Got an old carrier downdraft gas furnace, pilot light style. Runs normally
but at the end of the heat cycle after the fan shuts down then the fan will
very briefly try to startup a couple times. Everything works fine
otherwise. It uses a timer heat fan relay circuit for cool down. I'm
guessing that is where the problem is but it seems like a weird symptom.
Any ideas? TIA
The blower is controlled by a temperature sensor near the heat exchanger
in the furnace. The reason for this temperature sensor is to let the
furnace temperature rise before starting the blower so that cold air
isn't blown into the living area. Similarly, when the thermostat
signals the furnace to shut off the heat exchanger is still hot so the
temperature sensor keeps the blower running for a few minutes to extract
the remaining heat rather than loosing it to your basement or chimney.
It also prolongs the life of the heat exchanger by not letting it
overheat. The reason your blower is cycling after the furnace turns off
is that the temperature sensor has failed (or more likely) needs to be
adjusted so that the blower stays on a bit longer to extract more heat
from the heat exchanger.
Usually there is bimetal heat sensor which controls fan on, fan off
and safety cut off. Fan comes on and goes off after a certain delay
to prevent blowing cold air and to extract residual heat from heat
exchanger after flame is off. Some sensors are adjustable with dial and
set screws some are not. Maybe the sensor is just tired.
You didnt post enough information to know what your unit uses as a temp
control, but if its old enough, it will use a fan limit switch that is
adjustable, and since they dont get out of adjustment by themselves, it may
be telling you something.
If its got a true ICB on the unit, the board may be failing, as the ONLY
sensor for that setup is an overheat sensor, and if THAT is the case, then
besides board failure, it could be an airflow issue...you havent gone to
those 3M style filters recently have you?
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