Question about Null Heating & Cooling
Ok, my original problem was that my furnace would cycle on, the pilot would light, and then the blower would kick on without the main burner ever coming on. Additionally, the blower would kick on no matter if the thermostat was switched to off or run. This would happen with the A/C as well. The compressor would come on only when needed, but the blower stayed on constantly as long as the unit had power. I did some research and all signs pointed to my smart valve. I replaced the smart valve today, but that only took care of part of the problem. I replaced the valve (SV9501) and turned the gas back on and then switched my switch to give the furnace power. Now, the blower comes on as soon as the unit gets power, but now, at least, the burners come on. The burner now does not shut off even if the temperature is above the temp set at the thermostat. I am using my power switch to control the temperature in my house right now, but would love to find a permanent solution. I have not tried the A/C yet as it is a high of 50 today. Too much heat is better than no heat this time of year, but this will get old fast. Any help?
I can't stand the tempstar smart valve! Now I'm not sure what a dc90, but if you have a gas fire residential furnace I know what it is. The smart valve controls gas and flame proving only. The blower issue is probably one of the high temp safeties that has stuck. During a call for heat, your system verifies that all safeties are closed. If this sequence is not correct, then the unit assumes that the unit has overheated due to a high temperature safety issue, and brings the blower on to cool it down, normally for 90secs. Also if the unit is calling for heat when the power is turned off, still calling for heat when power is turned back on(like you said by throwing the switch), this will also bring the blower on for 90sec, then blower will/should turn off, and the heat cycle will start. So if your tstat is calling for heat(heat on and set temp higher than accrual temp), and you turn off power then turn power(main voltage) back on, this will bring blower on for 90secs. Now, the burners staying on is a different issue. I've not seen that in 15 yrs of service on a smart valve. There is probably 24volts feeding the "W" circuit, for some reason. On the control board there is a terminal block with R W G Y C. The W is your heat. This W receives a 24v signal from your tstat. Turn off power, remove W from the board, then turn power back on. If no heat then you've isolated your problem to the control wire coming from your tstat, or the tstat itself. If it continues to heat with W disconnected then its something in the furnace, circuit board? Hope this helps!
Posted on Mar 05, 2017
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Posted on Jan 02, 2017
SOURCE: I have a problem with
sounds like its not proofing flame. it has a sensor usually like a needle sticks out into the flame that tells the gas valve the flame lit and is burning so its ok to keep sending gas, if its fails to proof flame then burner shuts down, then it tries to relight 3 times then locks out on fail safe. the blower will sometimes run continually once that occurs to highlight a failed condition. every time you reset it will try again 3 times then shut down. lp gas is very dangerous, even i do not work on it as the gas acts like gasoline and lays around. not like natural gas that disperses well. if you see the flame sensor try gently cleaning it with a little emory cloth. also a loose ground can cause this. check tio be sure any green wires are snug , they actually flow volts to ground through the flame on some to proof, thats why no ground no flame. if that fails then the board is most likely problem. change wire for wire , or get generic replacemant and follow directions exactly. good luck.some ignitors act as a flamre proofer as soon as they are done igniting! rarely do they fail to perform both funtions.
Posted on Nov 21, 2008
If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.
Posted on Jan 06, 2009
I have serviced these furnaces for 20 years and I can tell you most of these models did not have diagnostic lite on them. (although some did, especially if the control board has been replaced) The problem is most likely a bad 3 wire pilot. This will cause exactly what you describe. 2 things to consider though. 1, If you were to replace the pilot don't get a cheaper knockoff. In my experience these do not last long at all. Get the right one from a carrier dealer. It will cost more but be much more reliable. 2 What is the condition of the secondary heat exchanger? Most if not all of the sx series had issues with the secondary heat exchangers rotting out and leaking. If the furnace is out of warranty you would be much better replacing the furnace as the cost of replacing the secondary heat exchanger will probably be almost the same as a new furnace. ( those babies are EXPENSIVE!)
Posted on May 01, 2010
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