I have an old gas furnace (circa 1981) being controlled by an electronic
I've added a relay that is energized when the thermostat calls for
heat. This relay is switching the fan contacts at the thermostat so
that the fan comes on the instant the thermostat calls for heat. I've
done this as a sort of safety feature, so I'm not relying only on the
furnace's internal bimetalic thermostat to turn on the fan.
Something else I was thinking of doing was putting one of these in
series with the gas valve relay control:
That seems to be normally used only to turn on the humidifier motor when
the furnace is actually running. Which is strange, since it would be
simpler to power the humidifier transformer from the furnace fan power
supply, which would insure the same result (that the humidifier motor
runs only when the furnace fan does).
I don't see why that air pressure switch couldn't be placed in series
with the gas valve wiring, so that the gas comes on only when the
furnace fan is started and develops a positive pressure in the plenum.
So the way this all would work would be:
1) Wall thermostat calls for heat, closes relay that energizes
gas valve. This also energizes new relay that starts fan.
2) Gas valve does not come on, because air pressure switch is
currently in the open position (no plenum pressure).
3) After a few seconds of fan operation, pressure builds up
in the plenum, closing the air pressure switch, which then
allows the gas valve to open and flame-rollout happens,
furnace generates heat, etc.
4) Wall thermostat at some point tells the furnace to turn
off by de-energizing gas relay. This shuts down the flame
and the relay that turns on the fan. But the fan will
still be running, because the furnace thermostat will still
be calling for the fan to operate until it reaches cool-down