Cleaning inside heating floor vent — Fireplace in Sandy, UT

When you switch on your furnace, you expect a gust of heated air to immediately begin regulating your home's indoor temperature. What you may not be expecting is the range of strange smells that waft in as your furnace runs.


Heating systems can give off strange and strong smells. Some of these odors will resolve on their own and shouldn't cause you any worry, while others can signify a potentially dangerous issue with the furnace unit. Identifying the likely cause of the scent can help you take the right steps to freshen your air.


In this blog, we list 10 common causes of abnormal furnace odors.


1. Cracked Components

Your furnace as a unit may last for significantly longer than some of its individual components. For example, many furnaces suffer cracked heat exchangers. When a heat exchanger becomes damaged, the gap releases a chemical smell that can resemble formaldehyde. Have the heat exchanger replaced right away since this issue can lead to furnace inefficiency, fires and carbon monoxide problems.


2. Damp

If your furnace sits in a damp part of your house, this moisture may settle on the unit. When the furnace turns on and gets hot, this dampness begins to burn off which can create a smell of dampness throughout your home. Luckily, this odor should resolve itself as the furnace continues to run.


3. Excess Dust

During the off-season, dust may gather in and on your furnace. As you begin to heat your home for the first time during the year, the furnace will likely burn this dust. If your heated air smells dusty for a short period of time, the odor should dissipate quickly. You can prevent this smell by dusting off the furnace before switching it on.


4. Gas Leaks

Natural gas used for heating is treated with sulphur to give it its distinct rotten egg smell. This odor is intended to alert homeowners to potential gas leaks as soon as possible. If you smell sulphur, leave the area and call an HVAC technician as well as your gas company.


5. Intake Contamination

Many furnace units intake air from outside the home. Anything that is within proximity to this intake system could potentially alter the characteristics of your heated air. For example, if your furnace smells like sewage, the odor likely comes from a sewer system opening that is near your intake system.


6. Mold

In some cases, dampness can lead to mold. Moldy furnaces are more common in areas with high humidity and in homes where the furnace stays cool and damp. Mold may smell similar to the moisture discussed in section two, but the smell generally won't go away as easily. Mold may also smell extremely musty or earthy.


7. Obstructions

If your furnace becomes obstructed in any way, one of the first signs that you're likely to notice is a change in air smell. The type of obstruction determines the resulting odor. For example, a blocked chimney may make your furnace smell like it's smoking, while a pest struck in the system would smell like decay. Generally, you'll need to turn off the system and have it inspected to resolve these issues without the risk of improper furnace venting.


8. Old Filters

Furnaces are fitted with internal filters that help regulate the air quality in your home. Over time, the filter can become clogged with dirt and debris. Not only can a blocked air filter affect the efficiency of your furnace, but you may also smell the obstruction as well.

An old furnace filter may smell somewhat like motor oil. However, you should be aware that if changing the filter does not get rid of the smell, the unit could have a serious oil leak, which requires professional attention.


9. Overheating

In addition to the gas leak-related odors discussed in section four, you should also never ignore electrical, hot metal or sparking odors. These smells typically indicate an issue with overheating and may point to a faulty auto-shutoff.


10. Vent Contamination

Like the area around your outdoor intake system, what's near your indoor vent registers can affect your furnace smells. One of the most common vent contamination issues is the smell of pet hair or urine. Check each vent register for a pet mess that needs cleaning. Taking care of the mess should resolve the smell as well.

In addition to the information provided, you should consult an HVAC expert if you have any concerns about the odor coming from your furnace. For example, if the smell appears suddenly, is particularly pervasive or persists for a long period of time, you may need a professional opinion to determine the true underlying cause.

Use this guide and the experience of your trusted HVAC technician to help you decide on the right course of action when your furnace starts to smell strange.

For reliable heating services, including routine and emergency furnace repairs, trust Hartman Heating, Air, and Fireplaces.