Furnace Fan Cycling

[Geordie97]

Our furnace works fine in keeping the house warm but when I come home (I work evening) around 2am, I can hear the fan cycling on/off constantly, regardless of the temp in the house or what the thermostat is set at.

The fan will cycle up for 3-5 seconds, then cycle down and almost shut off, then cycle up again. I have looked at the furnace (it's in the attic) and don't see any sign of the pilot light igniting but the fan just cycles up and down. Any suggestions??

Larry

[lonestarheir]

How old is the furnace - sounds like you either have a fan relay that is bad, or a control board that is bad

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Brandon
- Have you changed your filter this month? -

[Geordie97]

The furnace will be 12 years old in Dec. I change the filter on the 1st of every month. Is there any way to check the contol board? The funny thing is, it only does it at night. I'm usually up for 60-90 minutes after I get home (around 2am) and can hear it cycling (if I'm at the right place in the house) the whole time, unless it gets cold enough for the heater to fire up.

Could it have something to do with our auto thermostat?

Larry

[Geordie97]

Okay...figured out it's the exhaust blower motor. A friend suggested unplugging it for 3-4 minutes to reset the board. If that doesn't work, I'm going to try replacing the pressure switch that's hooked to the blower motor. Any other ideas???

Larry[)]

[lonestarheir]

Does the combustion motor come on when you turn it to heat? If not, try to spin the fan carefully by hand - have seen them hang up on some rust inside sometimes. If it does come on, it probably is the pressure switch that looks like a small right by the combustion motor that has a little Clear vinyl (soelling?) Otherwise it is a limit switch somewhere else.

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Brandon
- Have you changed your filter this month? -

[Geordie97]

Filter is clean. The furnace works fine except for the exhaust (combustion) fan cycling. I'm sure that, since it seems to work when it cycles, that it's working when the furnace has ignited. Will have to go up and check it when the furnace is on. Since unplugging it to recycle the circuit board didn't work, I will try to replace the pressure switch.

[Geordie97]

Okay...I feel officially stupid now. I decided to call the furnace mfg for information. I explained to them what the problem was and they referred me to the Technical Mgr. He asked if I had an auto thermostat and I told him we did. He said to replace it as that was what was causing the problem. Okay...simple enough.

I decided to put the old thermostat back on to see if it fixed it. Took the cover off the auto thermo to get to the screws and there were two batteries with instructions to replace them once a year. I thought back to the 6 or 7 years we had the thermostat and couldn't remember replacing them but the low battery signal never came on. What the heck...I replaced the batteries and came home tonight and the furnace is working fine. Boy...do I feel stupid...but I got away cheap!!

Thanks for all the suggestions.

Larry

[lonestarheir]

Most of the time it is the things that we overlook that get us in trouble



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Brandon
- Have you changed your filter this month? -