Abstract:
A furnace includes an induced draft blower; an inducer motor driving the induced draft blower; and a furnace control determining a modulation percentage, the furnace control controlling RPM of the inducer motor in response to the modulation percentage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application of U.S. Provisional Patent Application No. 61/389,868 filed Oct. 5, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein generally relates to modulating furnaces, and in particular to a method and system for controlling an inducer in a modulating furnace. 
     Modulating furnaces operate through ranges of combustion airflow and gas rate in order to efficiently meet heat demand. Existing modulating furnaces run between 40% and 100% of total capacity based on heat demand and other variables. One challenge in operating a modulating furnace over a range of heating capacities is control of the inducer. The inducer draws air through the heat exchanger and sends the air out a vent. It is desirable to provide a smooth transition of inducer speed over a range of operating capacities without requiring additional components, such as pressure transducers. 
     BRIEF DESCRIPTION OF THE INVENTION 
     An embodiment is a furnace including an induced draft blower; an inducer motor driving the induced draft blower; and a furnace control determining a modulation percentage, the furnace control controlling RPM of the inducer motor in response to the modulation percentage. 
     Another embodiment is a method of controlling a furnace having an inducer motor, the method including determining a modulation percentage; and controlling RPM of the inducer motor in response to the modulation percentage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts an exemplary furnace having an evaporator coil; 
         FIG. 2  is a flowchart of a control process; and 
         FIG. 3  is a plot of inducer RPM versus modulation percentage in exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1 , the numeral  10  generally designates a gas-fired condensing furnace employing the blower motor control of the present invention. Condensing furnace  10  includes a steel cabinet  12  housing therein burner assembly  14 , combination gas control  16 , heat exchanger assembly  18 , inducer housing  20  supporting, inducer motor  22  and inducer wheel  24 , and circulating air blower  26 . Combination gas control  16  includes a hot surface igniter (not shown) to ignite the fuel gas. 
     Burner assembly  14  includes at least one inshot burner  28  for at least one primary heat exchanger  30 . Burner  28  receives a flow of combustible gas from gas regulator  16  and injects the fuel gas into primary heat exchanger  30 . A part of the injection process includes drawing air into heat exchanger assembly  18  so that the fuel gas and air mixture may be combusted therein. A flow of combustion air is delivered through combustion air inlet  32  to be mixed with the gas delivered to burner assembly  14 . 
     Primary heat exchanger  30  includes an outlet  34  opening into chamber  36 . Connected to chamber  36  and in fluid communication therewith are at least four condensing heat exchangers  38  having an inlet  40  and an outlet  42 . Outlet  42  opens into chamber  44  for venting exhaust flue gases and condensate. 
     Inducer housing  20  is connected to chamber  44  and has mounted thereon an inducer motor  22  together with inducer wheel  24  for drawing the combusted fuel air mixture from burner assembly  14  through heat exchanger assembly  18 . Air blower  26  is driven by blower motor  25  and delivers air to be heated in a counterflow arrangement upwardly through air passage  52  and over heat exchanger assembly  18 . The cool air passing over condensing heat exchanger  38  lowers the heat exchanger wall temperature below the dew point of the combusted fuel air mixture causing a portion of the water vapor in the combusted fuel air mixture to condense, thereby recovering a portion of the sensible and latent heat energy. The condensate formed within heat exchanger  38  flows through chamber  44  into drain tube  46  to condensate trap assembly  48 . As air blower  26  continues to urge a flow of air, upwardly through heat exchanger assembly  18 , heat energy is transferred from the combusted fuel air mixture flowing through heat exchangers  30  and  38  to heat the air circulated by blower  26 . Finally, the combusted fuel air mixture that flows through heat exchangers  30  and  38  exits through outlet  42  and is then delivered by inducer motor  22  through exhaust gas outlet  50  and thence to a vent pipe (not illustrated). 
     Cabinet  12  also houses a controller  54  and a display  56 . Controller  54  may be implemented using a microprocessor-based controller executing computer program code stored on a computer readable storage medium. A thermostat  55  communicates with controller  54  to designate operational modes and temperature. Thermostat  55  may be an intelligent device that communicates requested air flow rates as described in further detail herein. A pressure tap  58  is located at primary heat exchanger inlet  60 , a pressure tap  62  is located at condensing heat exchanger outlet  42  and a limit switch  64  is disposed in air passage  52 . In a non-condensing furnace, pressure tap  62  would be disposed at primary heat exchanger outlet  34 , since there would be no condensing heat exchanger  38 . To provide additional control, a pressure switch assembly (not shown) including low pressure switch, medium pressure switch, and high pressure switch may be coupled to pressure tap  58  and pressure tap  62  and in communication with controller  54 . 
     For cooling modes, a cooling coil  82  is located in housing  80  on top of furnace cabinet  10  and is the evaporator of air conditioning system. The cooling coil  82  has an inlet  84 , where subcooled refrigerant enters, and an outlet  86 , where superheated refrigerant leaves, as is conventional. In response to an input from heating/cooling thermostat, air blower  26  urges air flow upwardly through cooling coil  82  where heat exchange takes place. As a result of this heat exchange, cool air is delivered to the conditioned space and superheated refrigerant is returned to the outdoor condensing section (not illustrated) via outlet  86 . In the outdoor condensing section the refrigerant is subcooled and returned to inlet  84 . This cycle continues until the thermostat is satisfied. 
     In exemplary embodiments, the furnace of  FIG. 1  is operated through five ranges of heating capacity, referred to herein as low, low-medium, medium, medium-high and high. In certain ranges, the inducer motor is controlled as a function of the operating capacity and an RPM value. The operating capacity is referred to herein as the modulation percentage, with 100% representing full capacity. 
       FIG. 2  is a flowchart of an exemplary process for controlling the inducer motor  32 . The process is implemented by furnace control  54 . The process begins at  100  where it is determined if a heat demand signal is received from thermostat  34 . If not, the process cycles waiting for a heat demand signal. 
     Once a heat demand signal is received, flow proceeds to  102  where a determination is made whether the modulation percentage is 40% to 51%, which may be referred to as low range. The modulation percentage may be computed by furnace control  54  based on preloaded routines and prior heating patterns. Alternatively, the thermostat  34  may be an intelligent device and provide the desired modulation percentage to furnace control  54 . If so, flow proceeds to  104  where the furnace control  54  sets the inducer motor rpm to value of RPM 1 ×Modulation %/K 1 . RPM 1  is a first RPM value and may be based on a reference RPM, RPM REF , such as the inducer motor RPM when the medium pressure sensor is tripped before ignition when the inducer begins circulating air through the heat exchanger. RPM 1  may be mathematically derived from RPM REF . K 1  is a constant and may be a reference modulation percentage, such as 40%. 
     If the modulation percentage is not 40% to 51%, flow proceeds to  106  where a determination is made whether the modulation percentage is 52% to 65%, which may be referred to as low-medium range. If so, flow proceeds to  108  where the furnace control  54  sets the inducer motor rpm to value of RPM 2 . RPM 2  is a second RPM value and may be based on the reference RPM, RPM REF , such as the inducer motor RPM when the medium pressure sensor is tripped before ignition when the inducer begins circulating air through the heat exchanger. RPM 2  may be mathematically derived from RPM REF . RPM 2  is different than, and greater than, RPM 1 . 
     If the modulation percentage is not 52% to 65%, flow proceeds to  110 , where a determination is made whether the modulation percentage is 66% to 71%, which may be referred to as medium range. If so, flow proceeds to  112  where the furnace control  54  sets the inducer motor rpm to value of RPM 2 ×Modulation %/K 2 . K 2  is a constant and may be a reference modulation percentage, such as 65%. 
     If the modulation percentage is not 66% to 71%, flow proceeds to  114 , where a determination is made whether the modulation percentage is 72% to 90%, which may be referred to as medium-high range. If so, flow proceeds to  116  where the furnace control  54  sets the inducer motor rpm to value of RPM 3 ×K 3 . K 3  is a constant and may be 0.9. RPM 3  is a third RPM value and may be based on the reference RPM, RPM REF , such as the inducer motor RPM when the medium pressure sensor is tripped before ignition when the inducer begins circulating air through the heat exchanger. RPM 3  may be mathematically derived from RPM REF . RPM 3  is different than, and greater than, RPM 1  and RPM 2 . 
     If the modulation percentage is not 72% to 90%, flow proceeds to  118 , where a determination is made whether the modulation percentage is 91% to 100%, which may be referred to as high range. If so, flow proceeds to  118  where the furnace control  54  sets the inducer motor rpm to value of RPM 3 . 
       FIG. 3  is a plot of inducer RPM versus modulation percentage in embodiments of the invention. The values in  FIG. 3  correspond to the numerical values provided in  FIG. 2 . It is understood that other numerical values may be used, and embodiments are not limited to the values described herein. Further, the number of ranges and the modulation percentages defining each range may be varied. Embodiments provide control of inducer motor RPM in over the range of heating capacities without drastic steps in the inducer motor RPM. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.