What Is That Burning Smell from the Furnace? How to Prevent Unpleasant Odors

As temperatures start to drop, homeowners make the switch to heating to ensure the home remains comfortable. However, the warm air can often be accompanied by a burning smell that quickly permeates your space.

However, there are ways to prevent this odor in the first place and other methods for handling unfamiliar odors, like a musty or electrical burning smell. Use this guide to discover how to prevent the furnace smell when you turn it on for the season, other mysterious furnace smells you might encounter, and when to call a professional.

Why Does the Furnace Smell When You First Turn it On?

In most cases, when a furnace smells after being turned on for the first time in the season, there’s no need to worry. The smell of burning dust should fade quickly. But, if the smell persists or you notice a different smell from the furnace, it could be due to a serious issue with the HVAC system. Consider these common reasons why the furnace may smell when you first turn it on.

• Burning dust smells are relatively common when first turning on the furnace for the season. This is typically due to a buildup of dust on the burners, heat exchanger, air ducts, and other heating unit components. Generally, this odor will fade quickly as the thin layer of dust is burned away.
• Musty odors coming from the furnace usually indicate that there is mold or mildew trapped inside the HVAC system. It may be in the heating unit, the ductwork, the furnace filter, or even the humidifier pad.
• Electrical burning smells are not normal and should be addressed as soon as possible. Sometimes the smell is due to a foreign object stuck in the ductwork or vents of the HVAC system, but on other occasions, this smell may be due to burned or frayed wiring, or can also be a result of the furnace overheating.
• Rotten egg scents produced by the furnace are a sign of a gas leak. Since natural gas is odorless, the gas companies add a sulfur-based chemical to give off the distinct odor of rotten eggs as a way of warning homeowners that there is gas in the vicinity. Gas is flammable, putting the home at risk of a fire or an explosion, and it’s also harmful if inhaled, so you should vacate the home immediately.
• Smoke smells can occur if the exhaust vent is blocked, forcing the combustion exhaust from the furnace to escape into the home. This smell may also be a result of flammable material inside the ductwork, near heating vents, or too close to the furnace.
• Animal odors are often due to pets. Pet hair and dander are taken into the ducts through the cold air return, where they will remain in the ducts. Every time the furnace turns on, it recirculates pet hair and smells, until you have your ducts professionally cleaned. It’s also not unheard of for a pest to enter the ductwork of the home, where droppings and hair will produce a foul odor when the furnace is turned on.
• Oil smells can indicate that the filter on the furnace is clogged. This is typically due to a problem with the filter, which can be easily resolved by an experienced DIYer or a professional HVAC tech.
• Sewage stenches are powerful and off-putting, but can also help identify when there is a problem with the home sewage system. If the furnace starts to smell like sewage when it is first turned on then there may be a broken wastewater line or an open sewer line nearby that will need to be located and repaired.
• Chemical smells from the furnace should not be ignored. This could indicate that the heat exchanger is broken or not operating properly, which may lead to the release of odorless, invisible carbon monoxide gas; a known risk of gas-based heating systems.

How to Prevent the Furnace Smell When You Turn It On

Burning Dust Smell

While the burning dust smell that comes from the furnace when it is first turned on is relatively harmless, that doesn’t mean you need to just deal with it each year. Instead, you can have the furnace and evaporator coil cleaned by a professional before the cold season begins.

DIYers with enough experience and confidence can clean the evaporator coil in a furnace on their own by turning off the furnace, removing the access panel, then gently cleaning the coil with a soft cloth or brush, while being careful to avoid damaging the fins.

Wipe down the condensate pan and the surrounding areas, then replace the panel and turn the furnace back on. You should also take this opportunity to replace the air filter, where dust might be trapped from the previous season. If the burning dust smell persists, then it’s recommended to turn off the furnace and contact an HVAC professional to inspect and repair the HVAC system.

Musty Odors

Musty odors are typically produced by mold and mildew. In some cases, the mold or mildew is attached to dust or other debris, so it burns off relatively quickly, but if the musty odor doesn’t go away, then you should start by replacing or cleaning the air filters. Similarly, you can replace or wash the humidifier pad using a solution of water and vinegar to kill any mold or mildew. However, if the musty odor still doesn’t go away, then it’s a good idea to call an HVAC expert to professionally clean and inspect your system.

Electrical Burning Smell

If the furnace is giving off an electrical burning smell, start by taking out the registers and visually inspecting the ventilation ducts for any foreign objects. Remove any objects you find in the ducts, but if you don’t come across anything stuck in the ductwork the issue could be more serious. Turn off the unit and contact an experienced HVAC technician to inspect the furnace for burned or frayed wiring. The HVAC tech will also be able to determine if there is a different problem, like a worn bearing or an overheating blower motor.

Rotten Egg Scent

The smell of rotten eggs should never be dismissed. Since natural gas does not have an odor, the gas is scented to make gas leaks easier to detect. If the furnace is producing a rotten egg smell, do not try to locate the leak yourself. Leave the home and contact the gas company to locate and repair the leak. Do not return to the house until you have been told it’s safe to do so.

Smoke Smell

The furnace should not be making a smoke smell, since the exhaust from the furnace should escape through the exhaust vent. If the smell is relatively faint, then it may be a partial blockage. Turn off the furnace, open some windows for ventilation, and contact an HVAC professional. However, if the smell is powerful or seems to appear suddenly, then it’s best to turn off the furnace and leave the home until the technician can arrive to determine the source of the smell.

Animal Odors

Animal odors can be due to a couple of different sources. If you have pets, then spend a little time checking the registers in the home to see if they left a mess on, in, or nearby the vents. The smell of pets can also be as simple as recirculating pet hair and dander that gets drawn into the system and blown out when the furnace is on. If you don’t have pets or the registers appear to be clean, then the smell may be due to a pest trapped in the ductwork of the home. To get rid of the scent of burned hair and droppings, you should have the HVAC system professionally cleaned and contact an exterminator to eliminate the pests.

Oil Smell

Generally, you can get rid of an oil smell produced by the furnace by simply replacing the filter. However, in some cases, replacing the filter won’t have any impact, in which case, you should contact an HVAC professional to inspect the system and make any necessary repairs.

Sewage Stench

A furnace is not connected to the waste disposal system for the home, so if the furnace produces a sewage stench when you first turn it on, this is a sign that there may be a broken wastewater line or an open sewer line nearby that will need to be located and repaired. Some DIYers may be able to locate and repair the issues, though in most cases you will need to contact a plumber to assess the situation and fix the problem.

Chemical Smell

Chemical scents can be produced when the heat exchanger is not properly cycling heat from the combustion chamber into the plenum. If the issue is not fixed, carbon monoxide fumes can also be released, so if you smell chemicals, it’s recommended to turn off the furnace immediately, open some windows, and contact an HVAC professional. It’s also a good idea to leave the home until the HVAC technician can verify that it is safe to return.