What to know
Where it comes from
- Condensation on ducts running through an attic, crawl space or garage, where the surface is cold and the surrounding air is not.
- Humid air pulled in through leaks on the return side, particularly where returns run through a crawl space.
- A blocked condensate drain or a drain pan that has rusted or shifted out of level.
- Water coming from somewhere unrelated — a roof, a plumbing run, or a foundation — that finds the ductwork on the way.
Why it matters mechanically
Damp inside a duct gives dust something to stick to. What would otherwise move through the system settles, compacts, and narrows the run, which is a slow airflow problem that arrives without anybody noticing a single event.
Sheet metal that stays damp corrodes. Insulation that gets wet loses much of its value and does not reliably dry out in an enclosed space.
What actually keeps it out
- Insulate ducts that pass through unconditioned space, so their surface is not sitting below the dew point of the air around them.
- Keep the condensate drain clear and check the pan at the start of cooling season.
- Deal with leaks on the return side, which is where humid outside air gets pulled in.
- Run bathroom and kitchen extraction to the outside rather than into an attic.
- Keep gutters and grading doing their job, so the crawl space is not the wettest part of the property.
Signs worth following up
- Visible condensation or staining on duct exteriors in an attic or crawl space.
- Insulation that is compressed, sagging or discoloured on a run.
- A musty smell that is strongest when the system first starts.
- Standing water anywhere near the air handler.
What we would do
Look at the accessible runs and tell you what condition they are in, including whether insulation has failed and where the connections are letting air in. If the ductwork is holding material that has settled and compacted, cleaning it is a genuine improvement.
But if water is still getting in, cleaning is the wrong end of the problem. We would rather point you at the cause than take payment for treating the symptom, because you would be calling somebody again next season.