Blower constantly runs

Brad Bockin

I have a forced air gas furnace. When I turn it on the blower constantly runs even though the furnace isn't burning any gas. How long is the blower supposed to keep blowing air even though its not burning any gas?

Brad White

Depends on how it was set up.

Your thermostat may have a switch on it for "off-fan-auto".

I would start there. "Off" is "off" (imagine that!), "on" is for constant operation. "Auto" is to allow cycling with the burner.

Typically this latter mode has the boiler fire, the plenum comes up to temperature (120-150F adjustable) and the fan starts. When the burner stops the fan runs until the plenum thermostat sensed the temperature has fallen below setpoint. This prevents moving cool air and saves fan HP.

The "On" mode of constant circulation is sometimes selected for better comfort. Constant circulation makes for even temperatures. It is also used when humidification or filtration is desired as an ongoing feature.

If not the thermostat, it could be the fan relay is stuck or jumpered out. More ideas from field practitioners I am sure, but those are my thoughts.

Brad

Hvacman

Or it's a

Misadjusted fan/ limit control, or the summer switch on the fan/limit is in the "on" position. And, some furnaces run the fan if the high limit circut is open. How old is this beast?