Fan cycles on and off to often the burner stays on and the fan cy
I turned the fan to on position on the TS and watched the burner it stays on until temp. in house is reached then it turns off.
If I put the TS in auto position the burner comes on but the fan cycles every 30 to 40 seconds?
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Re: fan cycles on and off to often the burner stays on...
What is happening is that the heat exchanger has to get to a certain temperature before the fan comes on that way you have warm air blowing out of the grilles and when the heat excahnger gets cool it shuts off until it warms back up again.
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If you have a switch that can manually turn on the fan, try it to verify the fan works. This is what I think is happening. The normal operation should be thermostat calls for heat, inducer fan comes on, pressure switch closes, igniter glows, main burner ignites and then in 30 or so seconds the fan comes on and stays on until the thermostat is satisfied. Once the thermostat is satisfied the burner shuts off and the fan runs for approx 120 seconds then shuts off. I think the control board that automatically turns your fan on is faulty. On most modern furnaces if the auto fan control fails and the furnace gets to hot and limits, when the limit opens and shuts off the burner it also energizes the fan motor to cool the heat exchanger.
Nov 17, 2012 - Uploaded by grayfurnaceman
This furnace should light its pilot with a spark then the pilot proves its on ... Troubleshoot an ignition problem on a Lennox gasfurnace .... Pilot and no inducer but is the same model as that great video! ... You probably have dirty burners. ... How do you know spark module is not sending 24 volts to mv and pv ...
The first thing I would do is make sure the flame sensor is clean, this is a metal rod immersed in the burner flame on the opposite side of the igniter. If too much oxidation builds up on it, it will not allow the burner to stay lit. To clean, just turn off furnace and remove flame sensor and clean with steel wool and then reinstall. If this does not solve the problem there may be a problem with the gas valve.
Yes there is a sequence: 0:00 Dial for fan/limit switch is at 70 deg position. Furnace is cold. I turn T-stat to 80 deg to start furnace. 1:00 Burner comes on and dial begins to move slowly up. 2:30 Dial at 140 deg and fan motor comes on. Dial never goes above 150 deg. 3:00 Burner turns off. Dial slowly moves back towards 90 deg. 4:45 Burner turns on and dial is at 110 deg. 4:50 Fan turns off with dial still at 110 deg. 5:30 Burner off. Dial at 120 deg. 6:15 Fan on. Dial at 140 deg. Burner is still off. 7:00 Burner on. Dial at 120 deg. 7:40 Burner off. Dial still at 120 deg. 8:30 Fan off with dial at 110 deg., and it quickly snaps back to the 80 deg. position. 8:30 plus, cycle continues.....
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.
that makes no sense? the door on does exactly the same thing as you holding the switch closed and neither has anything to do with the problem you are describing. if door were off nothing would happen. if door was on ,nothing would happen unless the switch was closed. it may be coincidence, but sounds like the unit is not proofing the flame. look for a little needle like sensor that protrudes into the flame path in front of a burner. when the burners light it senses flame and tells gas valve burners lit and its ok to stay on. then heat builds and the fan is briught on. once in a blue moon the overtemp sensor trips open prematurely as soon as the burners start to heat up. it is a little wire in , wire out sensor above the burners that protreudes into the bulkhead above the burners. if it is cold and open its bad. if you jump it and burners light and stay lit and fan comes on its bad. fairly cheap and easy to replace. you may have an old fan limit switch the rotates like a spring to bring the fan on as it heats up, then turns off burner if its gets to hot or is set to low. it also turns fan of as burners cool down.