A black or fuzzy ring around the supply registers is a warning sign. So is a musty smell when the AC kicks on, or allergies in the family that get worse indoors. Any of these usually means mold is growing somewhere in the HVAC system. We find it most on the indoor coil and in the drain pan, followed by the blower compartment and the first few feet of supply duct.
Mold grows wherever it finds moisture and organic dust at 60 to 90 degrees. In Houston, your HVAC system offers both for most of the year. Below is where it grows and how it gets a foothold, what it does to your air, and the habits that keep it out.
Where mold grows in an HVAC system
The indoor coil
The evaporator coil is wet whenever the AC runs. Water condenses on it and drips into the drain pan, then leaves through the condensate line. Airborne particles from the whole house land on the coil too. Constant moisture plus organic dust makes a perfect bioreactor, and a coil nobody has cleaned in 5+ years often has 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 partly clogged or sloped wrong. Standing water and dust grow a microbial mat within weeks. We've pulled drain pans out of attic units that looked like the inside of a neglected aquarium.
The blower wheel
The blower wheel sits downstream of the coil, and every blade picks up a fine film of dust. With enough humidity at the blower, that film holds enough water to grow things. A coated wheel moves a lot less air. The system loses efficiency, humidity goes up, and the mold gets more of what it wants.
The first few feet of supply duct
Most ducts around here are insulated flex with a vinyl inner liner. When a system is mis-sized or short-cycling, moisture from the coil ends up in the first few feet of duct past the air handler. Mold can grow on the liner wherever it stays wet.
Filter compartments and return grilles
These are less likely but worth a look. In humid homes, damp dust collects on the underside of return grilles. Surface mold there is cosmetic, but it tells you the whole house is too humid.
How it gets there
Mold spores are in the air everywhere, so they will always arrive. What matters is whether your system gives them what they need to settle in. Five things do:
- High indoor humidity. Anything over 60 percent is mold-friendly. A properly sized AC running long cycles holds a Houston home at 45 to 55 percent. An oversized one short-cycles and can leave it at 65 to 75.
- A dirty or cheap filter. Fiberglass filters let dust through to coat the coil and blower, and that dust is the food.
- A clogged drain line. Backed-up condensate means standing water in the pan, and standing water grows something within a few weeks.
- A system left off in summer. Vacation homes and rentals with the AC off for weeks stay warm and humid. The coil dries with everything still stuck to it, then gets wet again at the next startup, and the colony takes off.
- Leaky ducts in a hot attic. Return leaks pull in 130-degree attic air, and when that hot humid air hits cool surfaces it condenses. Wet liner, then mold.
What it does to your air and your health
Spores from an active colony in the HVAC get carried to every room. Effects vary by person and by species. Common ones:
- Allergy symptoms (runny nose, sneezing, itchy eyes) that are worse indoors and better outside
- Persistent cough or throat irritation
- Headaches, fatigue, and "brain fog" in sensitive individuals
- Asthma flare-ups in people who already have asthma
- Skin irritation
Kids and older adults are more vulnerable, and so is anyone with a weakened immune system. If someone's symptoms follow a clear "worse indoors, better outside" pattern, a mold workup is worth considering.
The maintenance habits that prevent it
1. Filter discipline
Change a 1-inch fiberglass filter every 30 days during cooling season and every 60 days otherwise. A pleated MERV 8 to 11 filter changed every 90 days is better, and that range is what we recommend for most homes because it filters well without choking airflow. Set a reminder on your phone.
2. Annual coil cleaning
The indoor coil gets cleaned at the tune-up. We use a foaming non-acid cleaner, rinse with the system off and let it drain out the condensate line. The outdoor coil gets rinsed from the top down at garden-hose pressure to clear the season's debris. Both happen during one maintenance visit.
3. Treat the drain line
Every 3 months, pour half a cup of distilled white vinegar down the drain access port at the air handler. It kills biofilm before it can clog the line. Some people prefer pan tablets that release a biocide slowly. Either works.
4. Verify the float switch
The float switch in the drain pan shuts the system off if water backs up. Test it once a year. It's an inexpensive part, and it prevents thousands of dollars in water damage along with the wet conditions mold loves.
5. Run the system properly
Going on vacation? Set the AC to 78 to 80 and leave it on. Two weeks at 90 degrees and 80 percent humidity is exactly when mold moves in. A smart thermostat that holds 80 degrees with a 50 percent humidity setpoint while you're gone is even better.
6. Address oversizing
An AC that short-cycles in a house that stays humid while it runs is probably oversized. That gets fixed at replacement time, by sizing the new equipment with a Manual J load calculation. Variable-speed equipment helps too, since it can run long, low cycles that pull moisture out.
7. Seal the ducts
Aerosealing, or replacing leaky flex with sealed metal trunk lines, stops the attic-air problem. Consider it at replacement time, or sooner if a duct test shows leakage above 15 percent.
8. Whole-home dehumidifier in some cases
Some homes run cool and still never get under 60 percent. For those, a whole-home dehumidifier ducted to the return is the answer. Installed, figure $2,400 to $3,200 and up. You get comfort out of it, and the house stops offering mold the conditions it needs, without being overcooled.
9. UV light at the coil (in some cases)
A UV-C lamp installed at the coil keeps biological growth off the coil surface and the drain pan. It does that job well. Despite some of the marketing, it does not sterilize the air. We suggest it as a maintenance aid for high-risk households with allergies or asthma, or someone immunocompromised.
What to do if you already have mold
- Have us inspect. We can usually find the source within 30 minutes and tell you what is in the HVAC and what is broader, like a wall cavity or the attic.
- If the mold is on the coil or blower or in the drain pan, we clean and treat it. That can mean pulling and cleaning the coil, removing and cleaning the blower wheel, replacing the pan and treating the drain line, with a UV light if it makes sense.
- If duct liner is contaminated, we replace those sections. Cleaning contaminated flex duct rarely holds up.
- Fix the cause, whether that's sizing, humidity, the drain or duct leaks. Otherwise the mold comes back.
- If the contamination is heavy or someone has health concerns, we bring in an indoor air quality testing firm to confirm the cleanup worked.
A musty smell at startup never fixes itself. We'll find the source and tell you what it takes to clear it. Call 281-992-7866 or book online.