Musty Smell from Vents: Causes and How to Fix It

When you turn on your HVAC and smell something musty, mildewy, or like dirty socks, something is growing where it shouldn't.

What Causes Musty HVAC Smells

Mold or Mildew on the Evaporator Coil The evaporator coil gets wet during cooling. If drainage is poor or humidity is high, mold can grow on the coil.

Moisture in Ductwork Condensation in ducts, especially in humid areas or from poorly insulated ducts in unconditioned spaces, creates conditions for mold growth.

Dirty Evaporator Coil or Drain Pan Organic matter (dust, dirt, etc.) combined with moisture creates a breeding ground for mold and bacteria.

Clogged Condensate Drain Standing water in the drain pan becomes stagnant and produces odors.

Dirty Air Filter An old, damp filter can harbor mold and bacteria.

What to Do

DIY Steps 1. **Replace the air filter** immediately 2. **Check the condensate drain** for clogs or standing water 3. **Run the fan only** for a while to dry things out 4. **Check for visible mold** near vents or on visible duct sections

Professional Cleaning If DIY steps don't resolve the smell, professional cleaning is needed: - Evaporator coil cleaning - Drain pan cleaning and treatment - Duct inspection and cleaning if necessary

Addressing the Root Cause Beyond cleaning, address why moisture is accumulating: - Ensure proper drainage - Check that the AC is removing humidity properly - Improve ventilation if humidity is high - Fix any duct insulation issues

When It's Serious

If the smell is strong or you see visible mold, don't ignore it. Mold can affect indoor air quality and health, especially for people with allergies or respiratory conditions.

If you suspect significant mold growth in ducts, professional remediation may be needed.

Prevention

• Change filters regularly
• Schedule annual maintenance (coil cleaning is included)
• Run the fan a few minutes after AC cycles off to dry the coil
• Keep condensate drains clear
• Address humidity issues in your home