Where mold actually grows in HVAC
There are two places in a residential HVAC system where biological growth (mold, mildew, bacteria) is common and predictable:
- The evaporator coil. It is wet for most of the day in summer, sits at 38-45 deg F, and has thousands of square feet of fin surface area. Dust and pollen pull through the filter (no filter is 100 percent) and accumulate on the wet fins, creating a biofilm. That biofilm grows mold.
- The condensate drain pan. Standing water, even shallow, with organic matter from the coil drips into it. If the drain line is partially clogged, water levels rise and slime growth accelerates. We have pulled drain pans from Houston systems where the pan was visibly black with mold.
Less common but still seen: blower wheels (they cycle through humidity and dust), insulated duct interiors (especially if there is moisture intrusion from a leak), and humidifier components on systems that have humidifiers (not common in Houston).
What 'dirty sock syndrome' really is
Some homeowners describe a musty, locker-room smell that hits when the AC kicks on and fades after a few minutes. The technical term is 'dirty sock syndrome' (DSS). It is biofilm and microbial volatile organic compounds (mVOCs) released when the coil first cycles through cold-and-wet to slightly-warmer air. The smell tells you there is biological growth on the coil. It is not dangerous to most adults, but it is not healthy long-term and it indicates conditions that will worsen.
Coil cleaning - either chemical (with an EPA-registered evaporator coil cleaner) or physical with a brush and rinse - clears DSS in 90+ percent of cases. The exception is when the coil has been growing biofilm for years; sometimes the biofilm has degraded the aluminum fins enough that the coil itself needs replacement.
What UV-C lights do and do not do
UV-C lights (germicidal ultraviolet, ~254 nm wavelength) installed in the air handler kill mold and bacteria on surfaces the light hits directly. They have value, but the marketing oversells them. What they actually do well:
- Coil-mounted UV-C lights aimed at the evaporator coil DO inhibit growth on that coil's surface. We see noticeable improvement in DSS-prone systems.
- They DO inhibit growth in the condensate drain pan when positioned to illuminate the pan.
- They DO have a peer-reviewed effect on surface microbial loads in the airstream they directly illuminate.
What they do not do, despite advertising claims:
- They do NOT clean ductwork - the air moves through the duct too fast for UV-C to make a meaningful kill at typical residential bulb intensities.
- They do NOT 'kill viruses in the air' at any meaningful rate - residential systems do not have the residence time for that.
- They do NOT replace coil cleaning. They REDUCE the rate at which growth returns. They do not eliminate the need to clean.
What actually prevents HVAC mold
- Keep indoor RH below 55 percent. Mold cannot thrive in dry environments. This is the single most effective prevention - upstream of any equipment.
- Schedule annual coil cleaning as part of spring maintenance. Catching it before it becomes a problem.
- Flush the condensate drain at least annually. We use a wet-vac on the outdoor end and a clean water flush from the access tee.
- Replace your air filter on schedule. A loaded filter starts bypassing dust around the seal, which lands on the coil.
- Fix moisture problems immediately. Roof leaks, plumbing leaks, slab cracks - any chronic moisture source feeds mold growth in or near the duct system.
When to escalate beyond HVAC service
If you see visible mold on supply register grilles, smell musty odors that do not improve after coil cleaning, or have residents with health symptoms that align with indoor mold exposure (chronic congestion, headaches, asthma flares only at home), the issue is likely beyond the HVAC equipment. Duct interior contamination, drywall cavity moisture, and crawl space/attic mold all need a remediation specialist - not just an HVAC contractor.
We can tell you when an HVAC fix solves it (most cases) and when you need a separate IAQ or remediation contractor. We will not pretend a UV light installation fixes a structural moisture problem.