Texas summer humidity plus an AC system that runs for ten hours a day equals a perfect environment for the indoor unit to develop smells. Musty, dirty-sock, rotten-egg, burning — each one means something different, and a few of them are serious enough that you should turn the system off until they are diagnosed. Here is what each smell tells you and how to fix it.
Musty / Mildewy Smell — "Dirty Sock Syndrome"
What it smells like: A locker room, wet towels, or moldy basement.
What it actually is: Microbial growth (mold, mildew, bacteria) on the evaporator coil or in the drain pan. The evaporator coil pulls humidity out of the air all summer, and that moisture creates a continuously damp environment 18 inches from where dust is being deposited. Mold grows. The blower then pushes air over that mold and through your ducts.
Why it's especially common in Texas: Our humidity does not let the coil dry out between cooling cycles. Houses in coastal Texas (Galveston, Corpus) have it worst, but Pflugerville, Bastrop, Round Rock, Bryan, and the entire Central Texas corridor see it every summer.
The fix:
Replace the air filter if you cannot remember the last time. New filters reduce the dust feeding the mold.
Pour a cup of distilled white vinegar into the condensate drain line. Find the small access port on the line (usually a T-fitting near the indoor unit) and pour the vinegar in. It kills the algae and biofilm growing in the line.
Run the fan on AUTO, not ON. Fan ON keeps the blower running 24/7 and re-introduces moisture from the wet coil into your air. AUTO lets the coil dry between cycles.
For severe cases, call for a coil cleaning — a tech accesses the evaporator coil and treats it with a coil cleaner that kills mold and rinses away the biofilm. $200–$400.
If the smell comes back within weeks of cleaning, you may need a UV sanitizer installed in the air handler. UV-C light over the coil prevents microbial growth between maintenance visits. Installation runs $400–$700.
Rotten Egg / Sulfur Smell — STOP. CALL THE GAS UTILITY.
What it smells like: Sulfur, rotten eggs, or a sewer smell that comes through the vents.
What it might be:
1. A gas leak. Natural gas is odorless — utilities add mercaptan, which smells like rotten eggs, specifically so leaks are detectable. If you smell this from your AC vents and you have a gas furnace, gas water heater, or gas line near the air handler, treat it as a gas leak until proven otherwise. Get out of the house, leave the door open, call your gas utility from outside. Do not flip switches, do not run the AC, do not light anything.
2. A dead animal in the ductwork. Rats, squirrels, or raccoons sometimes get into ducts and die there. Decomposition produces sulfur compounds that smell similar to natural gas. The difference: a dead-animal smell tends to be localized to one room or one vent, while gas tends to be diffuse and stronger when an appliance fires. If you can localize the smell to one section of duct, that is the cause.
Either way, treat the rotten-egg smell as a gas leak first. It is cheaper to be wrong about a dead squirrel than to be wrong about a gas leak.
Burning / Electrical Smell — TURN IT OFF.
What it smells like: Hot wires, ozone, or melting plastic. Sharp and irritating, often only when the AC is running.
What it actually is:
1. Failing electrical component. Capacitor, contactor, or blower motor overheating. The insulation breaks down before catastrophic failure and produces the smell.
2. Wiring issue. Loose connection at the contactor or disconnect, often arcing intermittently.
3. Burning belt (older systems). Older blower motors had drive belts that could slip and burn. Rare on modern systems.
The fix: Turn the AC off at the thermostat AND at the breaker. Do not run the system again until a tech has diagnosed the source. An electrical fire in an attic-mounted air handler will spread to the whole house in minutes. Same-day diagnostic call. Capacitor and contactor replacements are $150–$300; blower motor is $400–$800.
Stale / Cigarette Smoke Smell from the Vents
What it actually is: Either the previous owner of the house smoked indoors and the residue is in the ductwork, or there is a current smoking source whose smoke is being pulled into the return air.
The fix: Duct cleaning is the most effective option. A professional duct cleaning runs $400–$1,000 depending on the size of the system. Sealing any return-air leaks (especially if the return duct passes through an attic) prevents future contamination.
Chemical / Sweet / Chloroform-Like Smell
What it actually is: Refrigerant leak. Modern refrigerants (R-410A, R-32) have a faint sweet or chemical odor. Most leaks happen at the evaporator coil or at fittings.
The fix: Refrigerant leaks are not just a smell issue — they reduce cooling capacity, freeze the coil, and damage the compressor. Get a tech out for a leak detection. Repair runs $400–$1,500 depending on where the leak is and whether the coil needs replacement.
Sewer Smell from AC Vents
What it actually is: The condensate drain line connects to your home's drain system. If the trap on that line dries out (common in Texas when the AC has not run for a few weeks), sewer gas backs up through the line and into the air handler, then out your vents.
The fix: Pour water down the condensate drain line to refill the trap. If it persists, the line may need a P-trap installed if it does not have one. $150–$300.
What to Do Right Now Based on the Smell
Musty: Replace filter, vinegar in drain line, switch fan to AUTO. Schedule coil cleaning if it persists. Not urgent.
Rotten egg / sulfur: Treat as a gas leak. Leave the house, call the gas utility from outside.
Burning / electrical: Turn off at thermostat AND breaker. Call same-day for diagnostic. Do not run again until inspected.
Sweet / chemical: Refrigerant leak. Schedule a leak detection — same-day if cooling has dropped.
Sewer: Pour water in the condensate line. Schedule a service call if it persists.
When to Call Us
Same-day AC diagnostic and odor remediation across Central Texas — Pflugerville, Bastrop, Taylor, Elgin, Georgetown, Hutto, Manor, Cameron, Rockdale, Brenham, and our full service area. Call (737) 260-7255. Texas license TACLB00027491E. Flat-rate pricing.