October 18, 2021

6 Common Furnace Smells and What They Mean

If your furnace stinks while heating, it’s trying to tell you something’s amiss. To address this, we’ve created a list of the top six most common furnace smells, what triggers them and how you can correct them.

  • Burning smell from furnace—dusty furnace
  • Furnace smells like burning plastic—electrical problem, overheated motor or cracked heat exchanger.
  • Gas smell from furnace—gas leak.
  • Furnace smells like car exhaust—blocked exhaust pipe.
  • Furnace smells like sewer—dry sewer trap or sewer leak.
  • Furnace smells musty—dirty ductwork.

You’ll also learn which problems you can fix yourself, and which ones require an HVAC technician. When you need a technician, Home Heating & Cooling is here to help you.

1. Burning Smell from Furnace

What Causes It: Dusty Furnace

If your furnace smells like burning, it’s probably happening when you switch on your furnace for the first time in the autumn. Like other appliances in your house, your furnace gets dirty over time. This dust burns off when you switch on the heat, causing the burning dust smell from your furnace. This smell is normal and isn’t something to be worried about.

2. Furnace Smells Like Burning Plastic

What Causes It: Electrical Issue

If your furnace smells like burning plastic or burning rubber, it might have an electrical problem. This smell of burning can be triggered by melting plastic wire casing, which is a safety hazard. If you’re noticing this odor, switch off your heating system right away and call a local HVAC contractor for furnace repair.

What Causes It: Overheated Motor or Cracked Heat Exchanger

An electrical burning smell could also mean your furnace motor is overheated or the heat exchanger is cracked. The heat exchanger properly exhausts carbon monoxide that’s normally produced during the combustion process.

If it cracks, carbon monoxide can get into your home. This invisible gas is deadly in large amounts. So, it’s important to have an HVAC pro do annual furnace maintenance and have functioning carbon monoxide detectors.

Other signs of a cracked heat exchanger include:

  • Soot on or inside the furnace.
  • Yellow burner flame, instead of blue.
  • Chemical odor from your furnace, such as formaldehyde.
  • Moisture near your furnace, assuming that the condensate drain isn’t plugged.
  • Feeling like you have the flu when you’re home but feeling better while away.

3. Gas Smell from Furnace

It’s typical for your heating system to let off a small amount of gas when it turns on, so you might experience a gas smell from your furnace. But it’s not normal to be smelling natural gas or rotten eggs throughout your home on a day-to-day basis.

What Causes It: Gas Leak

If your furnace smells like gas continuously, turn it off. If you know where your gas supply shutoff is, switch that off too. Then contact the gas company and Home Heating & Cooling for help, if you're in or near O'Fallon, IL.

4. Furnace Smells Like Car Exhaust

If your furnace smells like car exhaust or smoke, shut it off as soon as possible and let fresh air into your home. Your furnace uses an exhaust pipe to release combustion gases. If the pipe is blocked, the gases will flow back into your house.

What Causes It: Blocked Exhaust Pipe

If your heating system is high efficiency, it will have a white PVAC exhaust pipe on the side of your house. This pipe can get clogged by snow or birds, so be sure to regularly check that it’s clear.

5. Furnace Smells Like Sewer

If your furnace smells like sewer, or rotten eggs, you’ll want to ensure you don’t have a gas leak. If you don’t, the issue could be with your sewer lines and not your furnace. This is particularly true if it’s been very dry recently.

What Causes It: Dry Sewer Trap or Sewer Leak

If you think your sewer trap is dried out, you can try dumping some water down the floor drain in your basement. If that doesn’t fix the problem, you might have a sewer leak, so you’ll want to call a company that does sewer line repair.

6. Furnace Smells Musty

Sometimes you'll notice a bad smell from furnace exhaust. If your furnace smells musty, mold has likely been growing in it for some time.

What Causes It: Dirty Ductwork

Your furnace utilizes air ducts to spread warm air across your house. Mold can develop in the ductwork as time passes, especially if your home is too humid. You can eliminate the mold by having a Home Heating & Cooling HVAC technician do air duct cleaning.

You can keep your home’s humidity in check by:

  • Installing a smart thermostat, which will automatically tell you humidity levels. We recommend keeping humidity below 60% in the summer.
  • Using a dehumidifier.
  • Running your air conditioner regularly in the summer.
  • Frequently changing your air filter. We recommend monthly for flat filters and every three months for pleated filters.

Let Our Experts Handle Your Furnace Repair in O’Fallon

Troubleshooting your heating and cooling system on your own is hard. Your comfort is our top priority at Home Heating & Cooling, so our professionals are here to help you when you need us. When you need furnace repair in O’Fallon that you can depend on, contact us at 618-416-0111.

Communities We Serve

We serve the entire St. Louis area, including these communities and surrounding areas.
Home Heating & Cooling
1450 Frontage Road, O'Fallon, IL 62269 USA
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