What to know
Where to look
- Around the indoor coil and the drain pan, which is the dampest part of the system.
- Inside register openings, particularly on the return side.
- On duct surfaces in a crawl space or unconditioned basement.
- Anywhere insulation has been wet, which is often visible as staining or compression.
Take the register cover off and use a torch. Photograph what you see, because comparing photographs over time tells you more than a single look.
What gets mistaken for it
Ordinary dust and lint accumulation, which is grey and fibrous rather than patchy. Surface corrosion on older metal ducting. Staining left behind from moisture that has since dried. Dark insulation fibres at a boot.
A dark patch is not a diagnosis. If it matters to you, testing exists, and it is a different service from cleaning.
What the EPA actually says
The EPA's own guidance is worth knowing here, because it is more cautious than most advertising: duct cleaning has never been shown to prevent health problems, and studies have not conclusively shown that particle levels in homes rise because of dirty ducts. What the EPA does list as reasons to consider cleaning are substantial visible mould growth inside the ducts, vermin infestation, and ducts so clogged that debris is actually being released into the room.
The part that matters more
Mould needs moisture. The EPA and the CDC both frame mould control as moisture control, and the CDC's household guidance is to keep indoor relative humidity below roughly 60 per cent and dry water-damaged material within 24 to 48 hours.
So the useful question is not only what is growing, but where the water is coming from — a condensate drain that is not draining, uninsulated duct running through humid space, a crawl space that stays wet, or a leak somewhere else that found the ductwork.
What we can and cannot tell you
We can show you what is inside the accessible ductwork and what condition it is in. We are not a testing laboratory and we do not diagnose what a substance is, and any company telling you confidently what is growing in your ducts without testing is telling you something they do not know.
If what you have is a moisture problem, cleaning the ducts treats the symptom and the moisture will keep doing what it was doing.
Where to start
What is actually in your system is a question about your house, and it takes a look rather than a guess.
Our inspection is free, we will show you what we find, and if the honest answer is that nothing needs doing yet, that is what you will get told.