If you have a black or fuzzy ring around your supply registers, a musty smell when the AC kicks on, or family members with allergy symptoms that get worse indoors, you probably have mold somewhere in your HVAC system. The most common locations: the indoor coil, the drain pan, the blower compartment, and the supply ducts within a few feet of the air handler.
Mold grows wherever it has moisture, organic dust, and 60 to 90 degree temperatures. Your HVAC system, especially in Houston's climate, has all three for most of the year. Here is how mold gets in, where it grows, what it does to your indoor air, and the maintenance habits that keep it out.
Where mold grows in an HVAC system
The indoor coil
The evaporator coil is wet every time the AC runs. Water condenses on it, runs down into the drain pan, and out through the condensate line. The coil is also where every airborne particle in your house tries to land. Combine constant moisture with organic dust and you have a perfect bioreactor. Coils that haven't been cleaned in 5+ years often have visible mold and a musty smell at the air handler.
The drain pan
The pan under the coil holds standing water if the drain line is partially clogged or sloped wrong. Standing water plus dust equals a microbial mat within weeks. We have pulled drain pans out of attic units that looked like aquarium algae.
The blower wheel
The blower wheel sits downstream of the coil and gets coated in a fine film of dust on every blade. If humidity is high enough at the blower, that film holds enough moisture to support biological growth. A coated blower wheel reduces airflow 20 to 40 percent, which makes the system less efficient and worsens humidity, which feeds the mold problem.
The first few feet of supply duct
Most ducts in our area are flex insulated with a vinyl inner liner. The first few feet downstream of the air handler are where any moisture from the coil ends up if the system is undersized, oversized, or short cycling. Mold can grow on the inner liner where it's wet.
Filter compartments and return grilles
Less common but worth checking. Damp dust collects on the underside of return grilles in humid homes. Surface mold here is cosmetic but a sign that whole-house humidity is too high.
How it gets there
Mold spores are in the air everywhere. The question isn't how the spores arrive (they always do), it's whether your system gives them what they need to colonize. The drivers are:
- High indoor humidity. Anything over 60 percent indoor RH is mold-friendly. Properly sized AC running long enough cycles holds Houston homes at 45 to 55 percent. Oversized AC short-cycles and leaves humidity at 65 to 75 percent.
- Dirty filter. Cheap fiberglass filters let dust through to coat the coil and blower. The dust is the food source.
- Clogged drain line. Backed-up condensate equals standing water in the pan. Standing water always grows something within a few weeks.
- System left off in summer. Vacation homes or rentals where the AC is off for weeks at a time stay warm and humid. The coil dries with whatever was on it locked in place. Comes back wet next time and the colonization starts.
- Leaky ducts in a hot attic. Pulling 130 degree attic air into the return mixes hot humid air with cool surfaces. Condensation on the cold side. Mold on the wet liner.
What it does to your air and your health
Mold spores released by an active colony in your HVAC end up in every room of the house. Health effects vary by person and species, but the common ones are:
- Allergy symptoms (runny nose, sneezing, itchy eyes) that worsen indoors and improve outdoors
- Persistent cough or throat irritation
- Headaches, fatigue, and "brain fog" in sensitive individuals
- Asthma exacerbation in people with existing asthma
- Skin irritation
Children, elderly, and immunocompromised individuals are more vulnerable. Anyone whose symptoms have a clear "indoor worse, outdoor better" pattern should be considered for a mold workup.
The maintenance habits that prevent it
1. Filter discipline
Change a 1 inch fiberglass filter every 30 days during cooling season, every 60 days otherwise. A pleated MERV 8 to 11 filter every 90 days is better. Set a reminder on your phone. We recommend MERV 8 to 11 for most homes, balanced between filtration and not over-restricting airflow.
2. Annual coil cleaning
Indoor coil gets cleaned during a tune-up. Foaming non-acid cleaner, rinse with the system shut off, drain through the condensate line. Outdoor coil gets rinsed top down with garden water pressure to remove the season's accumulated debris. We do both during a maintenance visit.
3. Treat the drain line
Pour a half cup of distilled white vinegar down the drain access port at the air handler every 3 months. Kills biofilm before it clogs the line. Some folks prefer pan tablets that slow-release biocide. Either works.
4. Verify the float switch
The safety switch on the drain pan that shuts off the system if water backs up. Test it once a year. It is a $30 part that prevents thousands of dollars of water damage and stops the conditions for mold growth.
5. Run the system properly
If you go on vacation, leave the AC at 78 to 80, not "off." Letting the house go to 90 and 80 percent humidity for two weeks is exactly when colonization happens. Better: a smart thermostat that holds the house at 80 with 50 percent humidity setpoint while you are gone.
6. Address oversizing
If your AC short-cycles and your house is humid even when it is running, you have an oversized system. The fix is at replacement time. Right-size the new equipment with a Manual J load calculation. Variable-speed equipment helps too because it can run long low cycles that pull moisture out.
7. Seal the ducts
Aerosealing the duct system or replacing leaky flex with sealed metal trunk lines stops the attic-air problem. Worth considering at replacement time or if your duct test shows leakage above 15 percent.
8. Whole-home dehumidifier in some cases
For homes that run cool but never get below 60 percent RH, a whole-home dehumidifier ducted to the return is the right answer. Roughly $2,500 to $4,000 installed in our market. Pays back in comfort and prevents the conditions for mold without overcooling the house.
9. UV light at the coil (in some cases)
A 36 watt UV-C lamp installed at the coil keeps the coil surface and drain pan free of biological growth. Effective for what it does. Doesn't sterilize the air the way some marketing claims. Useful as a maintenance aid for high-risk customers (allergies, asthma, immunocompromised). Roughly $500 to $800 installed.
What to do if you already have mold
- Have us inspect. We can usually identify the source within 30 minutes and tell you what is HVAC and what is broader (wall cavity, attic, crawlspace).
- If the mold is on the coil, blower, or drain pan, we clean and treat. Coil pull and clean, blower wheel removal and cleaning, pan replacement, drain line treatment, UV light if appropriate.
- If duct liner is contaminated, we replace the affected sections. Cleaning contaminated flex duct is rarely effective long-term.
- Address the underlying cause. Sizing, humidity, drain, leaks, all of it. Otherwise the mold comes back.
- If the contamination is heavy or you have health concerns, we coordinate with an indoor air quality testing firm to validate the remediation.
"Customer in Pearland called us about a musty smell every time the AC kicked on. We pulled the air handler door and there was a half inch of black biological growth on the bottom of the coil and the entire drain pan. Their drain had been clogged for at least a year and the float switch had been bypassed by the previous installer. We replaced the pan, did a deep coil clean, replaced the bypassed float switch, and added a UV light. Smell gone in a day. Their daughter's allergies improved within the week."
Bottom line
Mold prevention is not complicated. Filter, drain, coil, sizing, humidity. Get those right and your HVAC won't grow anything. Skip them and Houston will eventually grow you a colony.
Smell something musty? We will diagnose and price the fix. 281-992-7866 or book online.