ODOR – On a unusual smell coming from the furnace or ductwork.
POSSIBLE PROBLEM & DESCRIPTION
It is normal to have a burning smell (and sometimes even a small amount of smoke) from the furnace the first time that it comes on in cold weather.
This is the result of heating the dust that has attached to the heat chamber over the summer months. The smell is very similar to that of a toaster oven.
Clogged Air Filter
A dirty or clogged air filter will prevent the furnace from delivering the proper amount of heated air to the space and will often cause the furnace to cycle on and off on its over-temperature device. The over-heating of the furnace may also produce a slight burning odor from the furnace.
If these symptoms are identified, try removing or changing the filter to see if the conditions improve.
Return Ductwork Issues
If the return air ductwork is run in a crawlspace, any openings in the ductwork can draw-in musty or moldy smells and introduce them into the living space when the fan is operating.
Bad Blower Motor - Qualified Technician Recommended
After the burners have ignited and proper ignition has been detected by the ignition control board, 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 about 1 minute after ignition. If the blower motor does not start (no air blowing out of the registers), it can be the result of the activating switch or the furnace control board, but often the problem is a bad blower motor.
If the blower motor does not start, the furnace may produce a burning smell and sometimes smoke.
One diagnostic step that can be taken is to turn the fan 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. If necessary, leave the fan in the "On" position so the heat can continue to operate (just be aware that the fan will run continuous).