My furnace smells like gas when I turn it on

Natural gas is dangerous to inhale, and it’s also highly flammable, which means if your smelling it you’ll want to take safety precautions immediately.

We’ll walk you through what to do.

First of all, let’s determine where you notice the smell:

  • Do you notice a faint gas smell near your furnace (the indoor unit) when it’s turned on? You probably don’t need to worry. It’s normal for your gas furnace to occasionally emit small amounts of gas when it’s cycling on. But if you notice this faint smell persistently, or it gets stronger, you may need a furnace repair.
  • Do you notice a strong, rotten egg smell odor coming from your air vents when you turn on your furnace? If so, it may mean your furnace has a gas leak, which is dangerous for you and your family.

In this article we’ll explain…

  1. What to do if you smell gas coming from your furnace
  2. What causes gas to leak from your furnace

What to do if you smell gas coming from your furnace

If you’re smelling a strong gas odor from your furnace or air vents, follow these steps:

  1. Turn the furnace off via the thermostat.
  2. Open windows to ventilate the gas.
  3. Turn off the gas main to shut off the gas supply to your furnace.
  4. Stay out of the house so you don’t breathe in the gas fumes.
  5. Call your gas company to inspect your home and make sure the problem is with your  furnace and not the main gas line or gas appliances like a stove, fireplace or water heater.
  6. Contact a furnace professional to diagnose and repair any furnace problems.

What causes gas to leak from your furnace

If you smell a strong gas odor when your furnace is running, it’s likely because of these 3 issues:

  1. A cracked heat exchanger
  2. A back-drafting furnace
  3. A gas line or valve leak

Let’s go into more detail about each of these causes.

Cause #1: A cracked heat exchanger

Your furnace’s heat exchanger is the component that actually heats your home’s air. Here’s how your heat exchanger is involved in the heating process:

  1. Gas combusts then enters the heat exchanger.
  2. Cold air gets blown over the exterior of the heat exchanger, which heats the air.
  3. The heated air enters your home via the duct-work and registers (vents).
Where your heat exchanger and blower are located inside your furnace.

Several things can cause a heat exchanger to crack, but the most common issue is overheating. When a heat exchanger cracks, combustion gases (including natural gas and carbon monoxide) can mix with the heated air and enter your home’s air-stream.

Since a cracked heat exchanger is a safety risk to your family, you’ll want to get it fixed ASAP. Unfortunately, a cracked heat exchanger is an expensive problem—often it’s more cost-effective to replace your entire furnace than to replace your heat exchanger.

Cause #2: A back-drafting furnace

Back-drafting is when combustion gases from your furnace leak into the surrounding area (or even your home’s air) instead of exiting your home safely via the exhaust flue.

The most common causes of a back-drafting furnace are:

  • Disconnected, leaky or clogged flue
  • Running too many exhaust fans in an airtight house
  • Leaky ducts

You’ll need to have a furnace repair pro inspect your furnace and fix any issues that may be causing back-drafting.

Cause #3: A gas line or valve leak

Natural gas is delivered to your furnace via gas lines. The gas valve controls the flow of gas entering into your furnace from these lines.

If either one of those components have a leak, you’ll definitely notice a strong gas smell.

Common reasons a gas line or valve could be leaking include:

  • Age (leaks due to general wear and tear)
  • Improper installation
  • Accidental damage

Again, you’ll need a furnace professional to fix your gas lines or gas valve if they are leaking.

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