Four-stage high efficiency furnace

Abstract

A four-stage furnace includes a gas flow control assembly operable to provide gas to the furnace at one of four firing rates. The furnace also includes a heat exchanger and an inducer blower for establishing a flow of combustion gas that generates a differential pressure across the heat exchanger. A furnace controller controls the rate at which the gas flow control assembly provides gas to the furnace and controls the inducer blower to establish one of four combustion gas flows that corresponds to one of the four firing rates.

1 . A four-stage furnace comprising:
 a gas flow control assembly operable to provide gas to the furnace at one of four firing rates;   a heat exchanger;   an inducer blower for establishing a flow of combustion gases that generates a differential pressure across the heat exchanger; and   a furnace controller for controlling the rate at which the gas flow control assembly provides gas to the furnace and for controlling the inducer blower to establish one of four combustion gas flows that corresponds to one of the four firing rates.   
   
   
       2 . The furnace of  claim 1 , wherein the gas flow control assembly comprises:
 a main gas valve having first and second open states; and   a throttling valve connected in fluidic series with the main gas valve having at least first and second open states.   
   
   
       3 . The furnace of  claim 1 , and further comprising:
 a pressure switch assembly including a first pressure switch and a second pressure switch, wherein the pressure switch assembly senses differential pressure across the heat exchanger and generates first and second pressure signals that vary in accordance with the differential pressure sensed across the heat exchanger.   
   
   
       4 . The furnace of  claim 3 , wherein the furnace controller controls the furnace to operate at first and second firing rates as a function of the first pressure signal from the first pressure switch, and wherein the furnace controller controls furnace to operate at third and fourth firing rates as a function of the second pressure signal from the second pressure switch. 
   
   
       5 . The furnace of  claim 1 , wherein the combustion gas flows established by the inducer blower for ignition at first and second firing rates are substantially equal, and wherein the combustion gas flows established for ignition at third and fourth firing rates are substantially equal. 
   
   
       6 . The furnace of  claim 1 , wherein the combustion gas flows established during steady state operation of the furnace at the four firing rates have different respective magnitudes. 
   
   
       7 . The furnace of  claim 1 , and further comprising:
 a circulating air blower that is controlled by the furnace controller for establishing a flow of circulating air from the furnace.   
   
   
       8 . The furnace of  claim 7 , wherein the circulation air flows established during steady state operation of the furnace at the four firing rates have different respective magnitudes. 
   
   
       9 . A four-stage furnace including an inducer blower for establishing a flow of combustion gas, a circulating air blower for establishing a flow of circulating air, and a heat exchanger, the furnace comprising:
 a gas flow control assembly for controlling a rate at which gas is supplied to the furnace to one of a first firing rate, a second firing rate, a third firing rate, and a fourth firing rate;   a pressure switch assembly including a first pressure switch and a second pressure switch, wherein the pressure switch assembly senses differential pressure across the heat exchanger and generates pressure signals that vary in accordance with the differential pressure sensed across the heat exchanger; and   a furnace controller for controlling the rate at which the gas flow control assembly supplies gas to the furnace, for controlling speeds of the inducer blower and the circulating air blower, and for receiving the differential pressure signals from the pressure switch assembly.   
   
   
       10 . The furnace of  claim 9 , wherein the gas flow control assembly comprises:
 a main gas valve having first and second open states; and   a throttling valve connected in fluidic series with the main gas valve having at least first and second open states.   
   
   
       11 . The furnace of  claim 9 , wherein the furnace controller controls the furnace to operate at the first and second firing rates as a function of the first pressure signal from the first pressure switch, and wherein the furnace controller controls furnace to operate at the third and fourth firing rates as a function of the second pressure signal from the second pressure switch. 
   
   
       12 . The furnace of  claim 9 , wherein a magnitude of the combustion gas flow established for ignition at the first and second firing rates are substantially equal, and wherein a magnitude of the combustion gas flow established for ignition at the third and fourth firing rates are substantially equal. 
   
   
       13 . The furnace of  claim 9 , wherein the combustion gas flows established during steady state operation of the furnace at the first, second, third, and fourth firing rates have different respective magnitudes. 
   
   
       14 . The furnace of  claim 9 , wherein the circulation air flows established during steady state operation of the furnace at the first, second, third, and fourth firing rates have different respective magnitudes. 
   
   
       15 . A furnace comprising:
 a main gas valve having low open state and a high open state;   a throttling valve connected in fluidic series with the main gas valve and having a low open state and a high open state; and   a furnace controller for operating the main gas valve and the throttling valve to supply a gas to the furnace at one of a low firing rate, a medium-low firing rate, a medium-high firing rate, and a high firing rate.   
   
   
       16 . The furnace of  claim 15 , wherein the main gas valve and the throttling valve are in their low open states for the low firing rate, the main gas valve is in its low open state and the throttling valve is in its high open state for the medium-low firing rate, the main gas valve is in its high open state and the throttling valve is in its low open state for the medium-high firing rate, and the main gas valve and the throttling valve are in their high open states for the high firing rate. 
   
   
       17 . The furnace of  claim 15 , and further comprising:
 a heat exchanger; and   an inducer blower controlled by the furnace controller for establishing a flow of combustion gas that generates a differential pressure across the heat exchanger that corresponds to one of the four firing rates.   
   
   
       18 . The furnace of  claim 17 , wherein the combustion gas flows established by the inducer blower for ignition at the low and medium-low firing rates are substantially equal, and wherein the combustion gas flows established for ignition at the medium-high and high firing rates are substantially equal. 
   
   
       19 . The furnace of  claim 17 , wherein the combustion gas flows established during steady state operation of the furnace at the four firing rates have different respective magnitudes. 
   
   
       20 . The furnace of  claim 15 , and further comprising:
 a pressure switch assembly for sensing pressure across the heat exchanger and including a low pressure switch and a high pressure switch, wherein the furnace controller controls the furnace to operate at the low and medium-low firing rates as a function of a pressure signal from the low pressure switch, and wherein the furnace controller controls furnace to operate at medium-high and high firing rates as a function of a pressure signal from the high pressure switch.