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Question DetailsAsked on 10/29/2017

Gas furn, pilot light on, burners come on when i set therm to heat, but go out when i turn on fan at the thermostat

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1 Answer


Is this a trick question ? Unless you have a really fancy thermostat, it usually has 2or 3 function settings on a slide switch - HEAT, A/C or AIR, and sometimes FAN - or similar titles.

HEAT obviously tells the thermostat to activate the furnace at the selected temperatures at the thermostat - ditto to the A/C setting telling the air conditioner it is the device in use. That switch, by having individual selection for each and only one at a time keeps both from being in use at one time (and fighting with each other).

The FAN setting is just to run the central air fan - to provide a bit of circulation, to air out odors in the house (like cooking odors), to distribute warmer or cooler air in certain rooms into the entire house, or to filter out dust like when dusting or vacuuming. The fan runs automatically as needed with the HEAT and A/C settings - putting it on FAN setting just turns on the central air fan only and shuts off the heater and the A/C.

In houses with whole-house ventilation fan, sometimes the FAN setting is connected to that fan instead, not the central air system.

If you have a fancy totally digital whole-house environment system like a NEST or such, you need to read the instructions - because some of them do allow continuous fan run with intermittent furnace or A/C operation as needed to keep the temperature in the range specified. And my sympathies. But in that case it would not have a simple HEAT, A/C, FAN switch - those would be autommatically selected by the system based on your selected operating parameters, or hve them as a push-button or remote-controlled function option. [And if wired wrong, I have seen these type units running fan, furnace, and A/C all at once - amusing to see them fight but more than a bit energy efficient and hard on the equipment].

Answered 2 years ago by LCD

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