Does a furnace smell indicate carbon monoxide? 

Is a furnace smell an indication of carbon monoxide, or is a natural gas leak or some less alarming concern. Although CO is odorless, combustion smells are warnings of danger. Remember, your pet will smell these warnings first.

By Rebecca Martin

Carbon monoxide is described as an odorless, tasteless gas which cannot be detected by smell. This does not mean that carbon monoxide is not a concern when the smell of gas is present. Carbon monoxide will be present any time there is incomplete combustion from a fuel-burning device. Smells generated by a furnace are a warning sign that the heating system is not operating as it should.

There are two times when smells might be noticed during the operation of a furnace; startup and operation. The smells that occur during startup are commonplace and generally of no concern. Those that persist during operation are warning signs that the heating system is not operating as it should.

As carbon monoxide itself is odorless, no mammal, including dogs and cats can smell it. But that doesn’t mean that a bad smell isn’t a warning of carbon monoxide.  Carbon monoxide from a furnace is the result of a failure to complete the combustion process –meaning oxidizing (changing the molecular nature through burning) all of the carbon fuel (the natural gas) with oxygen. When there is not enough oxygen for the amount of fuel that is being emitted from the burners on the furnace, carbon monoxide will form. Most times that carbon monoxide is created, there is also unburned fuel, the unburned hydrocarbons. Unburned hydrocarbons have some smell, depending on the percentage of these unburned hydrocarbons in the air. The greater the amount of carbon monoxide in the exhaust, the greater the amount of unburned hydrocarbons. When the unburned hydrocarbons get to one level, dogs (and perhaps cats) will begin to smell it. If they get high enough, the human nose will detect it. In our experience, it is when the concentration of carbon monoxide in the exhaust is above 4,000 ppm, that the human nose will begin to detect the smell of the unburned hydrocarbons. If you are smelling this is the house, it is deadly serious. 

A furnace smell can be a sign of danger, depending on the circumstances. Such smells should always be taken seriously.

New furnaces often emit an unpleasant smell when first turned on. This is the smell of the protective oil coating on the inside. This smell usually lasts for a day or two and then dissipates. Established furnaces may have a smell when first turned on for the winter due to the burning of accumulated dust on the vents, registers, and heat exchangers. The dust heats up and burns off quickly. There can even be a brief whiff of gas at startup as the gas begins and is ignited. This smell is usually detected when one is near the furnace and is quickly dispersed by opening a window.

Smells that persist after startup are a reason for concern. Smells during the operation of a furnace indicate that the system is not efficiently burning fuel and that situation can turn deadly if ignored.

Natural gas, which is the most common fuel used in the United States, has no odor on its own. Because of this, gas companies add a chemical called mercaptan. Mercaptan is a foul-smelling chemical which helps identify gas leaks so that they do not go undetected. It contains sulfur which can resemble the smell of rotten eggs. If your furnace is emitting an odor that smells like rotten eggs it is indicative of a gas line leak, though you might identify it as the smell of sewage or gas. This odor will not be eliminated by opening windows and doors. This is a persistent odor.

To further explain this topic, it is important to differentiate between three different portions of your home’s system to provide heat through a forced air furnace.

Furnace Smells from Gas Line

First is the gas line. The gas line brings the natural gas from either the public utility line or your propane tank, to your homes furnace. If the seal on these lines is bad, you will have this leak of natural gas. The biggest risk associated with such a leak is fire or explosion.

Furnace Smells from a Chimney or Flue

Flue Gases. The second is the ductwork to get the flue gases outside of your home. It may be easiest to think of this as your furnaces chimney but in more HVAC terms it is your furnace “exhaust flue.” While natural gas furnaces should never smoke like a woodburning fireplace does, the purpose of the exhaust flue is to get this invisible smoke out of your home. Woodburning smoke would make you cough, hurt your eyes. Natural gas smoke can kill you from carbon monoxide exposure before it would be sensed by either your lungs or your eyes. That is why ever furnace has some system, the exhaust flue piping, to get the flue gases out of your house.

Furnace Smells from Internal Ductwork for Forced Air

Forced Air Circulation. The third is the internal metal ductwork, airducts, which circulate the warm air that the furnace creates inside your home. There never should be exhaust inside of the warm circulating air. Inside of the furnace, there is something called a heat exchanger, which is warmed up by the dangerous exhaust gases and transfer this heat, while maintaining a strict barrier between the flue gases and the forced air, warm air.

What is a Heat Exchanger in a Furnace?

Think of a heat exchanger as a pot on a stove. The pot is warmed by the fire on the stove, and then warms the water within it. As long as the pot is watertight, the process works. If there were a leak or a crack in the bottom of the pot, the process would fail as the water would put out the fire. In a heat exchanger, the air from the fire warms the metal heat exchanger and then the circulating forced air, is warmed by the heat exchanger. The concern if there is a crack or a hole in the heat exchanger is not leaking of water, but the leakage of poisonous gases with air we are meant to breathe.

Gas Line or Connection Leaks

A gas leak is a leak in the gas lines entering the home. There are signs to watch for if you suspect a gas leak.

First is the odor of rotten eggs, just like the smell that could come from a malfunctioning furnace. You can smell a gas leak even when the furnace is not in operation. Leaks can occur both inside and outside of the home.

Hissing noises may be present and might only be heard when the furnace is off. These are caused by tiny cracks or holes in the lines.

Plants are indicators of gas leaks. Soil absorbs natural gas. The natural gas in the soil interferes with the plant’s root system. Dying plants can indicate that there is natural gas leaking into the home.

Extended exposure can cause flu-like symptoms.

If you suspect a gas leak, you should evacuate your home immediately and call your gas company or 911 and do not re-enter until directed.

Gas leaks outdoors can sometimes be revealed by bubbles in puddles around the home and again, dying plants.

Carbon monoxide detectors do not detect gas leaks.

Exhaust Leaks from Furnaces are Deadly

If there is a leak in the exhaust of your furnace into the breathable air within your home, that will inevitably lead to a carbon monoxide escaping into your home. If you smell flue  gases, that means the levels are deadly. Get out, then call for help. Other ways in which flue gases can escape into the breathable air of your home is through displacement, corrosion or cracks in the exhaust venting or chimney. Any signs of rust and corrosion on these pipes meant to get dangerous gases outside of your home, is a serious warning sign. For more on the connection between rust and carbon monoxide poisoning, click here. 

Ductwork Smells

Sometimes people suspect uncleaned airducts (the air that circulates within your house, as opposed to flue gases which must leave your home) as the cause for unwelcome smells but according to experts, this is unlikely. Airduct cleaning is a homeowner’s expense that has not been proven to make any difference in most homes unless the duct work is unusually and noticeably dirty, so it is unlikely that gas smells would be caused by dirty ductwork. Replacing filters regularly is sufficient for normal operation.

If there is an odor of mold or general mustiness coming from air ducts, mold can accumulate in air ducts, especially in humid climates. A professional can then clean air ducts accordingly. But if mold and mildew is growing in air ducts it should be determined why there is excess moisture. Mold distributed throughout the home is a health risk.

If the smell of burning plastic or rubber is detected, the likely cause is an electrical problem. This is a safety issue and requires professional help. However, the electrical burning smell could indicate that your furnace motor is too hot or that the heat exchanger is cracked.

Understand your heating system

It is important to understand where your vents enter and exit your home and make sure that those ingress and egress points are clear of debris and snow, especially in conjunction with storms. Fuel-powered machinery should never be left in operation adjacent to intake points. Keep in mind that intake points also include the normal airflow through windows, doors, and garages. This is why portable generators should be 20 feet from the home.

The most important thing is to commit to annual maintenance of furnaces with a thorough inspection of all the components. An annual inspection should include inspection of the venting system including air intakes, the heat exchanger, the burners (ignition and flame), filters, the gas line, the thermostat, and the drains. You should insist that a combustion analyzer is used at the time of inspection. An inspection of the placement and the condition of carbon monoxide detectors should be done at the same time.

Even with the completion of an annual inspection, take the warning signs seriously and have a safe winter.

Part of the job of the heat exchanger is to remove all carbon monoxide that is a by product of combustion. If the heat exchanger is cracked it can leak carbon monoxide into the home.

Other symptoms of a cracked heat exchanger include:

  • Soot on or inside the furnace
  • Yellow burner flame, instead of blue
  • Chemical smell from furnace, like formaldehyde
  • Water near your furnace, as long as the condensate drain isn’t plugged
  • Flu-like symptoms

Any indication that the heat exchanger is damaged or has failed is a serious problem that should be addressed immediately. The furnace should be shut off immediately and a professional should be called. If the situation is ignored, dangerous levels of carbon monoxide could enter the home.

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