Furnace Smells Like Gas – What Do You Do? – Furnace Troubleshooting

We have written a lot of articles lately on how to troubleshoot problems with your furnace, but today’s topic is a bit more serious.  When someone asks us why their furnace smells like gas, the answer is almost always due to a leak in your furnace’s heat exchanger, which can be serious for all sorts of reasons including possible fire hazards, explosions and carbon monoxide poisoning.  Don’t get carried away just yet though, because there are a couple of other things that we will have to rule out before we can diagnose a cracked heat exchanger.  For instance, is it the furnace exhaust that smells like gas or is the actual air that is blowing through your vents and into your house?  Those of you who read our articles regularly, know that we are a small, U.S. Veteran-Owned company in Southern California, and pride ourselves in giving people honest, straight answers to their questions.  This will be no different.  In this article, we will discuss what could cause a furnace to smell like gas, what you should do to troubleshoot when your furnace smells like gas, and how to prevent a gas-smell from coming out of your furnace in the future.

Natural Gas and Propane Furnaces Can Both Smell Like Gas

If you do not have a natural gas furnace or a propane furnace, then read no further – this article is not for you.  The question of why your furnace smells like gas is only applicable if you have one of these types of furnaces, so if your electrical heater starts to smell like gas, it probably isn’t your heater; check your stove!

What is the Difference Between Natural Gas and Propane?

Propane and natural gas have very similar molecular structures, and can best be thought of as being like fraternal twins.  They essentially work the same way but are a little different.  Natural gas, for instance, occurs naturally in nature (hence the name) and is a natural mixture of methane and other gases.  Propane, however, is not naturally occurring and is actually a byproduct of the refinement of petroleum, amongst other things.

Unsure as to whether you have natural gas or propane?  Ask yourself a couple of questions about what area you live in and how your fuel is delivered.  If you live in a cosmopolitan or suburban area, then you likely have natural gas, whereas if you live in a more rural area then you are more likely to see propane gas.  Another way to tell is by how the gas is delivered to your house: a gas line (metal pipe) that goes through a meter and is delivered directly into your house is a tell-tale sign that you have natural gas, whereas propane is delivered in liquid form and you will have an external holding tank if this is the fuel source for your furnace.

Differences Between Propane and Natural Gas Furnaces

One of the most notable difference between propane and natural gas as a means of heating has to do with their efficiency.  Propane delivers over 2,500 BTUs of energy per cubic foot, whereas natural gas only provides about 1,000 BTUs for the same volume.

It is also important to know that both of these gasses are odorless in their natural state.  They have a chemical odor added to them so that gas leaks are easy to detect.  If you are unfamiliar with the smell of natural gas or propane, it can most easily be equated to the smell of a rotten egg.

The price is also a major contributing factor in the decision one has to make between furnace fuels.  As you can see, natural gas is significantly cheaper than propane.  Usually it is about half as much but in some areas, it can even be six times cheaper.  Take a look:

Why Gas Leaks Are Dangerous

Before we go through the troubleshooting part of this article, it is important for you to understand that a gas leak should be taken very seriously.  If the gas-smell in your house is so strong that you can smell it very easily without having to sniff around, then I would turn your heat off completely and turn off your gas lines too if you know how (how to do so will be discussed later).  Your next best step would be to call the gas company.  If you are only catching a whiff of the gas-smell every once in a while, then you can continue on to our next section.

Carbon Monoxide Poisoning

Aside form the obvious dangers of having a combustible gas leaking into your house, there are others, the most notable of which is the possibility of carbon monoxide poisoning (aka “the silent killer”).  Carbon monoxide (CO) is an odorless gas that is more easily absorbed by your lungs than oxygen.  Over time, you can asphyxiate because your blood is saturated with carbon monoxide instead of oxygen.

Symptoms of carbon monoxide poisoning include drowsiness and a throbbing headache, so if you are experiencing any of these in conjunction with the smell of gas in your house then get out of the house immediately.  The nice part about carbon monoxide (if there is one) is that there are detectors available.  If you have a natural gas furnace or a propane furnace, don’t just buy a smoke detector – spend the money to have a combined smoke and carbon monoxide detector.  For more information on carbon monoxide poisoning, visit Carbon Monoxide Poisoning – Wiki. 

Fire Risks from Gas Leaks

Other hazards associated with a furnace smelling like gas include the risk of explosion and increased risk of fire.  Propane is denser than air, so it sinks to the floor and is therefore more likely to collect in quantities large enough to risk explosion.  Natural gas, on the other hand, is lighter than air and is more likely to disperse naturally into the air unless there is some type of improper ventilation.  Take note of what type of fuel your furnace uses and be mindful of your gas’ specific properties.

Troubleshooting – Why Does My Furnace Smell Like Gas?

To be perfectly honest, there isn’t a whole lot of troubleshooting that you can do when your furnace smells like gas.  This should always be treated as a serious problem, and unless it is just a passing whiff as your turn on your oven, etc, it is a cause for concern.  There are no normal circumstances under which your furnace should smell like gas.

1. Rule Out Other, More Obvious Options

It may seem like I am insulting your intelligence, but I’m honestly not – make sure that the gas smell is emanating from your furnace.  There are several other reasons for the sour, rotten egg smell associated with a gas leak, including compost heaps and trash cans near open windows.  Check the following obvious problems before you jump to conclusions – they may seem obvious, but a friend of mine once got called out for a gas leak only to find out that the man’s dog had been sprayed by a skunk, and he had tried to cover up the scent from his wife with some flower-scented spray (for the record, use tomato juice for a skunk).  Apparently, the combination smelled just like natural gas:

  • Compost Heap
  • Trash
  • Gas Stove
  • Propane Grill
  • Gas coming from your furnaces exhaust pipe and blowing back in through a window
  • Ask your husband if his dog was sprayed by a skunk

2. Ensure That the Gas Smell is Coming From Your Air Vents

If your furnace smells like gas and you have ruled out the other, more obvious options, ensure that the smell is coming from your air vents (called registers).  This will most likely diagnose a leaking furnace heat exchanger.  If the gas-smell is coming from the registers, then turn the unit off immediately, open the windows if practicable and call the gas company.  Get out of your house and stay at a neighbor’s place.  A gas leak may be more serious than you realize and leaks can happen even if the furnace is off.  On your way over to your neighbor’s, turn off the gas to your house from the outside.  Here’s a short instructional video on how to do this – How to Turn Off the Gas to Your House:

How to Prevent Gas Leaks From Your Furnace

The goal here is not to treat a gas-leaking furnace, but to educate you on how to prevent it.  There are two, and only two things that you can and should be doing to prevent your furnace from giving off a gas-smell when you turn it on: preventative maintenance and using carbon monoxide detectors.

Do Preventative Maintenance

This is honestly not a shameless plug because we do not do furnace inspections at All Systems Mechanical HVAC.  However, if you live in a cold-weather climate than I guarantee that your local HVAC contractor does.  If you live in a cold-weather climate like Bozeman, Montana, have your furnace inspected every year.  Make sure that your inspector uses specialized tools (endoscopic cameras, etc) and not just their eyes – you are paying them to properly test your furnace for leaks.  Ensure that they are licensed and have a good reputation.  For more information, here is a guide on How to Choose an HVAC Contractor.

Purchase Carbon Monoxide Detectors

If you have a natural gas or propane furnace, you must have one of these in every room.  Non negotiable.  They aren’t expensive, and you actually buy them as a combined smoke/carbon monoxide detector so no installation is required.  They are relatively cheap and run about $25 each.  As you might imagine, they give off a loud alarm (just like a smoke detector) if they sense smoke or carbon monoxide, alerting you and your family before you are incapacitated.  Remember, the scariest aspect of carbon monoxide poisoning is that if it happens while you are asleep, you probably will not wake up.  It’s worth the money.

Final Thoughts on When Your Furnace Smalls Like Gas

Many of our posts include troubleshooting tips that you can do yourself, but when your furnace smells like gas, there is not a whole lot that you can do.  The biggest thing I would recommend is to just make sure that the gas you are smelling is coming from your air vents and not your stove, etc.  Then get out and call the gas company.  I know this is not what you want to hear, but this is honestly one of those problems that you don’t want to deal with yourself.  For more information on problems that you can troubleshoot, try: Furnace Not Blowing Air, Heat Pump vs Furnace, Which Should I Choose? or Heater Blowing Cold Air.  Please also make sure to visit the ASM Air Conditioning Blog for more information on air conditioning and heating, as well as product reviews. 

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