Furnace Exhaust Smells Like Gas? : Easy Fix 2023

NOTE: If your furnace exhaust smells like gas strongly, or if you hear a hissing, then turn off your furnace and leave the house immediately with everyone. You should then call your local gas station or 911. Do not turn on any switches, appliances, or light any matches. 

How to Decide if it is a Gas Leak?


How to Decide if it is a Gas Leak?

Gas leaks are hazardous due to the possibility of carbon monoxide poisoning and the risk of explosion or fires. Consider the following questions, and if you answer YES to any of them, you should suspect a gas leak. Turn off your furnace and evacuate as soon as possible. Open the windows before you test this.

  • Is the gas smell coming from the air vents? 
  • Is the scent constantly there?
  • Can you hear a hissing or whistling sound near the furnace or another gas appliance? 
  • Is your CO2 detector going off?

It is very important to keep your doors and windows open in any case of a suspected gas leak. This is to prevent any gas accumulation inside the house, which might lead to immediate explosions. Turning off the gas is also important to prevent any further leaks. Do not enter the house again until the professionals have taken care of the issue and confirmed that the danger is eliminated.

Is It Normal for My Furnace to Smell Like Gas?

Is It Normal for My Furnace to Smell Like Gas?

It makes sense if you have a furnace running on gas as it would produce a faint gas smell. Having an electric furnace and if your smells like gas, then you should know that there is something else wrong in the system. You can start checking with the stove first.

The natural gas furnace smells like gas when running, just like the propane heaters. Even though both these gases are naturally odorless, a chemical odor is added to them by gas companies. This is done so that it is possible to detect gas leaks. Even if you are not familiar with the smell of propane or natural gas, it is recognizable when you smell it, as it has a rotten egg smell. Efficiency is the main difference between propane and natural gas as propane delivers more BTUs of energy per cubic foot. Natural gas delivers less than half of that per cubic foot.

What to Check?

What to Check when your furnace exhaust smells like gas

Here are some situations where your furnace exhaust smells like gas but might not be serious. However, if your situation is not relevant to any of the situations here, you should get the heaters checked by a professional as soon as possible.

  • When the gas from your furnace exhaust pipe blows back in the windows, you would smell the gas. Even though the gas is safely removed from the house, the wind brings it back inside. You can check this by paying attention to identify whether the smell always comes from the same direction. If so, you can close a window to overcome this issue.
  • The furnace smells like gas on startup the first time it is used in the winter. This is because dust and debris get built up in the furnace as it sits idle for months. The first time you turn it on, you can smell the gas odor as it clears out the cobwebs and burns up the dust.
  • There is also a possibility that the furnace smells like gas on startup every time it starts on each cycle. It would be a faint odor that goes out after some time. This might not be a major issue, but it’s a good idea to get your furnace checked to be sure.

How to Prevent a Gas Leak?

How to Prevent a Gas Leak?

There can be situations of gas leaks beyond your control. However, you can minimize the risk of a hazard caused by a gas leak by taking precautionary actions.

  • Buy a carbon monoxide detector – Since carbon monoxide is produced when fuel is burned, it would be the same when your furnace runs. As it is an odorless gas, it might be difficult to identify it at once. However, it is just as deadly as natural or propane gases. So it is a good precautionary measure to install a carbon monoxide detector to identify any possible gas leaks.
  • Maintain your furnace – You can keep your furnace healthy by having an annual checkup and making the relevant repairs as they’re needed. It would also be beneficial for the health of your family and to avoid any unexpected hazards.

Amount of Money Wasted Through Gas Leaks

In addition to safety issues, your money is also being wasted due to gas leaks. Most people use natural gas due to low costs than fuel oil or propane.

Fuel TypeCost per MMBTU
Natural Gas$19.62
Fuel Oil$34.60
Propane $41.87

Natural gas would mostly be around half of the price of propane in most areas. In some regions, it would even be six times cheaper than propane. However, propane has more heating ability than natural and is hence preferred by some people. If you are using propane to light up your furnace, then you might need to know the Cost to fill a 20 lb Propane Tank.

Common Furnace Odors and Causes

Common Furnace Odors and Causes

Some other things in your house could also smell like gas. It is better to know about them, to confirm the real situation at home.

  • Compost/ Trash – Sometimes, if your compost pile or trash can is located close to the window by your furnace, you would smell it and mistakenly consider it as a gas leak. 
  • Propane Grill – This would once again occur if your propane grill is near a window close to your furnace. So, there is a possibility that the propane tank has a leak or wasn’t closed properly after last use.
  • Burnt Dust – Your furnace would get accumulated with layers of dust and debris when it is not being used for some time. Once you light it up, it is normal that the furnace smells like gas on startup. This is due to the burning of dust and would eventually go away.
  • Musty Odors – If your furnace shares the same handle as the split air conditioning system, then you would experience that the furnace exhaust smells like gas. As musty odors occur due to dark and damp environments, clogged and unchanged air filters can cause your furnace to smell musty. This would not go away with time but increase more. So, it is better to get HVAC cleaning done on your furnace and its ductwork.

Other Smells to be Aware of

Other Smells to be Aware of

Gas smells are not the only odors that you need to be aware of. There may be similar situations of issues with furnaces and heaters that you need to be careful about. 

  • Oil Odors – Spilled oil when refueling your furnace would cause the smell to linger for days before dissipating completely. Improperly burned oil due to dirty or malfunctioning burners can also cause odors similar to gas leaks.
  • Noxious Fumes – Chimney blockages can also cause redirection of noxious exhaust odors back into your home. So if you have a chimney, make sure that there are no obstructions built inside them.
  • Metallic Odors – Furnace components made entirely from metal also cause strange metallic odors as they overheat. Using your furnace under this condition could damage the components and cause electrical shocks. Therefore get your HVAC technician to check the issues and repair them.
  • Burning Plastic – If you smell burning plastic in your furnace, it would indicate the failure of a plastic component in your furnace. This can also be due to heat-damaged circuit boards. So get your HVAC technician to inspect this and diagnose the cause.

Frequently Asked Questions

Frequently Asked Questions about Furnace exhaust

Why are gas leaks dangerous?

Gas leaks are extremely dangerous and could even be deadly. This is because gas leaks can cause fires and explosions. The harmful gas carbon monoxide might also be released during gas leaks or burning. Inhale of such poisonous gas can also result in intoxication and even death.

Is carbon monoxide harmful?

Carbon monoxide is a very poisonous gas known as the “Silent Killer.” Since it is colorless and odorless, you would not be able to identify its presence. However, it is easily absorbed into the lungs than oxygen, and the blood will get saturated with it instead of oxygen. A severe headache and drowsiness are the symptoms of being exposed to this gas.

Can gas leaks cause explosions?

Gas leaks can cause both fire and explosions. This is why you need to be very attentive if your furnace exhaust smells like gas. Since propane is denser than air, it would collect down near the floor and cause an explosion when it has gathered a large enough quantity. Since natural gas is lighter than air, it would float over in the air inside if the ventilation is not good enough. This also creates the potential for both fire and explosion when the gas is built up.


If your furnace exhaust smells like gas, you should take this situation seriously and get the problem resolved immediately. Piping installments should be done correctly to avoid damage from most situations that can occur. It is important to have the heaters and piping inspected from to time, with regular maintenance done to prevent any possible causes of gas leaks. Following proper precautionary measures and regular inspection and maintenance before your heater smells like a gas can prevent furnace problems, gas leaks, and fire hazards from occurring in the future.

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