Mold in Your AC Ducts: What to Do First

If you suspect mold in your air conditioning ducts, turn the system off before you do anything else. A running air handler is the fastest possible way to distribute spores through every room in the house — including rooms that had no moisture problem at all. Everything else can wait five minutes; that cannot.

How do you know there is mold in the ductwork?

Duct mold is usually diagnosed by smell and symptom before anyone sees it, because most of the run is out of sight. The signals worth taking seriously:

• A musty smell that gets stronger when the system runs and fades when it is off. This is the most reliable single indicator.
• Visible growth at the supply registers — dark speckling on the vent louvres or on the ceiling immediately around them.
• Symptoms that track the air conditioning: congestion, coughing or itchy eyes that improve when you leave the house or turn the system off.
• Standing water in the drain pan or a blocked condensate line at the air handler.
• A saturated or discoloured filter replaced far sooner than expected.

Why do AC ducts grow mold in the first place?

An air conditioning system is a dehumidifier that happens to also cool. As warm household air passes the evaporator coil, moisture condenses out. That condensate is supposed to drain away. When it does not — a clogged line, a tilted pan, a failed float switch — you have standing water sitting inside a dark enclosure at a comfortable temperature, with dust as a food source. Those are precisely the three conditions mold requires.

Two San Diego-specific factors make it more likely. Coastal humidity means air entering the system already carries moisture, so more condenses out. And a system that is oversized for the property cools the space quickly and shuts off before it has run long enough to actually pull humidity down — short cycling that leaves both the ducts and the house damper than they should be.

What should you do first? Five steps in order

1. Shut the system down. Off at the thermostat and, if you can do it safely, at the breaker. Stop moving air until you know what you are moving.
2. Do not clean the registers yet. Wiping or vacuuming visible growth without containment aerosolises it. It is the most common way a contained problem becomes a whole-house one.
3. Get it identified. Dust, soot and mold look similar at a vent. Testing tells you whether you are dealing with mold and what the concentration is, which determines the scope of work — and prevents you paying for remediation you do not need.
4. Find out what the ductwork is made of. Sheet metal can generally be cleaned and disinfected. Internally lined or flexible fibreglass duct is porous; once mold is established in the lining, that section usually has to be replaced rather than cleaned. This distinction drives most of the cost difference between two otherwise identical jobs.
5. Fix the water before you fix the duct. Clear the condensate drain, correct the pan, address the humidity. Remediation without solving the moisture source simply resets the clock.

Can you clean duct mold yourself?

A surface wipe of a metal register with detergent is within reason. Anything beyond that is not, for three reasons: you cannot see how far the growth extends, you cannot contain what you disturb, and porous liner cannot be cleaned in place regardless of technique.

The EPA’s general guidance is that mold growth covering more than roughly 10 square feet should be handled professionally. In a duct system, you almost never know whether you are above or below that line, because most of the surface area is inside a wall or ceiling. (Source: EPA — Mold Cleanup in Your Home)

The EPA also advises that if there is mold in your HVAC system, you should not run it, because doing so spreads contamination through the building.

Is it a health risk?

The CDC notes that damp and mouldy environments are associated with respiratory symptoms, and that people with asthma, allergies or weakened immune systems may react more severely. (Source: CDC — About Mold and Your Health) Duct-borne mold is worth treating promptly because it does not stay in one room — the system’s entire purpose is to move that air everywhere.

How do you stop it coming back?

• Keep indoor relative humidity between 30% and 50%; add a dehumidifier if the AC alone cannot hold it.
• Flush the condensate line at the start of each cooling season and confirm the drain pan empties and stays dry.
• Change filters on schedule, and use a filter with a MERV rating your system is actually specified for.
• Repair plumbing leaks and roof penetrations promptly — mold begins within 24 to 48 hours of material getting wet.
• Have the system inspected if it short cycles; an oversized unit is a humidity problem, not just an efficiency one.
• Insulate ducts running through hot attics so the surface does not drop below dew point and sweat.

Get the ductwork assessed properly

Smart Dry Restoration inspects and remediates mold across San Diego County following IICRC guidelines, with containment and documented moisture readings for insurance purposes. If the AC smells musty or you can see growth at a vent, turn the system off and call (858) 433-5100. Dispatch runs 24 hours a day.

Related reading: mold remediation in San Diego · what causes mold to grow