Trevor Boicey
> Am I missing something?
>
> My fan timing is controlled by my thermostat, as in
> "the digital control thing that lives upstairs". Lots
> of settings in it to compensate for various furnace
> types and personal preferences.
You are only missing a little bit.
The wall thermostat controls the temperature that the furnace is trying
to maintain. The ones that are in the furnace are the ones that control
the operations of the furnace itself. When the wall thermostat tells
the furnace to turn on, the gas (in the case of a gas furnace) starts
and begins heating the furnace. The blower motor doesn't start at this
time, since it's still cold in there, and will be for a while. When it
warms up sufficiently, a thermostat in the furnace senses that, and
turns on the power to the blower. The gas continues to burn, and the
blower pushes it through the ducting.
When the wall thermostat reaches its set point, it tells the furnace to
stop. The furnace then turns off the gas, but the insides of the
furnace is still hot. You don't want to waste that heat, and leaving it
in there is not good for things - so, the blower keeps running. Warm
air keeps coming out of the ducts, and the inside of the furnace cools
down. At a point when it's cool enough, the thermostat in the furnace
shuts off the power to the blower, and it stop blowing.
The trick is to have the internal thermostat set so that it doesn't turn
on until it's hot enough that cold air isn't blown out of the ducts, and
to not turn off until you've gotten all the warm air that you can get...
My fancy wall thermostat even has a setting on it to tell my fancy (but
cheap) high efficiency furnace how long extra to keep blowing the air -
that way, I can even get more of the warm air out of it...
Tim Mullen
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