the number of “stages” or heating rates of which the furnace is capable. However, implementation of the method of the invention by a furnace controller enables a furnace to be operated in an efficient manner with a thermostat capable of providing signals for operating the furnace at heating rates less than of which the furnace is capable.

Although a preferred embodiment of the invention has been described in detail herein, those skilled in the art will recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.