Monday, November 7, 2011

The mystery of the furnace fan that won't shut off

When winter's chill set into our house, my wife and I were glad to hear the furnace click on and start pumping warm air into our home.

But something was amiss. After about 15 minutes or so, the air coming out the vents was no longer warm. The indoor temperature had not reached the thermostat's setting, but all the furnace was doing was blowing out room-temperature air. No amount of button-pushing on the thermostat even the "off" setting would stop the fan from blowing.

The blower eventually did stop, but not before it had run for more than hour. When the furnace started again, the same cycle repeated itself warm air at first, then unheated air being blown out for over an hour.

Visions of an expensive furnace bill danced in my head as I tried to figure this out. At first, I thought the problem might be the thermostat, since it didn't seem to respond. But once the fan had shut off, the thermostat acted normally, allowing us to turn on the heat.

Checking online, I found a suggestion that the problem might be a "stuck fan switch." To investigate this, I climbed into our attic and opened the front panel on the furnace. Immediately, the blower shut off.  I thought I was onto something, but later realized I had just tripped a safety mechanism that turns the fan off when the panel is opened.  Further, I had no clue where this "stuck fan switch" might be.

Digging deeper online, I learned that when furnaces overheat internally they shut off the burners while the blower keeps running. Aha! This sounded right. Why might the furnace be overheating?  One reason would be a dirty air filter.

Now here was a repair I could handle! I removed the old, dirty filter, bought a new one and put it in. And the furnace now works perfectly.  Grand total for this repair: $10.  Boy, would I have felt foolish calling in a repairman simply to change the filter.


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