Abstract:
A controller for a two-stage heat source that can be used with a single stage or a two stage thermostat. The controller includes a second stage delay timer that initiates second stage heating when the timer is decremented to zero. The timer is reset when there is no longer a call for first stage heat, when there is a call for second stage heat, or if there is no call for second stage heat and the controller determines that it is connected to a two-stage thermostat. The controller can include an indicator such as a contact bar and jumper or a switch for indicating the type of thermostat connected to the controller. Alternatively, the controller can be programmed to “learn” whether it is attached to a single stage or two-stage thermostat, assuming that it is connected to a single stage thermostat, unless and until a call for second stage heating is received.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a controller for a two-stage heat source that can be used with either a single or a two-stage thermostat. 
     There are two types of commonly available, gas-fired, warm air furnaces in the marketplace: those with a single gas flow rate, and those with two or more gas flow rates. These are referred to as single and multistage furnaces, respectively. Multistage furnaces are frequently selected by homeowners for replacement furnaces because they offer increased performance and comfort. In retrofit applications there is typically a thermostat and wiring in place. It can be troublesome to install a multistage thermostat in a retrofit application with a single stage thermostat already in place because of the need to add additional wiring. For simple and economical installation, it is desirable to be able to continue to use a single stage thermostat and single stage thermostat wiring with a replacement multistage furnace. 
     Several attempts have been made to utilize a single stage thermostat with two-stage furnaces. One method, disclosed in U.S. Pat. No. 5,271,556, ties the first stage and second stage terminals on the controller together. The controller recognizes a simultaneous call for first stage and second stage heat as a single stage thermostat, and operates in the first stage only. If the demand for heat is not satisfied within a predetermined time (typically ten minutes), the controller switches to second stage heating. Another method, disclosed in U.S. Pat. No. 5,806,760, includes a time delay relay in the first stage terminal. After the expiration of the time for the time-delay relay, the controller switches to the second stage heating. 
     SUMMARY OF THE INVENTION 
     The controller of the present invention permits a multistage heating plant, such as a two stage gas furnace, to be operated with either a single or a multi stage thermostat. This allows fast, easy, and inexpensive replacement of an existing single stage heating plant with a modern multistage heating plant, without replacing the thermostat and/or adding additional thermostat wiring. 
     Generally the controller of the present invention has a first terminal for connection to a single stage thermostat, or the first stage of a multistage thermostat, and a second terminal for connection to the second stage of a multistage thermostat. The controller also comprises an indicator for selecting the type of thermostat (single stage or multistage) that is connected to the controller. This indicator can be, for example, a contact bar with three contacts that are appropriately connected by the installer to indicate to the controller the type of thermostat connected to the controller. Alternatively, the controller could be programmed to assume that the thermostat is a single stage thermostat, unless and until the thermostat receives a demand for second stage heating which indicates that the controller is connected to a multistage thermostat. 
     The controller initiates a second stage delay timer, and the timer continues to count down unless reset, until it reaches zero and second stage heating is initiated. The second stage delay timer can be reset by several events, including the satisfaction of the demand for first stage heat; a demand for second stage heat; or the absence of a demand for second stage heat when the controller is connected to a multistage thermostat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is schematic view of a controller constructed according to the principles of the present invention, shown as it would be connected to a single stage thermostat; 
     FIG. 1B is a timing diagram of the operation of the controller upon a demand for heating from a single stage thermostat; 
     FIG. 2A is a schematic view of a controller constructed according to the principles of this present invention, shown as it would be connected to a multistage thermostat; 
     FIG. 2B; is a timing diagram of the operation of the controller upon first stage and second stage heating demands by the multistage thermostat; 
     FIG. 3 is a flow chart of the logic implemented by the controller of a first embodiment; and 
     FIG. 4 is a flow chart of the logic implemented by the controller of a second embodiment. 
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a controller for a multistage heater, such as a multistage gas furnace, that can be used with either a single stage thermostat, or a multistage thermostat Such a controller is indicated generally as  20  in FIGS. 1A and 2A, where the controller is shown connected to a single stage thermostat  22  (FIG. 1A) and a two-stage thermostat  24  (FIG.  2 A). 
     As shown in FIGS. 1A and 2A, the controller  20  includes first and second terminals  26  and  28 . The first terminal  26  is adapted to be connected to the output of single stage thermostat  22  or the first stage output of multistage thermostat  24 . The second terminal  28  is adapted to be connected to the second stage output of multistage thermostat  24 . When used with single stage thermostat  22 , the second terminal  28  of the controller  20  is not connected to the thermostat. 
     The controller  20  preferably also includes an indication device with which the installer can indicate the type of thermostat (single stageor multistage) that is connected to the controller. In the first preferred embodiment, this indication device is a contact block  30  with first, second and third contacts  32 ,  34 , and  36 . When the controller  20  is connected to single stage thermostat  22 , a jumper  38  connects the first contact  32  and the third contact  36 , and when the controller is connected to a multistage thermostat  24 , jumper  38  connects the first contact  32  and the second contact  34 . 
     In an alternate embodiment, the contact block  30  is replaced with a two position switch (not shown) with a first position for indicating that the controller  20  is connected to a single stage thermostat  22 , and a second position for indicating that the controller is connected to a multistage thermostat  24 . In still another alternate embodiment, the microprocessor  40  of the controller  20  is connected to the second terminal  28 . The processor  40  assumes that the controller  20  is connected to single stage thermostat unless it receives a request for second stage heat on the second terminal  28 , which can only happen if the controller is connected to a multistage thermostat  24 . 
     The controller  20  also includes a second stage delay timer  42 , typically set for ten minutes. Ten minutes is an appropriate compromise delay for determining whether first stage heating will be sufficient to satisfy the call for heat, without causing undue discomfort. Once initiated, the second stage delay timer counts down unless reset, and when the countdown reaches zero, second stage heating is initiated. Events that cause reset of the second stage delay timer  42  include the absence of a call for first stage heat, the presence of a call for second stage heat; or the absence of a call for second stage heat, when the controller is connected to a multistage thermostat. 
     The method implemented by a first embodiment of the controller  20 , in which the type of thermostat connected to the controller is indicated with a jumper or switch, is shown schematically in FIG.  3 . At  100  the controller sets the second stage delay timer  42  to ten minutes. At  102  the controller  20  determines whether there is a call for first stage heat. If there is no call for first stage heat, at  104  the controller  20  turns off the first stage inducer and gas valve and the second stage inducer and gas valve (if they are on) and returns to start, resetting the second stage delay timer  42  to ten minutes. If there is a call for first stage heat at  102 , then at  106  the controller energizes first stage inducer and gas valve, and at  108  determines whether there is a call for second stage heating. If there is a call for second stage heating at  108 , the controller  20  energizes the second stage inducer and gas valve at  110 , and returns to start, resetting the second stage delay timer  42  to ten minutes. If there is not a call for second stage heating at  108 , then at  112  the controller then determines what type of thermostat is connected to the controller  20 . If the thermostat is a multistage thermostat, and there is no call for second stage heat, then at  114  the controller  20  turns off the second stage inducer and gas valve. If the thermostat is not a multistage thermostat, the second stage delay timer is decremented at  116 . The controller then determines at  118  whether there is any time remaining in the second stage delay timer  42 . If there is no time remaining on the second stage delay timer  42 , then at  120  the controller energizes the second stage inducer and gas valve, and returns to  102  to check whether there is still a call for first stage heating, and repeats the cycle. If, at  118 , the controller  20  determines that there is time remaining on the second stage delay timer  42  then at  122  the controller turns off the second stage heating, and returns to  102  to check whether there is still a call for first stage heating, and repeats the cycle. 
     The controller  20  can operate with either a single stage thermostat  22  or a multistage thermostat  24 . In the case of a single stage thermostat  22 , the thermostat is connected to the first terminal  26 , and the controller  20  is set for operation with a single stage thermostat. In one embodiment the controller  20  is set by connecting the first and third contacts  32  and  36  on contact block  30  with jumper  38 . In another embodiment the controller  20  is set by operating a switch to the single stage thermostat position. The controller  20  sets up the second stage delay timer  42  for ten minutes, and then checks for a demand for first stage heating on terminal  26 . If there is no demand for first stage heating, the controller  20  returns to  100  where the second stage delay timer is reset to 10 minutes, and the cycle repeated. If there is a demand for first stage heating on terminal  26 , then at  106  the controller  20  energizes the first stage inducer and gas valve. The controller then checks for a demand for second stage heating on terminal  28 . There is, of course, no such demand, so the controller  20  then determines whether the controller is connected to a multistage thermostat. The jumper  38  or switch enables the controller  20  to determine that the controller is not connected to a multistage thermostat. At  116 , the controller then decrements the second stage delay timer  42 , and at  118  checks the timer. If no time is remaining on the second stage delay timer  42  the controller  20  energizes the second stage inducer and gas valve, and returns to  102 . If there is still time remaining on the second stage delay timer  42 , the controller turns off the second stage inducer and gas valve, and returns to  102 . 
     The method implemented by a second embodiment of the controller  20 , in which the controller is capable of “learning” the type of thermostat connected to the controller, is shown schematically in FIG.  4 . In this embodiment the controller includes a two stage application flag which may be, for example, a memory location whose content is representative of the type of thermostat connected to the controller. At  98  the controller clears the two stage application flag. At  100  the controller sets the second stage delay timer  42  to ten minutes. At  102  the controller  20  determines whether there is a call for first stage heat. If there is no call for first state heat, at  104  the controller  20  turns off the first stage inducer and gas valve and the second stage inducer and gas valve (if they are on) and returns to  100 , resetting the second stage delay timer  42  to ten minutes. If there is a call for first stage heat at  102 , then at  106  the controller energizes first stage inducer and gas valve, and at  108  determines whether there is a call for second stage heating. If there is a call for second stage heating at  108 , the controller  20  sets the two stage application flag at  109  (because the call for second stage heat unambiguously indicates that the controller is in fact connected to a multistage thermostat). The controller  20  then energizes the second stage inducer and gas valve at  110 , and returns to  100 , resetting the second stage delay timer  42  to ten minutes. If there is not a call for second stage heating at  108 , then at  113  the controller then determines whether the two stage application flag has been set, indicating that the controller has determined that it is connected to a multistage thermostat. If the two stage application flag has been set, indicating that the thermostat is a multistage thermostat, and there is no call for second stage heat, then at  114  the controller  20  turns off the second stage inducer and gas valve. If the two stage application flag has not been set, indicating that the thermostat is not a multistage thermostat (or at least that the thermostat has not unambiguously been determined to be a multistage thermostat), the second stage delay timer is decremented at  116 . The controller then determines at  118  whether there is any time remaining in the second stage delay timer  42 . If there is no time remaining on the second stage delay timer  42 , then at  120  the controller energizes the second stage inducer and gas valve, and returns to  102  to check whether there is still a call for first stage heating, and repeats the cycle. If, at  118 , the controller  20  determines that there is time remaining on the second stage delay timer  42  then at  122  the controller turns off the second stage heating, and returns to  102  to check whether there is still a call for first stage heating, and repeats the cycle. 
     Operation 
     The controller  20  can operate with either a single stage thermostat  22  or a multistage thermostat  24 . In the case of a single stage thermostat  22 , the thermostat is connected to the first terminal  26 . In the first embodiment the controller  20  is set for operation with a single stage thermostat. In one embodiment the controller  20  is set by connecting the first and third contacts  32  and  36  on contact block  30  with jumper  38  or by operating a switch to the single stage thermostat position. In the second embodiment, the controller has a two stage application flag that is reset at start up and indicates that the controller is not connected to a multistage thermostat. If the controller ever receives a call for second stage heating, it sets the two stage application flag. Thus, in effect, the controller  20  “assumes” it is connected to a single stage thermostat unless and until it receives a demand for second stage heating on terminal  28  (which never occurs with a single stage thermostat). The controller  20  sets the second stage delay timer  42  for ten minutes, and then checks for a demand for first stage heating on terminal  26 . If there is no demand for first stage heating, the controller  20  returns to  100  where the second stage delay timer is reset to 10 minutes, and the cycle repeated. If there is a demand for first stage heating on terminal  26 , then at  106  the controller  20  energizes the first stage inducer and gas valve. The controller then checks for a demand for second stage heating on terminal  28 . There is no such demand, since the controller is connected to a single stage thermostat. The controller  20  then determines whether the two stage application flag has been set. Because the thermostat is a single stage thermostat, the two stage application flag is not set. At  116 , the controller then decrements the second stage delay timer  42 , and at  118  checks the timer. If no time is remaining on the second stage delay timer  42  the controller  20  energizes the second stage inducer and gas valve, and returns to  102 . If there is still time remaining on the second stage delay timer  42 , the controller turns off the second stage inducer and gas valve, and returns to  102 . 
     FIG. 1B illustrates the operation of the controller when connected to a single stage thermostat. After a demand for heat the controller waits for ten minutes and if there is still a demand for heat the controller provides high heat. 
     In the case of a multistage thermostat  24 , the thermostat is connected to the first and second terminals  26  and  28 , and the controller  20  is set for operation with a multistage thermostat. The controller  20  of the first embodiment is set by connecting the first and second contacts  32  and  34  on the contact block  30  with a jumper  38 , or by operating a switch to the multistage thermostat position. In the controller of the second embodiment, the controller “assumes” that it is connected to a single stage thermostat unless and until it receives a demand for second stage heating on terminal  28 , which will occur the first time that second stage heating is demanded on terminal  28 , and the controller “learns” that it is connected to a multistage thermostat. The controller  20  sets up the second stage delay timer  42  for ten minutes, and then at  102  checks for a demand for first stage heating on terminal  26 . If there is no demand for first stage heating, the controller returns to  100  where the second stage delay timer is reset to  10  minutes, and the cycle repeated. If there is a demand for first stage heating on terminal  26  then at  106  the controller  20  energizes the first stage inducer and gas valve. The controller  20  then checks for a demand for second stage heating on terminal  28 . If there is such a demand, the controller  20  energizes the second stage inducer at  110  and returns to  100 . If there is no such demand, then the controller  20  determines whether the controller is connected to a multistage thermostat. The jumper  38  or switch enables the controller of the first embodiment to determine that the controller is connected to a multistage thermostat. The two stage application flag enables the controller of the second embodiment to determine that the controller is connected to a multistage thermostat. The controller  20  then turns off the second stage inducer and gas valve and returns to  100  resetting the second stage delay timer  42 , and restarting the controller thermostat cycle. In the event that the controller  20  of the second embodiment does not yet know that it is connected to a multistage thermostat, the controller decrements the second stage delay timer  42 , and checks the timer. If no time were remaining on the timer, the controller  20  would energize the second stage inducer and gas valve at  120 , and return to  102 . However, this is not likely with a multistage thermostat. The more likely situation, if this step is reached at all, is that time does remain. If there is still time remaining on the second stage delay timer  42 , the controller  20  turns off the second stage inducer and gas valve, and returns to  102 . 
     FIG. 2B illustrates the operation of the controller when connected to a multi-stage thermostat. On initiation of a demand for low heat the controller provides low heat, and immediately upon a demand for high heat the controller provides high heat.