5 Things to Know Before Your First Aircon Chemical Wash

General servicing covers the basics — filter cleaning, a rinse of the coil surface, and a drain flush. A chemical wash goes further by dismantling the fan coil unit and soaking the evaporator coil and drain pan in a chemical solution that dissolves deep-set grime, mould, and biological buildup that a surface rinse cannot reach.

The distinction matters because a chemical wash addresses problems that accumulate inside the coil fins and the drain channel over extended use. If your unit has only been in service for a short period with regular general servicing, a chemical wash may not add much. It becomes relevant when cooling drops despite clean filters, or when odour persists after a general service.

The technician removes the indoor unit's front panel, fan blower, and drain tray to expose the evaporator coil fully. The coil is then soaked in a chemical cleaning solution — typically alkaline-based — that breaks down grease, mould, and oxidation that has bonded to the aluminium fins. After soaking, the coil is flushed with pressurised water to remove the dissolved residue.

The drain pan and blower wheel are cleaned separately. Once everything is reassembled, the technician runs the unit to confirm airflow and drainage are working properly. The process requires the unit to be off for the duration and creates some water runoff, so the area around the indoor unit needs protection.

A chemical wash for a single indoor unit typically takes between one and two hours depending on how dirty the coil is. A system with multiple indoor units takes longer because each fan coil is dismantled and cleaned individually. The technician needs access to the indoor unit and a power point nearby, plus space to lay out the parts.

Expect the unit to be out of service during the cleaning. Some water and chemical runoff is normal — technicians use trays and sheets to catch it, but the area directly below the unit may get damp. If your unit is installed above furniture or electronics, move those items beforehand.

The most noticeable change is airflow. A coil clogged with grime restricts the air passing through it, and once that buildup is removed, the fan can push air through more freely. Cooling usually improves because the coil can absorb heat more efficiently when the fin surfaces are clean.

Odour reduction is the other common outcome. Mould and bacteria growing on the coil and drain pan are the primary source of musty smells from the aircon. A chemical wash removes that biological layer. The improvement should be immediate — if the smell returns within a short period, the mould source may be deeper than the coil, such as inside the ductwork or the drain line.

A chemical wash addresses dirt and biological buildup on the coil and drain components. It does not fix mechanical or electrical faults. If the cooling loss is caused by low refrigerant, a failing compressor, or a faulty expansion valve, the chemical wash will clean the coil but the underlying problem remains.

The same applies to water leaks caused by a cracked drain pan or a broken condensate pump. Cleaning the drain path helps if the blockage is biological, but structural damage to the drain components requires repair or replacement. A technician should assess the root cause before recommending a chemical wash as the solution.

If your unit has not been serviced in a long time and the cooling has dropped gradually alongside a musty smell, a chemical wash is a reasonable starting point. The symptoms match the kind of buildup that a general service cannot remove.

If the cooling dropped suddenly, the unit makes unusual sounds, or the problem started right after a servicing visit, the cause is more likely mechanical or related to refrigerant. In those situations, a diagnostic check is more useful than a chemical wash. The diagnostic tells you what is wrong; the chemical wash only cleans what is dirty.

If you are booking a chemical wash for the first time, mention how long it has been since the last service and what symptoms you are experiencing. That information helps the technician prepare the right solution strength and allocate enough time for the job.

After the wash, pay attention to the airflow and cooling over the following days. A well-done chemical wash should produce a clear improvement. If the symptoms return quickly, something beyond dirt is driving the problem, and a follow-up diagnostic is the logical next step.