Why Does My AC Smell Musty? A Baton Rouge Guide

Where the smell comes from

The indoor coil

The evaporator coil is cold and wet every time the AC runs. Dust that gets past the filter sticks to it, and in Baton Rouge's humidity, that damp, dusty surface can grow mold and bacteria. The "dirty sock" smell many people notice when the system starts after a break often comes from here.

The drain pan and condensate line

Water dripping off the coil collects in a pan and drains away through a PVC line. If the pan is rusted, cracked or not sloped, or the line is partly clogged, water sits there and grows slime.

Ducts

Ducts that sweat, leak humid attic air, or were ever wet from a roof leak or flood can harbor growth. Return ducts that pull from a damp crawlspace or garage are another source.

The house itself

If indoor humidity stays high, mold can grow on closet walls, behind furniture and on registers, and the AC simply circulates the smell. The EPA recommends keeping indoor humidity below 60 percent (EPA, A Brief Guide to Mold, Moisture and Your Home).

What you can check

1. Replace the filter. A wet, dirty filter can smell on its own.
2. Look at the drain pan under the indoor unit if you can reach it safely. Standing water, slime or rust are clues.
3. Flush the condensate line with a cup of white vinegar through the cleanout.
4. Check the humidity with a hygrometer. Above 60 percent with the AC running points to a system or duct problem.
5. Look at the registers. Black spots usually mean condensation.
6. Set the fan to "auto." Running it continuously re-evaporates water from the coil.

How it is usually fixed

• Cleaning the indoor coil and blower, and cleaning or replacing the drain pan.
• Clearing and re-sloping the condensate line, and adding a cleanout.
• Sealing and insulating ducts; replacing ducts that were contaminated or wet.
• Correcting airflow and fan settings, or adding a whole-home dehumidifier.
• At replacement time, right-sizing the system for humidity.

Masking products and ozone generators do not fix the source. The EPA's guidance is to control moisture first, then clean up; extensive growth, more than about 10 square feet, or growth inside the HVAC system needs more careful work (EPA, A Brief Guide to Mold, Moisture and Your Home). Air cleaners can reduce particles but do not solve a moisture problem (EPA, Guide to Air Cleaners in the Home).

Other smells

• Burning or electrical: turn the system off at the thermostat and breaker.
• Rotten egg or sulfur: could be natural gas. Leave and call 911 and your gas utility from outside.
• Exhaust: a furnace venting problem or a car running in an attached garage. Get fresh air and check your carbon monoxide alarms.

Related services

• Humidity, Mold & Air Quality: Damp, musty houses, mold around vents, whole-home dehumidifiers, filters and air cleaners.
• AC Maintenance & Tune-Ups: Spring AC and fall furnace tune-ups, condensate line care, coil cleaning and filters.
• Ductwork & Airflow: Leaky and crushed attic ducts, flood-damaged ductwork, hot rooms and sweating registers.

Sources: EPA, A Brief Guide to Mold, Moisture and Your Home; EPA, Guide to Air Cleaners in the Home.