The basic principle is that an air conditioner’s outdoor unit must release heat into the environment before the indoor unit can cool the room. But when outside temperatures are extremely high, the outdoor unit cannot do its job efficiently, and so setting a very low temperature yields no real benefit.

"Whether you set it to 18 or 20 degrees, the room won't get any cooler because the machine is already running at full capacity," Professor Nguyen Duc Loi, formerly lecturer of the Hanoi University of Science and Technology, explains.

According to the delta T (temperature differential) principle, residential air conditioning systems are designed to achieve an optimal difference of 8-11 C compared to the outside environment.

Thus, when the outdoor temperature is 40 C, maintaining an indoor temperature of around 29-30 C is already the maximum the unit can manage.

Forcing the compressor to run continuously at 100% capacity risks mechanical failure. In lower-quality units, the machine may lose its cooling ability altogether.

When the outdoor temperature reaches 40 C, adjusting the remote setting is pointless because air conditioners can only optimize room cooling by 8 - 11 degrees compared to the outside temperature. Photo by Phan Duong

Energy organizations and experts recommend setting the air conditioner at 25-27 C. Loi advises that the indoor-outdoor temperature difference should not exceed 10 C, with 8 C being the recommended gap.

How quickly a room cools depends on the unit's capacity relative to the room size and installation position, and not on how low the thermostat is set. According to experts, every degree below 25 C can increase the electricity bill by 1-3%.

Associate Professor Nguyen Xuan Tien, vice president of the Vietnam Association of Refrigeration and Air Conditioning Science and Technology, says a 12,000 BTU air conditioner (consuming approximately 1.2 kW) running continuously for 8 hours during hot weather will use nearly 10 kWh of electricity. Raising the set temperature by just 1 C can reduce that consumption.

Ways to improve air conditioner

To keep an air conditioner running effectively in 40 C weather without overloading it, users can apply the following measures:

Protect the outdoor unit: Install an awning or place the outdoor unit in a well-ventilated spot away from direct sunlight. A mist spray system aimed at the outdoor condenser can bring the surrounding air temperature down to 30-35 C, improving cooling performance.

Combine with a fan: Run the air conditioner at 25-27 C alongside a ceiling or standing fan to circulate air and speed up sweat evaporation. The room will feel as cool as 23-24 C while consuming less electricity.

Ventilate and add humidity: Do not keep the room sealed continuously. Crack a window or door every 3-4 hours, or install a ventilation fan. Using a humidifier or placing a basin of water in the room helps keep mucous membranes from drying out and improves comfort.

Block heat sources: Draw heat-blocking, light-blocking curtains on west- and south-facing windows. Avoid using high-heat appliances such as ovens and irons during the day. Clean the air filter regularly to maintain good airflow.