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Why Does Your Heat Smell Musty? 4 Causes (and Fixes!)

Having the heat smell musty and spread that smell throughout your home is horrible, especially when you don’t know why it’s happening or how to fix it. Luckily, many people deal with this in their homes, so there are several ways to find the root of the problem and make the smell disappear.

Heat smells musty in homes for various reasons, including mold in the air ducts, excess dust build-up due to infrequency, and even dirty filters or evaporator coils. While there are multiple causes, fixing the smell will mainly involve cleaning air ducts, eliminating mold, or replacing equipment.

In this article, I’ll discuss possible reasons for your musty-smelling heat, other smells that could be the cause of different issues, how to fix the smell, and how to prevent the musty smell from happening again.

Why Your Heat Smells Musty

Your heat can smell musty for a few reasons. The best way to determine why your heat smells musty is to find out the cause of the problem. To do this, you’ll most likely need a technician to come out and identify the problem. Most of the time, you’ll know it’s just dusty air vents if the smell disappears within a few days.

However, there are other reasons why your heat may smell musty, some of which might need a technician’s help and others not. Here are four common issues:


A common problem, and one of the more dangerous issues people often face, is mold in their air ducts. Mold happens when moisture doesn’t drain properly, so it doesn’t leave the air ducts and eventually forms into the mold. This occurs when the air conditioning is running, as it causes a lot of condensation.

If you believe there could be mold in your air ducts, it’s essential to get a technician to come out and check. While you can try looking in your air ducts yourself, it can be a difficult task if it’s not something you’ve done before.

The Dangers of Mold

While smelling mold isn’t necessarily dangerous, over some time, it can cause respiratory issues. Some people are also more sensitive to mold than others, so it also varies depending on previous health issues.

Breathing mold over long periods can cause allergic and respiratory symptoms. This will significantly impact those with current respiratory conditions, such as asthma or those with a compromised immune system.


A musty smell when you turn on the heat is often attributed to dirt build-up in the vents. This is most common when the heat hasn’t been turned on in a while. Most of the time, the ducts will have a build-up of dirt and dust after such a long time.

If you haven’t used your vents in a while and you believe there could be dust or dirt build-up, you can clean the vents yourself. However, getting an HVAC technician out to clean them for you is a better option as they’ll know how to do it correctly and efficiently.

Dirty Evaporator Coils

The evaporator coil, also known as the EVAP coil, is a part of the air conditioning or heat pump unit that absorbs heat from the air in the house. The evaporator coil and condenser coils in homes work together to cool and heat the home.

Evaporator coils should be cleaned often, sometimes even monthly. When evaporator coils get dirty, they can hold dust or other particles, which could cause a musty smell. While this isn’t the most likely cause of a musty smell, it’s still essential to check your EVAP coils regularly.

Dirty Filter

A musty smell could also be a sign of a dirty air filter. Many people don’t realize or often forget to change the filters in their homes every few months. According to HVAC, the number of times you should change your air filter depends on how big your house is, whether you have pets, and the number of pets. Typically every 30 to 60 days is standard.

If you don’t change your air filter as often as you should, it can become clogged and won’t allow the air to flow through completely. The particles, dirt, and dust that build up in the air filter can create a musty smell throughout the home.

4 Easy Fixes for a Musty Heat Smell

There are easy fixes for each problem, all of which include cleaning the area where the problem resides. While most of these fixes can be resolved by calling a technician, there are ways to fix them on your own quickly that’ll hopefully save you some money.

After identifying the problem, you can take the necessary steps needed to fix it. Here are four easy fixes for each issue:

1. Kill the Mold or Mildew

If the problem causing the musty smell is mold, you’ll need to kill the mold in your air ducts. While this process will take a bit longer than the other possible problems, it needs to be done to prevent potential health issues and eliminate the smell.

Getting an HVAC technician out to clean the mold is the best option. However, you can clean the mold yourself as long as you take the necessary precautions.

How Can I Clean Mold?

You can clean mold out of a furnace by:

  1. Putting on a face mask and gloves.
  2. Fill a bowl with 1 gallon of water, 5 drops of dish soap, and a cap full of bleach.
  3. Use a soft cloth dipped in the bleach solution to scrub the area.
  4. Allow everything to dry.

This method will only work on nonporous surfaces. If any mold has gotten into the insulation, it will have to be replaced entirely.

However, if mold is present, there has to be a reason. So, finding the area where moisture is is essential to ensuring you don’t have to deal with mold again.

In addition to cleaning the area, consider applying a mold control spray. I recommend the DWD2 Home & Commercial Mold Treatment from The 8 ounce fogger enters the HVAC intake and moves throughout the system to clean the ductwork.

2. Clean the Ducts

If you suspect the problem causing the musty smell is dust or dirt, you’ll need to clean out the vents. Professionals will be able to use high-end equipment to clean out all of the ducts, especially if they’re dirty from not being used, as there will more than likely be places you can’t reach on your own.

While cleaning the ducts, professionals will also be able to check for any mold.

How Can I Clean Dust?

Clean the dust out of your HVAC system by using a vacuum and a pole with a dusting attachment. Use a long vacuum hose in combination with a duster to reach inside the vents and clear dust and debris. 

I recommend purchasing DocaPole Cleaning Kit from This product includes a 24-foot pole with multiple attachments for those hard to reach areas.

The amount you can clean yourself depends on the HVAC system in your home and how much of it you can physically reach.

3. Clean the Evaporator Coils

While dirty evaporator coils typically aren’t the main reason for heat smelling musty, they should be checked and cleaned often. Removing the excess dust from evaporator coils will help with mustiness and keep the unit clean and working correctly.

How Do I Clean Evaporator Coils?

To clean evaporator coils:

  1. Find the EVAP coil in the furnace.
  2. Shut off the power to the furnace.
  3. Expose the coil.
  4. Remove the access panel.
  5. Use a brush to get dust and dirt off of the coil.

While brushing off the excess debris helps, you may need something stronger for tougher jobs. Using an evaporator coil cleaner, such as the Nu-Calgon Evap Foam from can make your job easier. This cleaner works to remove dirt, grease, oil, or any other particles from the coil quickly and efficiently.

You spray the cleaner onto the coil and let it sit for around five minutes before rinsing or wiping off. Repeat the process as necessary.

If you’re more of a visual learner, Any Hour Services has an excellent Youtube video showing how to do it:

4. Replace the Filter

A musty smell in your home could be the result of a dirty air filter. Replacing air filters is necessary, especially if you live in a house with many animals. However, many homeowners either forget to replace filters altogether or don’t replace them as often as they should.

Every home will have different sized air filters, so the first thing you’ll need to do is find the correct size for your home.

After finding the correct size, simply remove the old filter and replace it with a new one.

How Do I Replace an Air Filter?

Replacing the air filter in your home is a job for homeowners. 

When replacing an air filter, make sure you turn off the HVAC system before trying to change the filter. After this, locate the air filter and determine which direction the airflow is. This is important because it will determine which direction to install the filter.

LRN2DIY has an excellent Youtube video showing how to replace a filter properly. Check it out here:

How Do You Get The Musty Smell Out of Heating Vents?

If the musty smell comes from the heating vents and not the heater itself, then it’s likely that the odor is emanating from the ductwork.

The ductwork is more difficult to clean because it runs throughout the entire home. Accessibility is one of the biggest drawbacks to cleaning air ducts yourself—especially if they run through the ceiling.

The difficulty of cleaning air ducts is due to the tools required for the job. Most HVAC technicians utilize high-powered vacuums and rotary brushes.

While it’s recommended to call a technician to ensure a thorough cleaning of your heating vents and ductwork, there are some things that you can try at home.

To get the musty smell out of heating vents:

  1. Unscrew the heating vents from the floor or ceiling.
  2. Use a pole with a duster attachment and run it through the vents.
  3. Hose out the excess dust and debris with a vacuum.
  4. Call an HVAC technician if all else fails.

Tips To Prevent Heat From Smelling Musty

The best way to fix your heat from smelling musty is to prevent it altogether. Even after fixing the musty smell, it could always come back. Therefore, you should take measures to ensure your air ducts and filters stay clean.

Cleaning the furnace every season, or at least once a year, should help with ensuring there isn’t any dust or dirt build-up in the ducts. This is especially important if the heating isn’t used that often, as more dust will accumulate. Therefore, regularly turning it on will also work to prevent dust from building up.

The best way to prevent any mold is to eliminate the cause of moisture for mold. This might take a while and require professionals to come out and look for the reason for moisture. However, it’s a necessary step to avoiding mold problems in the future.

What Other Smells Could It Be?

Aside from a musty smell, other possible odors you might notice in your home include burning, gas, or oil smells. These odors could indicate a more serious problem, such as overheating of the HVAC system, or a gas or oil leak.

Burning Smell

A burning smell or electrical odor is a cause for concern. This could mean that your HVAC unit is overheating. To check for this, you’ll need to turn off the electricity and have a technician come out and look at the wiring. Smells such as this are typically a wiring problem and should only be fixed by a qualified technician.

Gas Smell

A gas smell could mean there is a leak in the pipe. While this might seem like a problem you could fix, it’s best to have someone qualified come out to look at it, especially if you can only smell it. A technician would be able to repair it efficiently and safely and wouldn’t spend forever looking for the leak itself.

Another possible smell that’s contributed to a gas leak is a sulfur smell. This typically means there’s a natural gas leak somewhere. This is especially dangerous, and you should leave the house immediately and call a gas company. 

Oil Smell

Lastly, an oil smell could mean that there is an oil leak in your home. Oil leaks usually aren’t a severe cause for concern. However, the smell can be aggravating, and oil leaks can sometimes lead to more significant problems in the home, so getting it fixed is essential. Calling a technician about this problem is the best way to handle it.

Final Thoughts

Your heat smelling musty may not always be a severe cause for concern. However, the smell can be overpowering, and like most people, you’ll want to get it fixed as quickly as possible. A musty smell most likely means there is a build-up of dust or dirt, there’s mold, the evaporator coils need to be cleaned, or the air filter needs to be changed. 

You can fix these issues pretty easily, and you can even fix them on your own. However, having a professional come out to clean mold or clear the ducts of dirt is the best option.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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