gas furnace blower turns on/off(burner stays on while this happens)


Old 01-06-08, 09:52 AM
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gas furnace blower turns on/off(burner stays on while this happens)

my dad's old williamson gas furnace blower motor turns on then shuts off for about 20 seconds and then turns back on.(this all happens while the burner stays on).This happens about 50% of the time the furnace turns on.
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Old 01-06-08, 10:29 AM
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Are you talking about a Williams brand of wall furnace, by chance?

Gas furnaces have a "fan switch" which is a temerature sensitive switch like a thermostat that turns the fan on after the heat exchanger is hot and keeps it running after the burner is off until the heat exchanger has cooled off, extracting most of the heat, but still avoiding circulating cool air that might create drafts.

Usually such fan switches will stay hot enough once they start to avoid the kind of fan shutoffs you are noticing.

I take it that once the fan starts the second time it stays on then --- and appears to rund at full speed? A fan motor that is starved for oiling might be slow to start and tend to shut off. Checking to see if the fan can be oiled might be worthwhile in any case.

That kind of fan shutoff isn't likely to cause any harm. If it's a nuisance, you would probably need to have a repairman check it out, there could be several different things that could cause such a problem.
Old 01-06-08, 12:43 PM
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Fan switch faulty (if not the kind you can set to your liking). With some fan switches you can set the temp so this does not happen.

Flame has gotten smaller than it used to be, so now the heat exchanger is cooler, since the blower speed did not slow accordingly.

Blower motor rpm speed too great (as in soemone replaced motor and it is now running on higher speed)


Bottom line for the last 2 reasons is a temp drop is occuring in the furnace causing the blower to shut off when too cool, then comes back on again only after the fire has been running long enough with the fan off to build the heat back up in the furnace, to cause blower to run again, until once again it cools back down again insoide and the fan shuts back off again. Etc.
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