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7 Reasons Air Conditioner Smells Musty and How to Get Rid of It

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As an air conditioner unit ages, it produces all kinds of odors. Some odors are more gaseous, and some are more electrical.

Suppose you feel like you’re smelling something musty or mildewy. It may be tempting to ignore this problem, but usually, smells are signs of the presence of dangerous bacteria and airborne contaminants. 

To keep your family and yourself healthy and restore the air quality in your home, read on to learn why your AC smells musty and how to solve these problems.

To learn more about our maintenance services or request service today, call Cardinal or fill out our service form here.

The Most Common Causes of Musty Odors

1. Clogged Air Filters

When your air filters are filthy, they can become clogged with dirt, debris, dust, and many other contaminants. This blockage allows moisture and bacteria to build up in your air filters.

Dirty air filters can become breeding ground for contaminants. You will breathe in polluted air filled with dust and debris. No wonder it smells musty. Your air may be pushing through inches of blockage!


The solution is simple: replace your air filters often. Ensure that you have the correct size and shape so that you’re not missing any airborne particles traveling through your ducts. We recommend changing your air filters every 1 to 3 months, depending on many factors.

2. Clogged Drain Lines

An air-conditioning unit removes heat from the air entering your home and extracts water vapor. The condensate line drains the moisture away from your home. 

If your drain line has become clogged, it may freeze your  AC system, cause the drain pan to overflow, and lead to further damage. This buildup of moisture can cause a musty smell.


To prevent a clogged drain line, pour a cup of bleach into the opening of the drain line to clean it out and check it often to ensure there are no clogs. 

If you believe that your drain line is clogged, call a technician at Cardinal to clear the clog from your drain line.

3. Evaporator Coils

The evaporator coils are what air runs through to become cool. These coils can get filled with all kinds of moisture and particles in the air. Any contaminants that are on your coils may be flowing into the air of your home.


If you have an indoor AC unit turn off the power at your circuit breaker, then unscrew the coiling of the cooling coils from inside of your AC.

You don’t have to worry about removing the coils if you have an outdoor unit. Prevent danger by turning off the system before cleaning. 

Remove the top of your outdoor air conditioner and pull out debris. Use a coil cleaning spray to restore airflow and eliminate debris buildup. Spray foam away with a water hose.

4. Dirty Vents

AC vents pull particles through them, including any old and dirty buildup flowing through the air. All vents will have contaminants inside that will need to be removed.


Turn off the air conditioner before cleaning any air vents to prevent inhaling dust and debris. Remove the face plate and scrub it with an antibacterial spray. 

If you think more debris has become trapped inside the ducts, call a professional immediately to avoid damage to your ductwork.

Finally, ensure that the face plate is secure and screwed on tight; loose components can lead to leaks and openings for bacteria and other contaminants to invade your home.

5. Incorrect Size 

An air-conditioning unit that’s too small will not have enough power to filter out all airborne particles. Insufficient power will slow airflow and lead to moisture buildup, mildew, mold, and rot. 

An AC that is too large can put too much pressure on your ductwork. It will cause the components to warp, bend and fall out of place.


Talk to an HVAC professional to decide on the proper HVAC units for your home. At Cardinal Plumbing Heating and Air, experts can help you pick out a new HVAC system that will be energy efficient and save you money on your energy bill.

We will also be there with you every step, including installation and yearly maintenance for your new HVAC system.

6. Too Much Moisture 

All kinds of moisture buildup as your HVAC system works. It can build up in the drain pan, drain line, ducts, AC unit, and other places.

The moisture buildup becomes a breeding ground for all kinds of contaminants like harmful bacteria.


If your thermostat displays humidity levels in the home, try to monitor how much humidity is in your air.

If you feel like your AC unit is causing an excess of moisture in the air, call an HVAC professional to diagnose the problem right away. 

Consider investing in a dehumidifier or looking for any standing water around your units.

7. Pests

Because parts of your HVAC system are outdoors, it allows space for all kinds of pests and critters to invade your home. It may have been possible that there is an animal trapped up in your ductwork or that you may have had some kind of infestation in one of your outdoor units. 

The presence of pests can cause some pretty unpleasant odors.


Dealing with any pest is dangerous without the proper training and equipment; make sure to call a professional if you suspect that you may have an infestation.

Find Professional Furnace Maintenance in Ashburn, VA Now

If you’ve noticed a smell that seems a bit musty, it may be time to call an expert to check out the situation. 

A smell can be a warning sign of the buildup of bacteria and contaminants. So don’t put off this issue. The consequences can be disastrous for the health of your home.

If you’re looking for reliable and affordable HVAC repair, call Cardinal today. Request service and get in contact with us today here.

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