Is There Mold in the Ductwork?

Air handling systems create the conditions mold needs at one specific point, and it is not the ducts.

Where it actually grows

The evaporator coil. Cooling air below its dew point condenses water on the coil by design. A coil that stays damp between cycles, with dust settled on it, is an ideal substrate.

The drain pan. Standing water, particularly when the drain line is partly blocked. Any pan holding water between cycles is a problem.

The blower wheel and housing, which accumulate damp dust.

Insulation inside the air handler, especially older fibreglass liner that has become wet.

Ducts, genuinely, but usually only where they run through unconditioned space and sweat, or where a leak has wetted the lining.

The smell on start-up

A musty odour in the first minute of operation that fades afterwards points strongly at the coil.

It happens because the coil is wet at shutdown, growth develops on it overnight, and the initial airflow carries the products of that growth through the house. Nothing in the ducts produces that timing pattern.

What to check

Look at the coil, if it can be accessed. Uniform grey dust is normal; black or green patchy growth is not.

Check the drain pan for standing water and the drain line for a blockage. A pan that never empties is the single most productive thing to find.

Check the filter. A filter that is old, wet, or bypassed at the edges lets dust onto the coil, which is what growth needs.

Look for duct runs sweating in an attic or crawlspace, which indicates missing or wet insulation.

Fixes that hold

Clear the condensate drain, and keep it clear. This is routine maintenance that almost nobody does.

Change filters on schedule, using the correct size so air does not bypass them.

Improve run time. Short cycling leaves the coil wet without moving enough air to dry it. An oversized system is a common underlying cause of coil growth.

Control indoor humidity. A system that cannot keep the house below about 55 per cent relative humidity is fighting a losing battle, and a whole-house dehumidifier is sometimes the honest answer.

Insulate ducts in unconditioned space, so they stop sweating.

Duct cleaning

Worth doing when there is documented growth in the ducts, after a renovation that filled them with dust, or where vermin have been in them.

Not worth doing as a routine measure or as a response to a musty smell, since the smell nearly always originates upstream of the ducts. A contractor who proposes duct cleaning without inspecting the coil and pan is selling the wrong service.

UV lights

Marketed heavily. Properly sized and positioned lamps aimed at the coil surface do inhibit growth there, and general-purpose in-duct lamps do considerably less than the marketing suggests.

The coil still needs to be clean and the drain still needs to work. A lamp is a supplement to that, not a replacement for it — and the underlying principle is the one in a musty smell with nothing visible: find the wet surface rather than treating the air.

Testing the air

Frequently offered alongside duct cleaning and rarely useful, for the reasons set out in mold testing kits: spores are present in every building, and a count without a reference point does not identify a source.

Inspecting the coil takes ten minutes and answers the question a sample cannot.