Fan Doesn’t Run
FAN DOESN’T RUN - The furnace fires, but the blower does not start.
After the burners have ignited, the next step in the heating cycle is starting of the blower motor. This will be done either by a heat-activated switch or internal timing of a control board. In either instance, the blower motor should start within 30 - 90 seconds after burner ignition.
If the fan motor does not start, the furnace will overheat and the high-temperature limit switch should cause the burners to shut-off.
One diagnostic step that can be taken is to put the Fan switch to the “On” position at the thermostat. If the fan operates, this would indicate that the problem is most likely something other than the fan motor itself. It may be necessary to leave the fan in the "On" position so the heat can continue to operate until the problem is resolved.
POSSIBLE PROBLEM & DESCRIPTION
Bad Blower Motor - Qualified Technician Recommended
If the blower motor does not start (no air blowing out of the registers), it could be the result of a bad blower motor.
A failed blower motor will often be accompanied by a bad odor (like burning plastic or rubber).
If the fan does not operate automatically after the burners have ignited, or after turning the thermostat fan switch to the "On" position, this may indicate either a bad blower motor or a bad capacitor (below).
Bad Capacitor - Qualified Technician Recommended
Most direct-drive PSC fan motors require a capacitor which provides extra power for starting.
A fan motor with a bad capacitor will typically attempt to start but will fail while making a louder-than-normal humming noise.
After several seconds of humming, the motor will typically overheat and shut-off and then retry after about 30-60 seconds of cooling.
Sometimes a motor will still be able to start (with difficulty) with a bad capacitor.
A fan motor that runs normally after helping to start by turning by hand usually indicates a bad capacitor.
Heat-Activated Fan Switch - Qualified Technician Recommended
Some older gas furnaces use a heat-activated switch to energize the blower motor after the burners ignite and the heat chamber warms.
If the blower starts by turning the thermostat fan switch to the "On" position, but does not come on automatically after the burners have ignited, this may indicate a faulty heat-activated switch.
Control Board - Qualified Technician Recommended
Most newer gas furnaces use a computerized control board to activate the blower motor during the heating cycle. The furnace control board should start the blower motor within 1 minute after igniting the burners.
If the blower starts by turning the thermostat fan switch to the "On" position, but does not come on automatically after the burners have ignited, this may indicate a faulty control board.
Some older gas furnaces use a belt-drive blower motor.
If the belt becomes extremely loose or breaks, the blower wheel will not turn even though the motor is operating.