5 Things That Make Your Room Harder to Cool

Aircon sizing is based on room conditions — square footage, sun exposure, ceiling height, insulation, and internal heat sources. If any of these are worse than the assumptions baked into the BTU calculation, the unit will underperform even though nothing is mechanically wrong. The room is asking for more than the system was sized to deliver.

This is why two identical units in two identical-sized rooms can produce completely different comfort levels. The room facing west with full glass gets far more heat gain than the shaded interior room. Understanding these factors helps you decide whether the fix is a bigger unit, a room modification, or simply adjusted expectations.

In Singapore, the afternoon sun hits west-facing walls and windows hardest. Glass transmits solar heat directly into the room, and a west-facing window can add a substantial heat load that the original BTU calculation may not have accounted for. This is the single most common reason a room feels warm despite the aircon running properly.

Curtains and blinds help — thicker materials and lighter colours reflect more heat. Blackout curtains with a reflective backing are the most effective. Window film that blocks infrared radiation is another option that does not require curtains. The goal is to reduce the heat entering the room so the aircon does not have to fight it.

Condos with full-height glass panels look open and bright, but they also let in significantly more heat than standard windows. The glass surface area is larger, and unless the glass has a low-emissivity coating, it transmits solar radiation into the room throughout the day.

If the glass cannot be replaced, the most effective mitigation is a combination of solar film and heavy curtains. Running the aircon at a lower set temperature to compensate is possible but expensive — it forces the unit to work harder and uses more electricity. In some cases, upgrading to a higher-capacity unit is the more practical long-term solution.

An aircon sized for a bedroom will struggle if the bedroom door is left open to a larger living space. The unit cools the air in its immediate zone, but warm air flows in from the unconditioned area. The thermostat never reads the target temperature because the system is trying to cool a space much larger than it was designed for.

Open-concept kitchens connected to living rooms are the most common example. The living room aircon pulls the room temperature down, but heat from the kitchen — especially during cooking — flows into the cooled zone and raises the load. Closing a partition, using a kitchen exhaust fan, or sizing the aircon for the combined space are the practical solutions.

Gaps under doors, unsealed window frames, and thin walls that face direct sun all let warm air in and cool air out. In HDB flats, the front door and back service yard door are common leak points. Older sliding windows may not seal tightly. Even small gaps add up — the aircon replaces the lost cool air continuously, which keeps it running longer than it should.

Checking for air leaks is straightforward. Hold a thin piece of tissue near door edges and window frames while the aircon is running. If the tissue moves, air is escaping. Weather strips, door sweeps, and foam tape are low-cost fixes that can make a noticeable difference in how quickly the room reaches temperature and how long it stays there.

The outdoor unit releases the heat it has absorbed from the room. If the outdoor unit sits in an enclosed ledge with poor airflow, the hot exhaust recirculates back into the intake. The condenser cannot reject heat efficiently, which reduces the overall system capacity. The indoor unit keeps running but delivers less cooling.

This is common in condos where the outdoor unit sits in a recessed ledge with walls on three sides and another unit directly above or beside it. There is not much a homeowner can do about the ledge design, but keeping the area clear of obstructions — stored items, drying racks, plants — helps. If the ledge is severely enclosed, a unit with a higher efficiency rating can partially offset the penalty.