Why is your west-facing room still hot with the aircon on?

Why is a west facing HDB hot in the afternoon?

Meteorological Service Singapore describes "high and uniform temperatures, and high humidity all year round". It notes prominent daily variations from hour to hour, "indicating the strong influence that solar heating has on the local climate". In the inter-monsoon period from late March to May, it says "Hot afternoons are common (maximum temperature above 32°C)".

A west-facing room takes the low afternoon sun on its windows and outer wall. Sunlight through the glass warms the floor, curtains and furniture, while the wall soaks up heat for hours. When the sun goes down, that stored heat keeps seeping back in, so the room can stay warm into the evening. In effect, it becomes an afternoon sun room, hot later than the rest of the flat. The aircon must then remove that stored heat as well as cool the air.

Shade the windows before you lower the setting

Heat that never enters the room does not need to be cooled, so shading is the place to start. NEA's energy efficient interior design tips say: "Use curtains or blinds over windows, or solar window films to block unwanted heat from the sun." The same page says to choose light colours such as cream, white or beige for walls and large furniture, as they reflect heat better than dark colours.

The simplest way to block heat from windows is to put something between the glass and the room. Curtains and blinds can be drawn in the afternoon, while film stays on the glass and keeps the view. Our blinds vs curtains guide compares the first two.

If you add film, HDB's window works guidelines say placing film over the glass panel of existing windows or balcony doors needs no permit, and the film should be non-reflective or "not exceeding 20% reflectance". Installing venetian blinds or a curtain rail internally needs no permit either. If you rent, ask the owner before you fit anything.

Set the aircon sensibly

It is tempting to turn the remote down when the room feels warm, but that has a cost. NEA's energy saving tips say to set the temperature at 25°C, and note: "The lower the temperature setting, the more energy used by your air-conditioner." They also say to consider running the aircon for a short while and switching to a fan after that.

NEA's interior design tips say to install fans to keep cool and save on air conditioning, and to use partitions to minimise the space to be air-conditioned. In a flat with an open living and dining area, closing a door or sliding partition means the unit only cools the room you are using.

For more ways to use the remote well, see aircon settings that save electricity.

Is the aircon the right size for the room?

A unit sized for a shaded room can struggle in a sunny west-facing one. The afternoon sun adds heat through the glass and the outer wall for hours, so the unit has more to remove than the floor area alone suggests.

If the aircon cannot cool room air to a comfortable level by late afternoon, even with the curtains drawn and the filters clean, the unit may be too small for the room. Cooling capacity is given in British thermal units (BTU), and the guide to aircon BTU for room size explains how room size is matched to capacity.

When you talk to a technician, mention that the room faces west, its size and how many windows it has. Ask whether the current unit suits the room before you decide on a replacement.

Rule out dirty filters and a weak unit

Before you blame the sun, check the simple things. NEA's energy saving tips say to check the air filter once a month and clean or replace it when necessary, and to have air-conditioning units maintained regularly and inspected once a year. HDB's home care guide for air-conditioners says to clean the air-filters of condensing units to maintain optimal airflow.

A clogged filter lets less air through, so less cool air reaches the room. If you have never taken the filters out, see how to clean an aircon filter.

If the airflow still feels weak with clean filters, or the room is still warm after you have shaded the windows, look at the unit itself, as the next section explains.

When to get the aircon checked

Compare how the room feels in the morning and in the late afternoon. If it cools well in the morning but not after lunch, the sun is the main load, so start with the windows. If the air from the vents is not cold even in the morning, before the sun reaches the west side, the problem is the unit, not the sun. A technician can check it, and the aircon not cold guide explains what to look at next. The cost depends on the job, so compare companies' published prices before you book.

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