What kind of burning smell are you experiencing from your gas furnace?

• Does it smell like?

• Plastic Burning
  Musty Dirty Smell

In an ideal world your furnace would not convey a sense of smell at all. Usually the furnace is not what you are smelling, but something else entirely. This first one is actually from the furnace. Burning plastic smells in furnaces can be caused by the following:

• Transformer Burned Out
  Circuit Board Burned out
  Blower Motor Burning Out
  The Furnace Return Air Is Sucking in Air from the furnace cabinet or crawl space

Transformer Burned Out

If the transformer in the furnace, the part that takes line voltage of 120v-240v to 24 volts, burns out it lets a safety switch activate which turns the furnace’s main blower motor on. This is to keep the furnace cool. It also acts to distribute the very foul burning plastic smell that a transformer lets off throughout the home. Transformers burn out real easy. A slight short, a mis-wired thermostat or unbalanced power can cause a transformer to burn out and cause this problem.

Circuit Board Burned out

Circuit boards are notorious for burning out. There are lots of relays and resistors in them and any number of those relays and resistors can burn out causing a plastic burning smell. Often it is the back of the furnace circuit board, the part that you cannot see, which has suffered a short circuit and the heat from this short circuit caused the board itself to burn creating this smell. Often the furnace will exhibit intermittent firing and operating conditions as the circuit board short gets worse and worse. Once the power is burned through on the circuit board the 24 volt safety circuit in the furnace engages the blower motor to cool the furnace down and will not stop until the furnace is unplugged.

Blower Motor Burning Out

If the blower motor burns out it means that the windings have failed and have shorted to ground. This does not always trip the circuit breaker and occasionally the furnace will still operate for a length of time until the motor heats up too much and shuts down on thermal overload. This failure is usually caused by water from the air conditioning, water from a defective vent through the roof and a lack of maintenance. The lack of maintenance causes debris to build up on the fan blades creating a very heavy wheel for the motor to spin. This increased work that the motor has to perform causes it to consume more power than it is designed for and this translates into more heat which eventually burns the motor up.

The Furnace Return Air Is Sucking in Air from the furnace cabinet or crawl space

The vent on the furnace gets hot. The induced draft motor, the small motor near the top of the furnace on 80% AFUE furnaces and above, gets hot and they will let off smell if they get dirty. When the furnace return air ducting is not sealed properly a vacuum can exist where the furnace is and any smells around the furnace can be sucked into the home.

Musty Dirty Smell from the Furnace

We normally get lots of calls for musty dirty smelling furnaces just as soon as the yearly heating cycle starts. Those musty dirty smells are usually the result of debris buildup, mostly dead human skin, on the heat exchanger or firebox and the heat from the burners is cooling them. Yes, it’s gross an ye, it stinks. You should have your furnace cleaned and checked every year before the heating season starts.

The other cause of this musty dirty smell is mold and mildew buildup. This does occasionally happen in a furnace or evaporator coil, but usually it is the return air pulling air from a dirty location. The most common dirty location is under the home. Open return air ducts to crawl spaces and cavities outside the home is tantamount to a health disaster. Furnaces are not allowed to get their return air from any area that could possibly have objectionable odors. Yes, that is a code in the Uniform Mechanical Code book. There are very few places in your home that are dirtier than the under floor crawl space.