AC Odour

Situation and takeaway

In most cases it is not a refrigerant leak and not a broken machine. The usual cause is condensation on the cold evaporator coil during cooling, which mixes with dust, skin flakes and cooking grease. In the dark, damp inside of the unit this builds into a biofilm; microbes release odours that smell mouldy, sour or like dirty socks. To remove the smell you need to clear the film, dry the unit and control humidity — fragrance sprays and deodoriser stickers only mask it.

Hong Kong is hot and humid, so both split and window units are commonly affected. The smell is often strongest just after start-up, or when the set temperature is around 27°C and the compressor is not cooling (fan-only mode).

Smell checklist

Smell | Most likely source | DIY or call a tech Mouldy | Dust and moisture on coil / blower; biofilm | Wash filters + fan-dry first; if strong, deep clean Sour / dirty-sock / urine-like | Damp organic soil; microbes release acids and ammonia (often called dirty-sock syndrome) | Surface clean OK; lasting sour smell → clean blower and drain pan Drain / sewer | Blocked condensate drain or bad connection | You can clear a mild blockage; recurring smell → inspect Toilet / sewer gas | Drain tied into foul waste without a proper trap; gas backs up | Sprays will not finish the job re-pipe Burning / electrical | Wiring or motor overheating risk | Stop and unplug at once; call an electrician / tech New-plastic | New parts | Ventilate; if harsh after 1–2 weeks, ask the dealer

Do this now

1. Remove filters per the unit’s instructions, rinse dust from the back, air-dry fully before refitting (do not refit damp; do not bake in the sun). 2. Run fan only for 1530 minutes to dry the cabinet. 3. Switch to cool and check that the outdoor drain line drips; no water plus indoor dripping may mean a blockage or bad joint. 4. Before every shut-down, run fan only 15–30 minutes again; in humid weather 30–60 minutes.

Methods that actually help

A. DIY surface clean What you can do yourself is mainly the filters, visible coil surfaces, supply vanes, and a mildly clogged drain line. Remove and refit filters per the unit’s instructions, and only put them back when fully air-dried. When wiping visible surfaces and vanes, be gentle — do not force the casing open or dismantle internal parts yourself. Do not DIY-disconnect the power or cover the circuit board.

If the drain is only lightly blocked, start with an enzyme drain cleaner (follow the label), wait, then flush with water. White vinegar only helps light scale and is a weak disinfectant. For stronger disinfecting/odour control on the drain line, you may use household bleach diluted about 1:99, pour a small amount into the drain, wait, then flush with clean water. Do not use neat or thickened bleach, do not use caustic drain powder or foaming uncloggers, do not mix bleach with vinegar/acids (chlorine gas), and do not blast aluminium fins with bleach. Foam cleaners only reach the surface; the cross-flow blower and deep biofilm usually need a technician.

B. Choosing cleaners Prefer a neutral / mildly alkaline air-con foam cleaner (e.g. 3M Scotch air-con foam): spray per label, wait about 10 minutes, wipe with a damp cloth or soft brush. Labelled “disinfectant: no” means it is a cleaner, not a certified disinfectant. Bleach may be used at about 1:99 in a small amount down the drain, or for soaking filters; rinse well and air-dry; do not blast aluminium fins, boards, or mix with acids. Fragrance products only mask odour. Registered coil disinfectants are for pros and must be metal-safe — ask your tech rather than hunting registration numbers yourself.

Also: consumer advice in Hong Kong is to clean filters and vents at least every two weeks in summer, and book a professional deep clean before the season. US EPA suggests indoor relative humidity around 30–50% and not staying above 60% long-term; killing mould is not the same as removing it — debris still needs clearing.

C. When you must call a tech Sour smell after washing filters, black specks in the airflow, weak airflow, dripping, more than a year without a deep clean, or infants / elders / allergy at home. Pros must clean evaporator fins, cross-flow blower, drain pan and drain line. Filters only is surface work. Sewer smell from a foul connection without a U-trap needs re-piping. Burning / electrical smell: stop at once — do not open the compressor, add refrigerant, or use unknown chemicals.

Upkeep checklist

When | What Every shut-down | Fan-dry 15–30 min (30–60 in humid weather) Every 2 weeks in summer | Wash filters, wipe vanes Monthly | Check the drain runs clear Season restart | Fan-only for a few hours first Before each summer | Professional strip-clean; pets / roadside / frequent smell → every 6 months Day to day | About 2526C room set

Three lines to remember

1. AC smell comes from biofilm and standing water — not refrigerant. 2. Masking does nothing; drying and removing soil works. 3. You can wash filters; the blower, drain pan and drain line clear sour smells.

Related service: air-conditioner cleaning.

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