Thaw cycle in condensing style gas furnaces

Abstract

A condensing gas furnace having an inducer for drawing exhaust produced by a burner through an exhaust path and venting the exhaust from the furnace. A pressure sensor positioned within the exhaust path to determine if the inducer is creating a negative pressure in the exhaust path preventing accumulation of exhaust within the exhaust path. The inducer includes a supplemental sensor for monitoring rotation of a fan of the inducer. The burner is not ignited when both sensors fail to indicate proper venting of the exhaust path. The burner is operated for a predetermined period of time when only the supplemental sensor signals rotation of the fan to thaw any condensate on the supplemental sensor.

What is claimed is: 
     
         1 . A gas furnace, comprising:
 a burner operable to produce an exhaust containing products of combustion;   an exhaust path for receiving exhaust from the burner;   a primary heat exchanger having an inlet operably connected to the exhaust path and an outlet operably connected to the exhaust path, wherein the primary heat exchanger is operably receive exhaust from the exhaust path and return exhaust to the exhaust path;   a working fluid stream operably connected to the primary heat exchanger to receive heat from the exhaust received within the primary heat exchanger;   an inducer, including a fan, operable to exhaust the products of combustion through the exhaust path;   a pressure sensor configured to measure pressure within the exhaust path and generate a pressure sensor signal;   a supplemental sensor configured to detect rotation of the fan and generate a detection signal indicating rotation of the fan;   a controller communicatively connected to the burner, the inducer, the pressure sensor, and the supplemental sensor, the controller being configured to:
 rotate the fan of the inducer, 
 receive the detection signal from supplemental sensor, 
 operate burner upon receiving the detection signal from the supplemental sensor, and 
 extinguish the burner if the pressure sensor signal is not received within a predetermined burn time. 
   
     
     
         2 . The gas furnace of  claim 1 , wherein the controller is configured to:
 increment a counter after each extinguishing of the burner.   
     
     
         3 . The gas furnace of  claim 2 , wherein the controller is configured to:
 ignite the burner in response to detecting rotation of the inducer fan from the supplemental sensor if the incremental counter value is less than a threshold;   extinguish the burner if the pressure sensor signal is not received within a predetermined burn time.   
     
     
         4 . The gas furnace of  claim 3 , wherein the threshold is five. 
     
     
         5 . The gas furnace of  claim 3 , wherein the controller is configured to:
 maintain ignition of the burner and rotation of the fan of the inducer upon receiving a pressure sensor signal from the pressure sensor.   
     
     
         6 . The gas furnace of  claim 5 , wherein the controller is configured to:
 extinguish the burner if the detection signal from the supplemental sensor ceases.   
     
     
         7 . The gas furnace of  claim 6 , wherein the controller is configured to:
 prevent ignition of the burner when the counter exceeds a threshold cycle limit.   
     
     
         8 . The gas furnace of  claim 7 , wherein the controller is configured to:
 start a timer upon initiating rotation of the fan of the inducer.   
     
     
         9 . The gas furnace of  claim 8 , wherein the controller is configured to:
 ignite the burner when the timer exceeds a predetermined purge time.   
     
     
         10 . The gas furnace of  claim 9 , wherein the purge time is 30 seconds. 
     
     
         11 . The gas furnace of  claim 9 , wherein the controller is configured to:
 continue rotation of the fan of the inducer for a post-purge period of time after the burner is extinguished.   
     
     
         12 . The gas furnace of  claim 11 , wherein the controller is configured to:
 stop rotation of the fan of the inducer following an inducer shut down time.   
     
     
         13 . The gas furnace of  claim 1 , wherein the supplemental sensor transmits the detection signal when rotation of the fan exceeds a first threshold speed. 
     
     
         14 . The gas furnace of  claim 13 , wherein the supplemental sensor comprises a switch configured to close a circuit when rotation of the fan exceeds the first threshold speed. 
     
     
         15 . The gas furnace of  claim 14 , wherein the supplemental sensor comprises one of a centrifugal switch, a sail switch, a Hall effect switch activating when a rotational speed of the inducer motor exceeds a threshold, or a current sensing relay activating when a motor current of the inducer exceeds a threshold. 
     
     
         16 . The gas furnace of  claim 15 , wherein the supplemental sensor includes a hysteresis such that transmission of the detection signal continues until rotation of the inducer fan is less than a second threshold, the second threshold being lower than the first threshold. 
     
     
         17 . The gas furnace of  claim 16 , wherein the first threshold is 2,000 revolutions per minute. 
     
     
         18 . The gas furnace of  claim 1 , wherein the pressure sensor transmits the pressure sensor signal when the measured pressure within the exhaust path exceeds a threshold pressure. 
     
     
         19 . The gas furnace of  claim 18 , wherein the pressure sensor includes a switch configured to close a circuit and thereby transmit the pressure sensor signal to the controller when the measured pressure within the exhaust path exceeds the threshold pressure. 
     
     
         20 . The gas furnace of  claim 19 , wherein the pressure sensor comprises one of a diaphragm switch, a differential switch, or a sail switch. 
     
     
         21 . The gas furnace of  claim 19 , wherein the pressure sensor is located in a port of a housing surrounding the fan of the inducer. 
     
     
         22 . The gas furnace of  claim 1 , wherein the burn time is 60 seconds. 
     
     
         23 . The gas furnace of  claim 1 , wherein the gas furnace comprises a condensing gas furnace. 
     
     
         24 . The gas furnace of  claim 1 , further comprising a secondary heat exchanger having an inlet proximate to an outlet of the primary heat exchanger and an outlet proximate an inlet of the inducer. 
     
     
         25 . A method for operation a condensing gas furnace with a thaw cycle, comprising:
 rotating a fan of an inducer;   receiving a signal from a sensor indicating rotation of the fan of the inducer;   initiating an ignition sequence for a burner upon receiving the Hall Effect signal;   operating the burner for a first period of time;   monitoring for a pressure sensor signal;   ceasing operation of the burner if the pressure sensor signal is not received within a second period of time;   repeating operation of the burner for the first period of time for a specified number of times until pressure sensor signal is detected.   
     
     
         26 . A condensing gas furnace with a thaw cycle, comprising:
 a primary heat exchanger;   a gas manifold assembly including at least one burner and at least one gas valve, wherein the gas manifold assembly is positioned at the inlet of the primary heat exchanger;   an igniter for initiating combustion of fuel dispensed from the gas manifold assembly;   a secondary heat exchanger coupled to the outlet of the primary heat exchanger;   an inducer having a fan coupled the secondary heat exchanger, wherein the fan is rotatable to generate airflow through the first and secondary heat exchangers;   a pressure switch located in-line with the inducer;   a sensor for detecting the angular velocity of the fan the inducer; and   a controller configured to:
 start an inducer, 
 receive a signal velocity from the sensor, the sensor indicating that the rotational velocity of the fan exceeds a predetermined rotational, 
 initiate an ignition sequence for the burner upon receiving the signal, 
 operate the burner for a first period of time, 
 monitor for a pressure sensor signal; 
 cease operation of the burner if the pressure sensor signal is not received within a second period of time; 
 repeat operation of the burner for the first period of time for a specified number of times until pressure sensor signal is detected.