Should you always close the doors when air conditioner is on?

"Why are the doors open when the air conditioner is on?" Oanh shouted at her children inside their apartment in a building in District 2, Ho Chi Minh City.

Even when only the living room unit was running, she insisted that all bedroom and bathroom doors be closed, even stuffing cloth into door gaps to prevent cool air from escaping.

In Hanoi’s Thach That District, Dinh Cu viewed sealing doors as "common sense" when using air conditioning. His home, equipped with older wooden doors, allowed air to leak, so he applied foam strips along door edges. "If the door is ajar, the machine has to run constantly to cool the room, and it could overheat or blow a capacitor," he said, repeating what he often tells his family.

The debate has gained traction on social media in recent weeks. One post discussing whether to keep doors closed while running air conditioning attracted nearly 100,000 likes and over 40,000 shares in just a few days.

Most air conditioner manufacturers advise closing doors to boost cooling efficiency and save energy. However, experts caution against taking this practice to extremes.

A person uses air conditioner. Illustration from Pixabay

Do Hoang Chuong, product quality director at electronics firm Casper Vietnam, said completely sealed rooms can lead to a buildup of carbon dioxide, which may cause headaches, dizziness, or a feeling of stuffiness.

To maintain fresh airflow, he recommended installing small fans or ventilation systems. After 3–5 hours of continuous use, rooms should be ventilated for 10–20 minutes. In homes without exhaust fans, residents can drill small vents high on the wall or use doors with air slits. If none of these options are available, briefly opening a door after several hours can help refresh the air.

Electrical appliance expert Nguyen Minh Tien said sealing a room completely is not inherently dangerous, despite some online claims. "In a typical 15-square-meter room, oxygen levels are sufficient for a family of three to five to sleep through the night," he said.

Feelings of discomfort, he explained, are usually caused by rising CO₂ levels rather than a lack of oxygen. Over time, poor ventilation can disrupt sleep and impact overall health. Tien noted that office buildings, where doors are often kept closed, are typically equipped with mandatory fresh air systems, underscoring the importance of ventilation.

Experts agreed that extreme practices, such as stuffing gaps or completely sealing a room, are unnecessary. Slightly open doors or small ventilation slits will not significantly reduce air conditioning performance or damage equipment. Instead, pairing air conditioning with low-speed fans or passive airflow creates a more comfortable living environment.

Dang Truc Linh, head of air conditioning development at Comfee, said some modern units include airflow-enhancing technologies such as long-range cooling and four-way swing fans that help circulate air more evenly.

She also advised against setting the AC more than 10 degrees Celsius below the outdoor temperature, as doing so can strain the unit and negatively affect health. Smart models now offer features that automatically adjust temperature and airflow for more restful sleep.