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gadha 02-13-2010 08:46 AM

Gas Heater -- Fan with Call for Heat??
I recently replaced our thermostat. We have a gas heater and a regular air conditionner.

The new thermostat gives me the option of turning on fan at same time as call for heat (apparently used by electric heater), or not turning on fan at same time as call for heat (apparently used by gas furnace).

From what I could tell, the existing thermostat was not installed at the time the system was installed and might have been wired improperly. (It enabled fan with call for heat)

Why is there the distinction? Does enabling fan during the initial call for heat disrupt the ability of the gas burner to ignite?

To me, it seems a bit scary to have the gas furnace on (even for 1 minute) without the fan blowing air through it. [Wouldn't it get extremely hot?]

For now, I have the thermostat set to enable fan at the same time as call for heat... I have noticed that about 30-60 seconds after I hear the gas furnace turn on (after igniting), I hear a momentary variation in the fan speed which makes me think that the furnace controller is internally switching some relays.

(I have verified that after some time, the furnace does automatically turn the fan on while heating even if the thermostat does not do so. I've just been a bit chicken to find out exactly when that happens)

So: To fan or not to fan with call for heat?

Whether it is nobler to maintain ordinary temperatures inside the furnance or nobler to test the limits of its engineering....

Marty S. 02-13-2010 08:56 AM

Gas furnaces bring on the fan after the burners ignite to prewarm the heat exchanger. Too cool and the moisture from combustion condenses and can rust the heat exchanger out prematurely. Best to set it up the way it's designed to operate.

AllanJ 02-16-2010 02:03 PM

Also, not turning the fan on until the heat exchanger is good and hot reduces electricity consumption (running the fan).

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