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Osh 06-22-2013 03:57 PM

furnace exhaust fan
We turned on the AC for the first time today. It's a 90% efficient furnace so it has a pvc "chimney" out the wall. Walking past it on the driveway I noticed the exhaust fan was coming on and off every 5 seconds or so. The fan feeding the cool air to the house is working fine. Any thing that I should look for first? It does not do it when the heat is on.

beenthere 06-23-2013 07:19 AM

What brand and model number thermostat do you have.

Osh 06-23-2013 08:42 AM

It's a White Rodgers 1F80-51 programmable. Could it be the batteries?

old_squid 06-23-2013 09:18 AM

When did you install the thermostat?

beenthere 06-23-2013 09:26 AM

It could be the batteries. Replace them. If it still does it. Remove the wire from the W terminal on the furnace board and see if it still does it. If it does, then its something with the furnace board, or a furnace safety.

Osh 06-23-2013 10:15 AM

It was the batteries. Weird that it did it only with the AC on and not the heat. The thermostat is well over 20 years old and I can't remember when the batteries were changed last, the display was still working.

Thanks , gotta luv DIY

beenthere 06-23-2013 10:25 AM

While I do make money coming out and putting batteries in people's thermostats. I prefer for them to try new batteries themselves first. So they don't fall over when they see what it cost for me to come out and do it.

Osh 06-13-2015 04:58 PM

Bringing up a 2 year old thread
Same problem as before but remembered this thread. Replaced the batteries in the thermostat again but didn't cure the problem. Removed wire from the W terminal as suggested last time but then the compressor didn't run. Any other suggestions? Furnace board kaput? It's a Armstrong ultra SX 90 furnace.

yuri 06-13-2015 05:03 PM

You got some mixed up or shorting wiring. W is ONLY for the heat. Y is for cooling. Been~ mentioned disconnecting W to see if it was calling for heat and running the exhaust fan. Should have nothing to do with the AC not working. That may be a pretty old thermostat and shorting or acting up. Try a new Honeywell one from HDepot. Otherwise you may have a wiring problem or short. Easier to try a new thermostat.

Osh 06-13-2015 05:48 PM

The AC is actually working, just that the exhaust fan in running intermittently every 5 seconds or so. I only noticed it while walking down the driveway where the pvc " chiminy' comes out the wall.

Osh 06-21-2015 10:36 AM

Just a FYI. After my wife reminded me the thermostat was 26 years old I went and bought a new Honeywell programmable t-stat. Less than 10 minute install and all is good. $50 vs a $150 service call minimum.

yuri 06-21-2015 12:09 PM

Now that you are $100 ahead she deserves a nice restaurant Dinner. Red Lobster is good.:yes:

Glad you found it and fixed the problem.

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