Musty smell from chiller AC vents — what causes it?
We're on district cooling (chilled water) in Abu Dhabi and lately there's a damp, musty smell every time the AC starts up. It fades after a while but it's unpleasant. Is this dangerous, and what's the fix?
3 answers
That smell is almost always biofilm / mould in the FCU (fan-coil unit) — specifically the coil and the drain pan where condensate collects. In a chilled-water system the coil is cold and damp all day, which is perfect for microbial growth. It's a hygiene issue more than a danger, but it can bother allergies.
The fix is a proper FCU service: clean the coil, disinfect and clear the drain pan and condensate line (often partially blocked, which is why water sits and smells), and replace/clean the filter. A blocked condensate drain is also the usual cause of that other classic complaint — water dripping from the ceiling near the AC. Ask specifically for drain-pan cleaning; a filter-only service won't touch the smell.
Had exactly this. The technician showed me the drain pan and it was… grim. Cleaned out and disinfected, smell gone the same day.
If the musty smell seems to come from the vents/ducting rather than just the unit, it's worth checking whether the ducts are sweating — there's a newer thread on AC ducts condensing and dripping in the ceiling where failed duct insulation lets the cold surface sweat in the humid ceiling void, which then breeds the mould you smell through the vents. Different cause from the coil/drain biofilm, and the fix is above the ceiling.
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