Derek ColeGM, Simmons One Hour Heating & Air · AI HVAC Answers

715. Why Does My AC Smell Musty? What That Smell Is Trying to Tell You

26 Aug 2026 · 3 min

Home and Life Comfort with Derek Cole

26 Aug 2026 · 3 min

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In this solo‑host episode of Home and Life Comfort with Derek Cole, the host explains why residential air‑conditioning systems can emit a damp, musty odor and what that smell indicates about the system’s health. He describes how an AC unit removes heat by passing indoor air over a cold refrigerant coil, which causes moisture to condense and drip into a drain pan. When the drain pan does not drain properly, standing water creates a dark, damp environment where mold and bacterial growth can develop, producing the characteristic musty, moldy smell. Cole emphasizes that even the most resilient microbes die within 24 hours when exposed to UV light, so introducing UV illumination into the indoor coil (via a “coil scrubber”) can prevent microbial growth and eliminate the odor. He reinforces this point with a simple analogy: a damp mat left in sunlight dries and stays clean because UV light inhibits growth, whereas the indoor air handler lacks such UV exposure. The episode’s core message is that a musty AC smell signals moisture‑related microbial growth, and installing UV light on the coil is an effective remedy.

Chapters

Home and Life Comfort with Derek Cole. Want to prove that point? Everybody at home has that mat at the front or back of the house that has the mildew and it's kind of wet and it's kind of damp. Take that mat, stick it out in the middle of the yard in the sunshine for a period of time, and that will be clean and dry because nothing can grow when UV is present, right? Musty. Your home smells musty. Yeah, musty is that kind of wet, damp, dirty socks, bottom of the hamper kind of smell. So that is musty. The reason it smells that way is because in air conditioning you have to remove moisture to get humidity out of the air. So air conditioning is all about removing heat, right? So you're taking heat out of the air, it's going over a cold coil which has refrigerant in it, and then it's coming back out Cooler air than what came in. So the byproduct of that is water. So that's why you have drain lines, that's why you see water dripping outside. So what will happen is when you have that hot air going across that cold coil, you get condensation and that's going to drip down to the drain pan. Well, if that drain pan is not draining or something is the matter, things will start growing in that drain pan because you have— Mold. Cold water sitting in a damp, dark spot. So the thing that happens is growth, right? So even the most— the strongest bacteria in the world lasts 24 hours. So if it's not in the air, or if it's a bug, or it's a virus, or whatever it is, it's going to die and it's going to sit at the bottom in that drain pan for a period of time. So you start getting— the first time you turn on your heat or your air conditioning, Especially you get that kind of damp, musty, moldy kind of smell. There's like all kinds of mold, so we don't call it mold, we call it growth. So the way to prevent that from happening is to introduce UV into that system somehow. Now we do it here with the coil scrubber, which shines on the indoor coil, has the UV power, so nothing can grow in UV. If you want to prove that point, everybody at home has that mat at the front or back of the house that has the mildew and it's kind of wet and it's kind of damp. Take that mat, stick it out in the middle of the yard in the sunshine for a period of time, and that will be clean and dry because nothing can grow when UV is present, right? So in the indoor air environment, in the air handler, that is not present. So we have to make it present with UV light. UV light will solve that problem. So the same power of the sun, the same energy that holds, we can bring it into your system and that'll stop that damp, dusty, musty smell. Home and Life Comfort with Derek Cole.