ODOR – On a unusual smell coming from the air handler, furnace or ductwork.
POSSIBLE PROBLEM & DESCRIPTION
It is normal to have a burning smell (and sometimes even a small amount of smoke) from an electric furnace or air handler the first time that it comes on in cold weather.
This is the result of heating the dust that has attached to the heaters over the summer months. The smell is very similar to that of a toaster oven.
Sometimes the smell will not show-up until the coldest day of the year when all stages of heat are required.
Clogged Air Filter
A dirty or clogged air filter will prevent the air handler from delivering the proper amount of heated air to the space and will often cause heaters to cycle on and off on their over-temperature device. The over-heating of the heaters may also produce a slight burning odor from the air handler (like burnt toast).
If these symptoms are identified, try removing or changing the filter to see if the conditions improve.
Open Return Air Ducts
The blower on a heat pump should pull air from the conditioned space (through the return air duct system), heat it, then deliver it back to the space (through the supply air duct system).
Open return air ductwork, especially in a crawlspace may cause bad odors to be drawn-in and blown into the conditioned space.
A test during cold weather conditions to see if your return duct system is open:
- At the thermostat, turn the system switch "Off" and the fan switch "On".
- Check the temperature of the air coming out of your supply registers. In ideal conditions, the air temperature should be the same as the temperature in the conditioned space. If the temperature is much lower than the conditioned space temperature, this would indicate openings in the return duct system.
This is a rare, but actual, condition where the indoor heat pump coil emits a sour odor through the duct system similar to the smell of old, wet gym-socks. The smell will manifest itself most often in heat pumps that switch from cooling to heating mode or when switching into defrost mode. Nationwide studies suggest the number of homes that report this condition range from less than 0.5%, in dryer climates to as high as 2% in more humid areas.
There has been no specific cure for this problem, but in all cases where we have observed the symptoms, the smell has shown-up early in the heating season and goes away after several occurrences.
In looking for similarities, we have noticed that in each instance a new heat pump has been installed on existing, old, metal ductwork. Duct-cleaning appears to eliminate the problem, but it is difficult to say with certainty as the smell tends to go away on its own with time.
Bad Blower Motor - Qualified Technician Recommended
If a blower motor fails on an air handler the electric heaters will continue to operate until the over-temperature safety device is activated. This may cause a slight burning smell (similar to a burnt toast).
Some motor failures will actually cause the electrical windings in the motor to burn. This will generally be associated with a burning smell from the ductwork, similar to burning rubber.
If the blower motor hums but does not start, it is most likely a bad motor or a bad capacitor (below).
If a heat pump system is attempting to operate without the blower motor running (no air from the supply registers), you should turn the system "Off" and contact a qualified technician to diagnose.