Before stopping AC use in winter: check the unit condition first, then arrange maintenance
Contents
As autumn and winter arrive, temperatures gradually cool and the air conditioner that has been running nonstop through the whole summer can finally rest for a while. Many Hong Kong households simply turn off the power, thinking that since the air conditioner will not be used for several months in winter, there is no need to spend money or time maintaining it, and plan to wait until the weather warms up next May or June before rushing to find someone to clean it. By the time spring arrives or the summer reopening comes, a pungent sour mouldy smell shoots out of the vent, sometimes with black mould flakes falling out, while the airflow is weak and the pipe leaks water frequently.
Many users are often puzzled: the AC was not used for an entire winter, so why has it become even dirtier? In fact, leaving an AC that has accumulated a whole summer of skin flakes, cooking fumes and fine dust untouched, allowing grease and moisture to ‘overwinter’ inside the sealed unit, carries hidden mechanical corrosion and health risks that are far more serious than expected.
If it's not used in winter, does it not need cleaning? The truth about leftover organic matter and mold breeding inside the unit over winter
Many Hong Kong residents have a blind spot when caring for household appliances: they assume that an appliance only gets dirty when it is turned on, and that when it is off it stays still and clean. However, in the context of air conditioning operation and the micro-environment inside the unit, this is a highly dangerous misconception. Hong Kong’s hot and humid summers mean that over months of continuous cooling, the indoor unit acts like a powerful dust collector and dehumidifier.
As air circulates repeatedly in a room, fine skin flakes naturally shed by the body, hair, bedding lint, pet sebum, and small amounts of cooking fumes drifting from the kitchen into the living room and bedroom are continuously drawn into the unit. When these organic impurities meet the condensation on the surface of the evaporator fins, they quickly adhere to the fin gaps, the cross-flow fan blades, and the lower drain pan. When the unit is switched off suddenly in autumn, the remaining moisture cannot be drained in time, and the inside of the unit forms a sealed microenvironment at room temperature with high humidity and abundant organic nutrients.
During the four- to five-month winter period, fungal spores such as black mould and penicillium can spread deep inside the dark interior of the indoor unit, turning the entire AC into a huge breeding ground for mould.
Why does the AC smell even worse after being left unused all winter? Uncover the acidification and fermentation of grease sludge and the hardening of the fan wheel into clumps
Many neighbours are hit by a strong sour smell the first time they switch on the AC in early summer, often leaving them dizzy and overwhelmed. This involves the 'chemical acidification and fermentation' of organic sludge. The skin flakes accumulated inside the AC are mainly made of keratin, while grease fumes are rich in various free fatty acids. When these organic contaminants mix with condensed water and ferment for months inside a sealed unit with little air circulation, anaerobic microorganisms break down the proteins and fats, releasing trace volatile organic acids, sulphides, and amine gases with a strong pungent irritant smell, creating a heavy sour, stale odour.
A more serious physical consequence is the drying and hardening of oily sludge. In the relatively dry microclimate of winter, the damp mould sludge attached to the back of each small plastic blade of the cross-flow fan (blower drum) gradually loses moisture and hardens with dust, forming a black, armour-like biological mass. This hardened deposit not only greatly increases the fan's rotational air resistance and motor load during operation, causing a sharp drop in airflow, but also destroys the fan's factory dynamic balance;
When it is switched on at high speed again next spring, hardened mould may break apart and be scattered like black snow over the sheets and sofas in the whole home, and may even cause severe shaking and noise faults in the unit.
Use supply air to dry it, or remove and wash the filter?
Before routine shutdown, you can follow the standardised protection steps below. First, fully dry the moisture inside the unit: on a dry day before proper shutdown, switch the AC mode to fan mode only and set the fan speed to maximum, then let it run for two to four hours. Use the moving air to dry and evaporate residual moisture on the evaporator fins and fan wheel surfaces, removing the water source needed for mould growth. Next, remove and wash the external dust filter: take out the front plastic filter, rinse off surface dust with room-temperature water and a soft brush, then let it dry completely in a cool, shaded place before reinstalling it.
Third, cut off standby power by turning off the dedicated wall power switch, and remove the dry batteries from the remote control for safe keeping to prevent moisture from causing leakage and corrosion of the metal contacts over time. However, do not buy off-the-shelf no-rinse chemical foam spray and blindly spray it into the air outlet. Without thorough rinsing with high-pressure water, the remaining chemical surfactants can mix with deep grease and sludge into a sticky paste, which will instead speed up oxidation and corrosion of parts. If the fan wheel already has extensive black stains inside, wiping with a cloth will not reach the gaps; it should be handled by a professional technician.
Clean it when winter starts, or wait until next summer: which AC maintenance habit best protects the unit?
Different maintenance timing has a decisive impact on the lifespan of air-conditioner parts, power consumption, and hygiene and health. To help Hong Kong families plan appliance maintenance with better reference, the following is a detailed comparison of deep cleaning before winter and leaving it for urgent cleaning in summer, so you can accurately assess which strategy best suits your unit's condition:
AC Cleaning Timing and Maintenance Habits ❄️
Component maintenance and anti-corrosion performance 💡
Booking schedule and service experience ⚠️
Professional advice strategies from the AC cleaning team ✅
Deep cleaning before winter shutdown (recommended)
Remove seasonal dust, grease and grime, and help prevent acidic substances from corroding the aluminium fins and copper pipes
Autumn and winter are off-peak service seasons, so technicians have more available slots and can work carefully and thoroughly.
Strongly recommended to keep the unit clean over winter, helping prevent mould and corrosion.
Left unused all winter and left until summer for an urgent clean
Oil and sludge ferment inside the unit for months, causing dirt to harden into clumps and mould roots to penetrate deeply.
The schedule is extremely tight during the peak season in May and June, and you often have to wait for several weeks
It can easily cause the fins to oxidise and turn black, and the unit may give off a bad smell when switched on again the next year
Only cleaning the surface dust filter yourself
Zero cost, keeps the AC’s front air intake clear
Heavy residue remains on the fan wheel and drainage pan, so the anti-mould effect is limited
Suitable as a basic supplementary care routine every two to three weeks
Never cleaned for years until dripping and breakdown
The radiator fins are sealed by heavy dirt, and the compressor has been running under overload for years
Often accompanied by serious water leakage and tripping, with heavy repair and electricity costs
Highly not recommended; one of the main reasons for reduced AC performance
As shown in the table above, leaving dirt and oily sludge inside the air conditioner over the winter can easily cause corrosion of the heat exchanger fins and deep-set mould hardening. Taking advantage of the off-season before winter to carry out a deep clean can avoid the rush of summer and help the unit maintain the best heat-exchange efficiency for longer.
Seek help from a professional team
If these acidic organic contaminants are left trapped inside the unit throughout autumn and winter, the copper tubing and aluminium fins can oxidise, leading to tiny pinhole refrigerant leaks. Rather than endure a musty smell and long waiting times next summer, take advantage of the off-season winter window and let our professional AC cleaning team handle the deep-cleaning work, remove mould and water-drip risks, and help keep the unit operating properly.
Frequently asked questions
Q1: If an air conditioner is not used at all for over a month in winter, why can the dirt inside harden and clump together?
A1: Because during summer there is continuous condensate inside the unit keeping it damp, the sludge is in a soft state. After the unit stops in autumn, the remaining moisture slowly evaporates, and without fresh water to rinse it away, the mixture of grease and dust gradually dehydrates and dries out, firmly sticking to the plastic fan wheel and metal aluminium fins, forming hard deposits that are extremely difficult to brush off by yourself.
Q2: After cleaning the air conditioner before winter, if it is not used for the whole winter, will it still collect dust over the months?
A2: it will not build up serious grime. After a deep clean, the inside of the unit is dry and free of water and organic matter, so mould loses the conditions needed to grow. As long as the indoor unit is covered with a breathable dust cover after cleaning and drying, airborne dust can be kept out, and the unit can still remain clean and fresh when used again in spring and summer the following year.
Q3: Why do many experienced technicians recommend cleaning the AC before autumn turns to winter, instead of waiting until summer?
A3: First, removing the grease and skin flakes accumulated over summer before winter helps prevent acidic substances from corroding the copper pipes and fins inside a sealed unit. Second, autumn and winter are the off-season for AC cleaning, so technicians have more time and booking is more flexible, allowing the deep clean and rinse process to be done more carefully and calmly, with better value for money.
Related reading and services
Need professional help?
Need to know about Residential air conditioning service? First check the service scope and arrangements, then provide the model and site details for enquiry