Biological material can cause a musty smell from your heat pump.

What’s That Musty Smell?

If you’ve had a heat pump for a long time, you may have noticed a musty smell coming from your vents at some point. Many people usually notice the odor when their unit first starts up, not after it has been running for a while. Maybe you’ve smelled this in your home and feel concerned about it; it happens quite often when you switch to cooling after heating or keeping the unit off for a long time.

Although the smell isn’t pleasant, it doesn’t typically indicate that anything too serious is going on. We’re going to explain why your A/C might smell musty and how an HVAC technician can help.



The short (and somewhat gross) answer is that a bunch of bacteria and biological material dries up on the indoor coil inside your air handler. We call that coil the evaporator, and it has an A-frame shape. The evaporator absorbs heat from the air and cools it.

When we run units in cooling mode, the evaporator gets a bit wet because moisture in the air condenses onto it. Whenever we remove heat from the air, that air can hold less water than before. So, some of that water will condense to a liquid, like dew on the grass. However, when we run units in heating mode, the coil is far less likely to become moist. That’s because we aren’t using that coil to remove heat anymore.

Therefore, when we shift the unit back into heating mode with dried gunk on the coil, the blower fan forces that nasty odor through the ducts. Then, that smell goes through the vents and into our living spaces.



The solution is quite simple: an HVAC tech just has to clean the coil.

A good technician will inspect the entire area. They may check for dirty filters, standing water around the unit, signs of pest intrusions, and gaps in the ductwork; all of those could potentially cause odors separately from the coil.

Cleaning an evaporator coil is NOT a good DIY project. HVAC technicians are specially trained to use chemicals to clean the coil. The chemicals we use at Kalos are non-toxic and made from natural enzymes. We also try our best to inspect and clean much more than the coil; we’ll also check the cleanliness of the drain pan, blower wheel, and the return air box.



UV-C lights can control the biological growth on your evaporator coil.

The best thing we can do is prevent that biological material from getting on the coil in the first place.

Some unit manufacturers have taken it upon themselves to put protective coatings on their coils. Those coatings prevent biofilm from attaching to the coil, and we’ve noticed that these coatings are relatively reliable.

However, it might be more practical to combine three things: regular cleaning, good filtration, and UV-C lighting. Regular cleaning is self-explanatory and will be covered in a maintenance program. Good filtration means that you must use an appropriate filter (usually MERV 8 or better) and change it at least monthly; a good rule of thumb is to change out your A/C filter whenever you receive your power bill.

UV-C lighting is also an interesting but effective solution when used correctly. UV-C lighting prevents biological materials from growing on the coil, but it’s only effective if it’s within a certain range of the surface. In other words, the solution isn’t as simple as slapping it inside the air handler and never touching it again. The UV bulb must be installed close to the coil and replaced at least yearly to get the most of your light. At Kalos, we offer Top Tech UV lights as installation add-ons; you can learn more about those bulbs’ germicidal properties HERE.


As gross and embarrassing as that musty smell may seem, it’s actually relatively common where we live. These odors are also relatively easy to control and prevent as long as you have a good HVAC technician taking care of your unit. If you have an A/C or heat pump issue and want Kalos to help, you can call us at (352)-243-7099 or read more about our residential HVAC services HERE.