[Window Unit Dust Protection] Covered for winter but not cleaned first? A guide to avoiding mould growth
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As autumn and winter arrive, Hong Kong’s north winds begin to strengthen. To block the piercing cold air seeping in through the gaps around the air conditioner, and to prevent dust and vehicle exhaust from accumulating from the street outside, many families buy a plastic or cloth dust cover for their window unit from a home goods shop and simply put it on the machine for winter. By the time the weather turns warm again the following May or June, when the cover is lifted, what comes out is a pungent sour, stale mouldy smell;
After carefully shining a flashlight inside, the air outlet, guide vanes and even the internal foam were covered with tiny black-green mould spots, and the base tray still had sticky black muddy water stains! I had originally thought that putting on a cover would protect the appliance from wind and rain, but it instead turned the air conditioner into a sealed bacteria greenhouse. Before putting a dust cover on a window-type air conditioner for winter storage, why must it be thoroughly cleaned and completely dried first?
Thinking of using a dust cover for insulation and warmth? The hidden greenhouse trap of moisture not fully dried and becoming mouldy
Many Hong Kong residents, when they stop using the air conditioner in autumn and winter, casually put on a protective cover and overlook the unique integrated-structure trap of window-type units. A window air conditioner integrates the evaporator, compressor, blower wheel and base pan inside the same metal casing. During the entire summer of operation, the heat-exchange fins continue to condense water, which drips constantly into the base pan. In many Hong Kong buildings, window units are required to connect to a drain pipe, but the base pan is often designed so that splashing condensate helps cool the condenser. As a result, there is usually several millimetres of stagnant water left inside for a long time.
When the autumn temperature drops suddenly and the unit is switched off, water deep in the base pan and moisture inside the machine do not evaporate and dry naturally. If you then rush to cover the air conditioner with a non-breathable plastic dust cover, you are trapping a large amount of moisture, water droplets, and the body flakes and fine dust absorbed during summer in a dark, sealed space. As daylight heats the window frame and creates a greenhouse effect, the humidity inside the cover rises to near saturation, allowing anaerobic Aspergillus niger and Penicillium to multiply rapidly over the three to four months of winter, turning what was once a clean unit into a heavily mouldy breeding ground.
Why is the sour, stale smell even stronger when the unit is switched on in the second year? Uncover the blind spots of stagnant water in the drain pan and the moisture absorption of foam
Many residents are puzzled: why does the unit smell even worse when the cover is removed and the AC is turned on, although it did not blow dust while covered? To understand this properly, we need to examine a key insulation component inside window AC units: high-density foam. To prevent condensation on the casing caused by contact between indoor cool air and outdoor warm air, the air passages inside a window unit are lined with thick foam all around. The surface of the foam is full of tiny pores, which easily absorb oily particles in the air, food odours and moisture.
When an AC is sealed in a dust cover with a whole summer’s worth of dirt trapped inside, the mildly acidic gases released by stagnant water in the base will continuously seep into the deep pores of the foam plastic; at the same time, the sticky sludge on the heat-dissipation fins will sour and deteriorate without airflow. When mould filaments penetrate deep into the foam plastic and form irreversible black hardened patches, the whole unit becomes like a mouldy sponge soaked in sour water.
When the fan is powered on again next summer, the high-speed airflow will suddenly blow out mold spores and stale acidic odors that have been trapped deep inside the foam and blower wheel. Not only is the smell pungent, it can also easily trigger skin allergies and respiratory discomfort.
Use supply air to dry it, or wipe the casing? Know the safe DIY scope for anti-moisture checks before fitting the cover
For routine handling, you can follow the standard maintenance steps below. First is the key internal drying procedure: one to two days before fitting the cover for proper shutdown, choose a dry sunny day, switch the AC mode to fan mode only (Fan Mode) and turn the fan speed to the highest setting, then let it run for three to four hours. Use the moving air to dry surface moisture on the heat-exchange fins and residual dampness on the air duct surface. Next is removing and washing the external dust filter: rinse off fine dust on the filter with clean water, then let it dry completely in a ventilated shaded place before reinstalling it.
Third, choose a breathable dust cover. Do not use a sealed plastic bag. Use a thick non-woven cover with breathable micro-perforations so any small amount of internal moisture can dissipate naturally. Finally, remember to switch off the wall power and remove the remote batteries. However, do not try to clean a window AC unit by pushing it out of the window without safety protection. Also, never use a household showerhead to flood the inside of the unit. If water backflows into the circuit board or drips to the street below, it can cause short circuits, tripped breakers, and even complaints to government departments.
Winter protection methods for window units compared: how far apart are a simple cover and a deep clean?
There are different ways on the market to prepare a window-type AC for winter, and different procedures can vary significantly in anti-mould performance, component protection and air quality when the unit is turned on again next year. To give Hong Kong residents a clearer budget direction when maintaining their homes, the following provides a detailed comparison of common winter protection methods to help you accurately assess which strategy best suits your practical needs:
Winter protection mode for window units ❄️
Internal drying and anti-mould mechanism 💡
Common limitations and potential risks ⚠️
Professional advice strategies from the AC cleaning team ✅
Put a sealed plastic dust cover on directly
Blocks outside cold drafts and may seem clean and time-saving at first
Trapped water and moisture in the base pan can easily lead to heavy black mould and odours
Highly not recommended; the number one culprit behind black debris blowing out when switched on the following year
DIY air-drying with a breathable non-woven cover
Removes surface water; the micro-porous fabric allows moisture to escape
Old grease sludge and foam mould accumulated in the drain tray still remain
Suitable for newer air conditioners that have just had a deep clean in summer.
No cover at all; left idle for the whole winter
Helps maintain natural ventilation and makes sour odours less likely to build up.
It is easily affected by strong outside wind backflow, and dust can easily build up inside the unit
Suitable for those who are positioned in a sheltered location and plan a deep clean before next summer
Professional deep high-pressure cleaning before winter
Thoroughly flush out stagnant water in the drip tray, black grime on the fan wheel, and dry it completely
A professional needs to be booked for on-site work, with the relevant service fee paid.
Strongly recommended to remove season-long oil and dirt, ensuring a stable and dry winter
As shown in the table above, covering the unit with a plastic cover without cleaning and drying first often does more harm than good. Carrying out fan drying properly, or taking advantage of the off-season before winter for a deep clean together with a breathable dust cover, is the wise way to protect the unit and respiratory health.
Seek help from a professional team
If the air conditioner is sealed without cleaning, dirt and moisture trapped inside may cause parts to rust, and contaminants may be blown out the following year when it is used again. It is recommended to arrange a professional cleaning team to handle it before the season changes, to ensure the unit is clean and properly protected.
Frequently asked questions
Q1: After covering a window-type AC with a dust cover, can it be switched on directly if the weather suddenly turns hot in winter?
A1: Do not switch it on directly! When the dust cover is fitted, both the air intake and outlet of the AC are completely sealed. If you force it on for cooling without removing the cover, the compressor and fan motor will operate without proper heat dissipation or exhaust, which can easily trigger overheating protection or even burn out the motor. The dust cover must be fully removed before switching it on.
Q2: If I find that the inside of the window-type unit covered last year has already developed mould after being opened, how should I deal with it before turning it on?
A2: Do not blow air directly into it. First, use a slightly damp cloth to gently wipe off any visible mould on the air outlet surface, and remove and wash the dust filter. If a torch shows black spots on the fan wheel blades and foam, it means the internal mould has spread deep into the air ducts. It is recommended to stop using it and book a professional team for deep high-pressure washing and steam disinfection to prevent mould spores from spreading throughout the home.
Q3: When buying a dust cover, should I choose a full-cover type or an indoor half-cover type?
A3: For window-type units, it is recommended to choose an "indoor half-cover breathable cover" that only covers the indoor panel first. The outdoor casing needs to keep the bottom drainage holes clear so that occasional rainwater can drain out. If the outdoor section is completely wrapped tightly in plastic sheeting, rainwater may accumulate on rainy days and cause serious rusting of the casing.
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