Fault-finding
The AC smells musty when it starts
A damp musty smell is biological growth on the indoor coil and in the drain tray, which our humidity encourages every monsoon. A burning smell is electrical and means switching off immediately. The first is a cleaning job; the second is not something to run and see.
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Musty means the coil and tray need cleaning
The indoor coil stays wet whenever the unit runs, and in a humid season that surface plus trapped dust grows mould and bacteria. The smell is strongest at start-up when air first moves across it. A proper deep clean of the coil, blower wheel and tray removes it; spraying an air freshener into the vent does not.
Why filter cleaning alone does not fix it
Filters are the easy part and most people do clean them. The growth is behind them on the coil fins and inside the blower wheel, which needs the unit opened and washed properly. This is the difference between the quick service that gets sold and the deep clean the unit actually needed.
A burning smell is different in kind
Hot plastic or electrical burning means switching the unit off at the isolator and leaving it off. Overheating windings, a failing capacitor or a scorched terminal are what produce it, and continuing to run the unit risks the fault progressing. This is not a wait-and-see symptom.
Prevention that actually works
Run the unit in fan mode for a few minutes after cooling, so the coil dries rather than sitting wet. Keep the drain clear so the tray is not holding standing water. Two proper services a year — before the season and after the monsoon — is what keeps it from returning.
Why does my AC smell bad?
Is a musty-smelling AC a health problem?
It can be for anyone with asthma or allergies, because the unit is circulating spores from the coil through the room. It is not an emergency, but it is a genuine reason to get the coil cleaned properly rather than living with it — and it is one of the few AC faults where the health case is stronger than the comfort case.
Will an antibacterial spray fix the smell?
Only briefly, and it treats the symptom. The growth is physically on the coil fins and inside the blower wheel, so it needs washing off rather than covering. Sprays sold for this generally mask the smell for days and then it returns, which is a reasonable way to get through a week but not a fix.
Why does it only smell for the first few minutes?
Because the coil is wet and still when the unit is off, and the first air across it carries the accumulated smell into the room. Once airflow is established the concentration drops. That pattern is a reliable sign it is coil growth rather than something in the ducting or the room.
Rather have it done?
A service is a deep clean of both coils, the filters and the drain, plus a set of measurements that show whether the unit is actually healthy. Servicing starts at PKR 3,000, and gas refilling starts at PKR 6,000 — but a unit needing gas has a leak, and refilling without finding it is a yearly bill, not a repair.
AC servicing and gas refilling in Islamabad — prices and scope