Guide

Mold in HVAC Systems and Ductwork

The coil is the wet part, so the coil is where it starts

An air conditioner cools by running warm indoor air across a cold evaporator coil. Water condenses out of that air onto the coil, runs into a drain pan and leaves through a condensate line. That gives you a permanently damp metal surface with a continuous supply of dust blowing across it in the dark, which is close to ideal growing conditions.

Two failures turn that from normal into a problem. A clogged condensate line backs water up so the pan stays full, and a filter that is undersized, cheap or overdue lets dust cake onto the coil fins and hold moisture there. Both are inexpensive to correct and both are widespread.

Ducts in an unconditioned space are the second cause

A lot of Adams County homes have supply runs through an unconditioned attic or crawlspace. In summer, cold supply air inside a poorly insulated duct turns the outside of that duct into a cold surface sitting in humid air, and it sweats. In winter the same run leaks conditioned air into the space around it. Either way the insulation gets damp and stays damp, and the duct becomes a moisture problem rather than just a delivery pipe.

Flex duct with a torn outer jacket is the common version. Once the vapor barrier is open, the insulation inside takes on water and there is no way to clean it. That section gets replaced rather than cleaned.

How to tell a real HVAC problem from a scare

Duct cleaning is one of the most aggressively upsold services in the trade, so it is worth knowing what actually indicates a problem.

• A musty smell that appears within a minute of the system starting and fades when it stops, which points at something in the airstream rather than in the room
• Visible growth on the coil, in the drain pan, or on the inside surface of the plenum near the air handler
• Standing water in the drain pan, or a condensate line that does not drip outside on a humid day
• Water staining on the ceiling directly below an attic air handler
• Symptoms in the household that consistently track with the system running

A photo of a dirty duct interior on its own is not diagnostic. Every duct in every house has dust in it.

Fiberglass duct board cannot be cleaned the way sheet metal can

Sheet metal duct has a hard, non-porous surface. It can be cleaned, dried and put back in service. Internally lined duct board is porous, and once growth is established in it, cleaning removes what is on top and leaves what is inside. Material like that is removed and replaced. This distinction matters because it is the difference between a few hundred dollars of cleaning and a duct replacement, and it is worth knowing which one you have before the quote arrives.

An oversized air conditioner causes mold

This one surprises people. Dehumidification is a function of run time, not of capacity. A unit that is too large for the house satisfies the thermostat in a short burst and shuts off before it has pulled much moisture out of the air. The house ends up cool and clammy, indoor humidity sits well above the 30 to 50 percent range the EPA recommends, and condensation appears on cold surfaces throughout the building. If a house feels cold and damp at the same time, oversizing is a genuine suspect.

What we do with the HVAC side of a job

During remediation the system is normally shut down and the affected supplies and returns are sealed, so that running the fan does not distribute spores through the rest of the house. Once the room work is done, we look at the coil, the pan and the drain line, check the filter size and rating, and check any duct running through an unconditioned space. Where the mechanical work is beyond cleaning, that is an HVAC contractor's job and we will say so rather than quoting it.

See preventing mold for the humidity targets, and signs of mold for what to look for elsewhere in the house.

Common questions

Is duct cleaning worth paying for?

The EPA's position is that duct cleaning has never been shown to prevent health problems and should be done when there is a specific reason, such as visible growth inside the ducts, vermin, or heavy debris. If there is visible growth, the cause has to be found and fixed too, or the ducts will be dirty again.

What is the black stuff around my ceiling vents?

Often it is not growth at all. Dark deposits at a supply register are commonly soot and dust settling where cold air meets a warmer surface, a pattern sometimes called ghosting. It is worth looking at, but it is not automatically mold.

Do UV lights in a system stop mold?

A UV lamp aimed at the coil can suppress growth on the coil surface, which is genuinely where the problem usually starts. It does nothing for growth already inside a duct run or for a drain pan that is not draining, so it is an add-on, not a solution.