5 Reasons Your Aircon Smells Even After Servicing

A general service focuses on the filter, the visible coil surface, and the drain line. These are the areas that accumulate the most dust and affect airflow. But odour-causing bacteria and mould can live in places a standard service does not touch — deep inside the coil fins, the drain pan grooves, the blower wheel, and the ducting behind the unit.

If the smell was caused by a dirty filter, a general service will fix it. If the source is deeper, the smell returns as soon as the mould or bacteria starts producing again — often within days.

The evaporator coil has hundreds of thin aluminum fins packed closely together. Dust and organic matter work their way between the fins over time. A wipe or brush during a general service cleans the front face but does not penetrate the fin gaps where mould colonies establish themselves.

This is the most common reason a smell persists after servicing. The coil looks clean from the outside, but the source of the odour is between the fins where airflow carries it into the room. A chemical wash dissolves the embedded matter and flushes it out through the drain.

The drain pan sits underneath the evaporator coil and collects condensation. It stays damp constantly, which makes it an ideal surface for mould and biofilm. During a general service, the drain line is flushed, but the pan itself is not always scrubbed — especially on units where the pan is difficult to access without partial disassembly.

A mouldy drain pan produces a stale, damp smell that gets pulled into the airflow. If the smell is strongest when the unit first starts up and fades after a few minutes, the drain pan is a likely source. The initial burst of air picks up the stagnant moisture and carries the odour into the room.

In units that have gone years without a chemical wash — or units in kitchens, smoking environments, or high-humidity rooms — the coil fouling goes beyond normal dust. Grease, nicotine residue, and thick biofilm bond to the coil surface and resist even a chemical wash applied from the front.

When this happens, a chemical overhaul is the next step. The indoor unit coil is removed from the housing, soaked in chemical solution, and cleaned from all sides. This level of cleaning reaches areas that are completely inaccessible when the coil is mounted in position.

The drain line carries water from the indoor unit to an outlet — usually a floor trap or an external discharge point. Biofilm and bacteria can colonize the inside of the drain pipe, especially in horizontal sections where water sits between cycles. The smell travels back up the pipe and into the indoor unit.

This smell is often described as sewage-like or sour. It tends to come and go because it depends on moisture levels inside the pipe. A drain flush during servicing helps temporarily, but if the biofilm is well-established, it grows back. Enzymatic drain treatments or a more thorough mechanical flush can address it more permanently.

For ceiling-concealed and ducted systems, the ductwork behind the indoor unit can accumulate dust, mould, and debris over time. The air passes through the duct after leaving the coil, so even a perfectly clean coil sends air through a contaminated path. The smell enters the room from the duct outlet, not the unit itself.

This is less common in wall-mounted split units because they blow air directly into the room. But in any ceiling or ducted setup, the duct interior is worth inspecting if the smell persists after the unit itself has been thoroughly cleaned. Duct cleaning is a separate service from a coil chemical wash.

Identify the smell type and when it happens. A musty smell that is constant points to the coil. A damp smell on startup points to the drain pan. A sewage smell that comes and goes points to the drain line. Knowing the pattern helps you and the technician target the right area.

If your unit was recently serviced and the smell did not improve, let the service provider know. A good technician will want to understand why the issue persists. The answer usually leads to a chemical wash, a drain treatment, or — for severe cases — a chemical overhaul. Each targets a different depth of contamination.