Should you leave the fan window OPEN on hot days? Science of Cooling Revealed

THE WAY you used your fan to keep cool this summer could be completely wrong.

According to air quality experts, there are a number of rules you should follow to avoid overheating and sleep better during the hotter months, including where to put your fan and whether you should leave your window open.

A fan can definitely cool you down - but are you using it correctly?


A fan can definitely cool you down – but are you using it correctly?Photo credit: Unsplash

The July heatwave, which is currently in full swing across the UK, has caused Brits a lot of problems, particularly when it comes to sleeping. And the worst of the extreme heat isn’t over yet. Temperatures are expected to rise even higher next week, reaching 35C.

Ensuring that bedroom temperatures are kept to a minimum by using a fan or an air purifier can help keep your body temperature down, thereby relieving sleep problems.

But what is the right bedroom setup for a fan or air purifier?

According to Andrew Persily, an indoor air quality engineer at the National Institutes of Standards and Technology In Maryland, USA, this can be accomplished by simply making sure that air in the room is blowing on your skin.

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‘The feeling of [air speed] improves thermal comfort in a chilled situation,” says Persily quartzand adds that this is because sweat, as it evaporates into the air, takes heat with it and makes us feel cooler.

This works because, even when we’re not perspiring noticeably, our bodies are constantly generating small amounts of moisture to cool itself off, says Persily. Therefore, air flowing over our bodies, whether from a fan or wind from a window, speeds up this evaporation process, making us feel cooler.

Should I leave my window open?

A safe and easy way to cool down a room and increase ventilation is to open a window.

This can be helpful when there is a breeze outside. However, since the air is actually warmer during the hotter months, opening a window may just let the cooler air into your room and let warmer air in.

In this case, it is better to position your fan so that it blows on you, but with the window closed.

Opening windows can mean bad air quality is getting in from the outside, especially if you live near a high-traffic area.

So when it comes to refrigeration cleaners, use the device in a closed environment as they work more efficiently there.

Where to place your fan in a room

Fans and air purifiers are usually designed to work in individual rooms. Therefore, when setting up the machine, make sure that there is at least a few meters of space on all sides so that air can circulate effectively.

If the air is cooler outside than inside, Persily says you should put your fan by your window, so it’s facing in. In this way, the cooler air is blown against your body from the outside, which makes the sweat evaporate even faster.

However, during the current heat wave, it’s probably best to keep the windows closed and the fan pointed toward your body while you sleep so the air movement over your skin can more effectively speed up the sweat evaporation process.

Close your curtains and blinds during the day

There are many other tricks you can use to stay cool this summer, one in particular is useful for preventing your room from getting warm in the first place.

Evan Stevens, Head of Environmental Care at Dyson, recommends closing curtains and shutters during the day to ensure sunlight doesn’t create a greenhouse effect

“Up to 30% of unwanted heat gain comes from windows,” he said. “So keep windows, curtains or shutters closed during the day to minimize the amount of sunlight entering the home.”

At night, summer temperatures tend to drop, so make a habit of opening windows at night to let in cooler air, he adds.

“Then be sure to close them in the morning before temperatures start to rise and open them again in the evening when temperatures start to drop.”

Reduce heat sources at home

Electronic devices can also generate excess heat that contributes to the ambient temperature of the room.

“Conventional light bulbs, for example, use up to 90% of their energy to generate heat, with only 10% being converted into visible light,” says Stevens.

Therefore, it is best to opt for energy-efficient lights and make it a habit to turn off and unplug all electronic devices that are not in use.

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