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Gas Smell from Furnace: Is it Normal?



John Abraham
Hey I'm John. I write about Manufacturing, Metalworking, CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com


Gas smell from furnace (Source: JM Heating & Air Conditioning)

If you detect any suspicious gas smell coming from your furnace, it’s essential to address the issue immediately to prevent potential fire hazards.

While it is essential to be aware of the potential issues, it is also possible that the gas smell coming from your furnace may not be a cause for alarm.

A gas smell from the furnace is normal for new furnace installations or if you turn on the furnace after a long period of inactivity. However, if the gas smell is persistent and accompanied by other sounds it could indicate a gas leak which is a potential hazard.

If you suspect a gas leak, turn off the main gas valve, open all windows and doors, and evacuate the building immediately. Call a professional to inspect and diagnose the furnace properly.

Also, avoid using any electrical switches near the vicinity where the gas leaked.

This article will explain reasons why a gas smell could be coming from your furnace, the signs of a gas leak, and how to prevent it.

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Gas Smell from the Furnace: Should it be a Concern?

When the Gas Smell Is Not a Concern:

  • If the smell comes from a gas furnace that has just been installed, it might not be a cause for concern. You may experience such a gas smell for the first few hours of operation.
  • If you turn on the furnace after a long period of inactivity, you may smell a burning odor similar to gas as the dust and debris accumulated in the furnace burns off.
  • If you smell gas, but there are no other signs of a gas leak, such as a hissing sound, it is likely not a cause for concern.

When the Gas Smell Is a Concern:

  • If the smell of gas is intense or is accompanied by a hissing sound, it is a sign of a severe gas leak and should be addressed immediately.
  • If the smell persists for a long duration or gets stronger, it is likely a sign of a serious gas leak.
  • If you are experiencing symptoms such as headaches or nausea after smelling gas, it is likely a sign of a severe gas leak.
  • If you are using an old heating system, it might have a pilot light on the furnace that uses a small amount of gas to stay lit. If the pilot light goes out, you’ll smell gas. It’s not a serious threat if you fix it shortly. But if not, over time, the gas can accumulate in closed spaces near the furnace, causing serious fire hazards.

Situations When Gas Smell from the Furnace is Normal

A gas smell is normal and not a matter of concern in the following cases.

Normal Ignition

It is normal to smell a faint, temporary, and localized odor of mercaptan when your gas furnace starts up, as it takes a few seconds for the burners to burn off the initial gas supply.

However, if the odor persists or is particularly strong, it could indicate a problem with your furnace, and it is important to diagnose and fix the issue.

Running your furnace after a long time

During the first few times you turn on your furnace in the fall or winter, it is common to smell a faint mercaptan odor.

This is because the heat from the furnace can cause accumulated dust in the furnace or ducts to burn off, and the burning dust can react with the mercaptan added to the natural gas.

To help minimize these early-season odors, you can have your furnace and ducts cleaned by a professional.

Additionally, it is a good idea to have a trusted technician perform the initial start-up of your furnace each season to ensure that it is functioning correctly.


Ductwork for a heating system
Ductwork for a heating system

The furnace exhaust system is responsible for safely releasing fumes from the furnace to the outside open environment.

If the exhaust system is not functioning correctly, it may release small amounts of unburned gas into your home, and you may be able to smell small traces of gas.

This is known as back-drafting and can be caused by various factors, including a malfunctioning exhaust system or a blockage in the exhaust pipe.

However, the faint gas smell of the exhaust is not a cause for concern as the exhaust pipes are used for releasing the used gases from the furnace.

But if the issue persists and affects the air quality, you’ll have to fix the exhaust system.

Defective Heat Exchanger

A heat exchanger
A heat exchanger (Source: Madares)

A cracked heat exchanger in your furnace could also cause a gas smell in your home. 

Heat exchangers can crack due to overheating, preventing the complete combustion of gases.

This results in hazardous carbon monoxide and unburned gas entering your home or office space.

Additionally, if your furnace generates a lot of soot, that may be another indication that your heat exchanger is damaged.

Other Possible Reasons for Gas Smell

While the smell of mercaptan is often associated with the gas used for home heating, there are other potential sources of the rotten egg smell. 

Garbage disposals and trash bins can be familiar sources of such odor. 

Compost bins, typically kept outdoors, can also contribute to the rotten egg smell near the home. 

Sometimes, a skunk may have sprayed outside the home, carrying the odor through an intake vent. 

However, it is more likely that the smell is coming from the furnace.

Signs of a Furnace Gas Leak: Potential Hazard

If you notice the following signs, it could indicate a furnace gas leak and that’s a potential hazard.

Persistent Smell

Natural gas is an odorless, tasteless, and colorless gas often used in furnaces.

Therefore, Federal law requires gas companies to add a harmless sulfur-based chemical called mercaptan to natural gas, making it easier to detect gas leaks.

Mercaptan has a strong and unpleasant smell that is often described as similar to rotten cabbage, garlic, eggs, or skunk spray.

If you notice a persistent sulfur smell in your home and cannot identify the source, the gas is likely leaking from the furnace or any gas lines.

The smell of mercaptan may be faint and only noticeable from the vent registers in some cases.

In other instances, if there is a significant leak, you might only be able to smell it next to the furnace.

Change in Flame Color

Change in flame color
Change in flame color

If the flames in your furnace appear flickering or yellow instead of a clear blue, it signifies incomplete combustion of gas, which causes the gas smell.

Strange Sounds 

Another sign of a gas leak is a roaring, whistling, or hissing sound coming from your furnace or gas lines.

If you cannot locate the source of the hissing sound, turn off your furnace and use a cloth soaked in soapy water to run along any exposed gas lines around your furnace and the heat exchanger.

If there is a gas leak, you will see bubbles form in the soapy water at the point where the gas is escaping.

Alarm from Carbon Monoxide Detector

A gas leak can often be accompanied by incomplete combustion, producing carbon dioxide and significant amounts of carbon monoxide.

If you’ve installed a carbon monoxide detector in your home, it will detect the carbon monoxide from the gas leak.

However, you cannot rely on this detector as it is not made to detect gas leaks. It’s only helpful if carbon monoxide is also a part of the released gas.

Dying Plants

If there is a gas leak in your home, it can cause houseplants to wither and die, so it is a good idea to pay attention to the health of your plants. 

Additionally, gas leaks may happen underground in the gas pipes and slowly find their way to your home, resulting in a yellowing of the greenery on your lawn.

Health Related Symptoms

Exposure to gas leaks can also cause health-related problems like nausea, headaches, fatigue, and irritation of the eyes and throat.

If the exposure to gas is more severe, it can cause dizziness, confusion, and in extreme cases, unconsciousness and death.

Moreover, if pets are in your home, it is essential to be aware that they may be more sensitive to the effects of gas than humans and may show symptoms more quickly. 

If you notice that your pets exhibit strange behavior, such as vomiting, trouble walking, or lethargy, it is crucial to take immediate action. 

This includes evacuating your home and contacting the gas company to have their technicians check your home for gas leaks.

How to Prevent a Gas Leak from the Furnace

Proper Installation and Maintenance of Furnace

It’s crucial to adhere to all manufacturer’s instructions and regional building regulations while installing a furnace.

Additionally, you should maintain the furnace according to the manufacturer’s recommendations, which may include replacing filters and cleaning the furnace regularly.

Regular Manual Assessment

Over time, furnaces can experience wear and tear, which can lead to gas leaks. 

If you notice any worn or damaged parts on your furnace, you must have them replaced whenever possible.

This includes things like burners, valves, and gas lines.

Have the furnace inspected and serviced annually

Having a professional inspect and service your furnace at least once a year is a good idea.

During this inspection, the technician will look for any issues that may cause gas leaks and make necessary repairs.

Install a Carbon Monoxide Detector

Carbon monoxide is an odorless and colorless gas that faulty gas appliances, including furnaces, can produce. 

If you have a carbon monoxide detector installed, it can raise the alarm in the presence of this gas, allowing you to evacuate the area and call for help.

Final Thoughts

In heating systems, a gas leak occurs when there is a problem with the gas supply to the furnace or the furnace itself, causing gas to escape from the system. 

This can be a severe issue, as gas leaks can lead to fires, explosions, and the release of toxic fumes, such as carbon monoxide.

If you suspect a gas leak in your furnace, it is vital to leave the area immediately and call a professional for help.

If you don’t know what you are doing, do not try to repair the furnace, as this can be dangerous.

A qualified technician will be equipped with the necessary skills, experience, and tools necessary to identify the problem safely and fix it accurately.

Frequently Asked Questions

How long does it take to get poisoned by carbon monoxide?

The amount of time it takes for an individual to become poisoned by carbon monoxide (CO) depends on the concentration of CO in the air and the duration of exposure. However, symptoms of CO poisoning can occur within a matter of minutes, potentially as soon as five minutes after exposure.

Can a furnace leak carbon monoxide without producing any symptoms?

Yes, a furnace can leak carbon monoxide without producing any immediate symptoms. Therefore, it is better to have working carbon monoxide detectors in your home, as they can alert you to the presence of carbon monoxide even if you are not experiencing any symptoms.

What are the long-term effects of poisoning from carbon monoxide?

Long-term effects of carbon monoxide poisoning from a furnace can include neurological problems, such as memory loss, difficulty with concentration, and learning disabilities. Carbon monoxide poisoning can also increase the risk of heart attacks.

John Abraham
Hey I'm John. I write about Manufacturing, Metalworking, CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com