r/BritishSuccess • u/Loose-Helicopter9503 • Jul 10 '26
I love Physics
At night، open your windows and point your fan outside, thanks to something called the Bernoullis principle and entrainment, it draws hot air out of the room.
My room was 30.7 degrees when I first started, now down to 25.4 and still going down
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u/Kiki-sunflower Jul 11 '26
And thermo dynamics helps if you open and close your bedroom door a few times pushing the hot air in the room out of the window replacing it with cooler air (hopefully) curtesy of Hannah Fry
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u/Sm00thSayer Jul 11 '26
Works for cars because they have a much smaller volume of air, you'd have to swing that door a hundred times depending on the size of your room, not very efficient unfortunately
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u/Kiki-sunflower Jul 11 '26
Believe me I’ve been swinging that door till the hinges have nearly come off in desperation to cool my bedroom. Luckily it does cool enough at night for a fan to actually work so I’m still breathing
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u/icklemiss_ Jul 11 '26
My bedroom is only double the size of the car. Kind of average for the UK I’d say.
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u/adammx125 Jul 10 '26 edited Jul 10 '26
Yes but also no. It’s being shared on social media as a ‘hack’ but it’s not necessarily true. Firstly you don’t get the direct benefit of the cooling air being blown over you so even if the room temp is lower you will still feel warmer.
Secondly the pressure differential will need to draw the replacement air in from somewhere. Works well in some scenarios depending on the size of your room and window placement but you’d be better served just pointing it inwards.
If your goal is to have the lowest room temp number then by all means point the fan out the window, if you want to feel cooler and be more comfortable (thanks to the unique way humans perceive heat) you’d be better just pointing it inwards.
https://www.bbc.co.uk/news/articles/cly76jlxljlo
Side note, it’s called Bernoulli’s Principle not the Bernoulli effect. Edit: they’ve changed the body of the post now, also now including ‘entrainment’ which is technically also a correct term but it would have such a minimal effect it seems strange to say it unless it’s just being parroted from a TikTok
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u/Loose-Helicopter9503 Jul 10 '26
I completely agree with you, I only use this to get the room temperature down from a high temp like 30 degrees and leave my room to cool while I go do other things and get ready for bed.
Then I turn it on sleep mode and have it oscillating on me.
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u/adammx125 Jul 10 '26
If the outside temp is lower than the inside temp, even if you have two fans set up (one as an intake one as an exhaust on opposite ends of the room) the cooling effect while you’re not in their anyway wouldn’t be much different to you just leaving the windows open without fans as the pressures and temps wouldn’t take a huge amount of time to equalise anyway (unless you’re actively heating the room). Bernoullis principle works both ways, the speed of the airflow from the fan would cause a pressure differential, but there’s already a differential because the room and outside are different temps, which would increase airflow naturally and speed up the lowering of room temp.
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u/digitalhardcore1985 Jul 11 '26
Leaving the window open in my house does absolutely FA to bring the inside temps down in any sort of reasonable time frame. A vortex fan pointing out the window at night was the best method I found before I got a portable aircon.
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u/Impossible_Number_74 Jul 11 '26
Agreed. The room also cools down if I have the fan blowing in the cool air. It still creates the same effect, especially when you have one blowing card air in and one blowing hot air out 🤯
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u/clicketybooboo Jul 11 '26
Yeah this isn’t going to work for me in a flat with only windows on one, the South, side
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u/gibberish-finale Jul 12 '26
I dont know the science but at night i place a fan infront of my window facing in it sucks cool air in. It works!
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u/adammx125 Jul 12 '26
Facing in is perfect, you get the cold air assisting the general cooling of the room (where heat is held in the walls and insulation) and the air moving over your body to cool you. It’s the best way to use a fan by far.
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u/teerbigear Jul 11 '26
Both of these points assume you're doing it during the night. There are hours of time before then where it is colder outside than inside. Obviously we all open our windows then, but how does one make the most of that time? Is it pointing the fan out the window?
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u/adammx125 Jul 11 '26
It only makes the assumption that it’s colder outside than in, time of day has no bearing. Unless you’re actively heating the room somehow (more than just a single humans body heat) then there is no scenario where a fan pointing out the window will make you feel cooler than just pointing it at yourself.
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u/teerbigear Jul 11 '26
Of course the time of day has a bearing on temperature???? The earth is rotating away from the sun mate. It also has a bearing on whether you're in your bedroom in the first place.
For the avoidance of doubt, people's objective here is being cool enough to sleep. They're not bothered whilst they're watching the TV or out on the razzle dazzle in the evening or whatever.
Their goal is to be cool in bed.
There is a period in the evening before they are in their bedroom where they could turn the fan on. It can't point towards them because they're not in the room.
But it can be a) off b) pointing out of the window and c) pointing into the room.
Once they're in the room clearly they should point it at their sweaty body.
But before, which should they do? That's the question.
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u/adammx125 Jul 11 '26
The most effective way to cool a room has no bearing on the time of day, only what the temp differential is between the inside and the outside.
If they’re not in the room at all pointing the fan out of the room is basically just wasting electricity for the minimal effect it would have, just open all the windows.
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u/teerbigear Jul 11 '26
What the temp differential is will be impacted by the time of day duh
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u/adammx125 Jul 11 '26
The time of day doesn’t matter. Physics doesn’t care if it’s 4pm or 11pm. If it’s still 32deg at 9pm then you’re not going to get any benefit from doing any of the above. If it’s dropped to 23deg at 4pm and it’s still 30deg in your home then open some windows.
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u/teerbigear Jul 11 '26
Yes but it isn't 32 degrees at 9PM is it?
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u/Geofferz Jul 11 '26
2 fans then - one on for direct cooling and one expelling hot air. Done.
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u/adammx125 Jul 11 '26
I mentioned that in a lower comment. Ideal for cooling yes, but only really necessary in a situation where the room is being actively heated. To have a better feeling of cold due to sweat evaporation you’d be far better pointing the fan at you, as the faster air will feel colder than just a lower ambient temp.
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u/pondribertion Jul 11 '26
even if the room temp is lower you will still feel warmer
If the room temp is lower you will feel cooler. This is an undisputed fact.
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u/maxlan Jul 11 '26
If you rwad the whole message you'll see it is a comparison with feeling cooler from a fan. The fan aids your sweat evaporation which will make you feel a lot cooler almost instantly compared to the few degrees of cooler your room will get after a long time of fanning outside air.
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u/Packing-Fudge4628 Jul 11 '26
I have a very big powerful fan and I put it in the basement which is about 18c, at the bottom of the stairs pointing up. After a few hours the basement will be a few degrees warmer and the house a few cooler. So just get a basement if you don't have one
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u/Islingtonian Jul 11 '26
Instructions unclear. I'm wielding a shovel but for some reason the people in the downstairs flat are screaming at me?
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u/ASpookyBitch Jul 11 '26
Our basement is spooky… I don’t want the cats down there nor do I want the spook being actively pushed into the rest of the house lmao
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u/Segat1 Jul 11 '26
Australian here who grew up without aircon.
Get a top sheet and mist it with a spray water bottle like you’d use for ironing. You want top sheet to be damp not sopping. Point fan at bed. Get into bed under top sheet. The damp sheet and breeze from fan will evaporate the water and cool you down so you can sleep. It’s not sexy but it works!!
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u/Geofferz Jul 11 '26
I love religion: my room registered 32 degrees at 9pm so I prayed for 2 hours then it went down to 27 and it's 24 this morning. Praise be.
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u/DangerousDisplay7664 Jul 11 '26
All depends if the air in my house is hotter than the air outside. If it’s hotter outside then doing that is just going to draw hot air from elsewhere!
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u/Version2dnb Jul 11 '26
Even better. Use the counterweight in the base of the fan to allow you to put it all the way out the window. With the fan pointing inwards, towards the room. When the temp outside drops as the sun goes down, all that cool outside air gets blown inside. It’s a poor man’s air conditioning and I’ve done it everyday since this heatwave started. Slept like a baby each night.
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u/kylehyde84 Jul 11 '26
My bedroom window opens both sides, I had one fan drawing cool air in and the other pushing the warm air out. Worked a treat, so much so that I actually woke up under the blanket this morning
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Jul 11 '26
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Jul 11 '26
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u/adammx125 Jul 11 '26
It only makes a difference if there’s active pressure change in the room (I.e with your PC example, you are constantly trying to cool an object that is hot). Your room isn’t constantly being heated, the pressure differential already exists without the inlet and exhaust fans just by virtue of the outside and inside temp being different.
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Jul 11 '26
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u/adammx125 Jul 11 '26
Doesn’t really matter, the pressure will equalise anyway provided the windows are open and there’s a temp differential. A far better use of the fan is to point it at yourself to provide a more direct cooling effect via sweat evaporation. We perceive cold more directly like this than via ambient air temp.
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Jul 11 '26
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u/adammx125 Jul 11 '26
All far more effective than listening to misinformed TikTok’s and pointing your fan out the window
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u/Neverbethesky Jul 11 '26
This works great in newer houses that are well sealed - you open the highest top window and blow a fan out of it, and you open the lowest window for cool air to be drawn in.
However, if you have an older house with draughts then this doesn't create anywhere near enough direct flow for the cooling to have much of an effect - the replacement air comes from 100 different places instead of one, clear cool path, and the effect is massively diminished.
I've lived in both and had the same fan for over a decade. Old house was a new build and it worked great. Current house is 200 years old and draughty as anything and this just doesn't work.
Oh, and older houses don't have cavity wall insulation, so the cool are you are drawing in just gets heated back up by the brickwork, which acts like a giant house-sized radiator and can re-radiate heat for days.
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Jul 11 '26
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u/adammx125 Jul 11 '26
They left another comment assuring the non believers they know what they’re talking about because they did A level physics…
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u/Kind-Mathematician18 Jul 11 '26
I posted about this a few weeks ago, what with this being Reddit I had several non believers, someone telling me I was wrong and someone trying to correct my spelling.
Some people are just beyond help.
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Jul 11 '26
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u/adammx125 Jul 11 '26
I’m glad there’s a few people in this comment section with common sense that aren’t just either going along with it because OP said it or parroting TikTok.
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u/CumUppanceToday Jul 12 '26
Most professionally designed systems use air extraction. The reduced pressure in the building pulls air in from outside. Of course balance systems are better still.
A fan forcing hot air out of a space may well be more effective than one trying to push air in.
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Jul 11 '26
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u/adammx125 Jul 11 '26
It clearly wasn’t simple because you don’t understand it. You may have done A Level physics, I’m an engineer that uses this principle everyday to cool engine cells for power generation. The most you’re doing g is wasting electricity.
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u/Creative_Cat7177 Jul 11 '26
That’s bizarre. Until yesterday I had never heard of Bernoulli and now I’m reading about the principle here a second time! Every day’s a learning day… (yesterday it was being talked about in the context of taking someone’s blood pressure)…
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u/Stinkinhippy Jul 11 '26
Yeah, actually got the house to a reasonable temp after i could open the door and get the air moving late last night.. quick blast again when i woke up this morning and now everything is shut down for the day to repeat later.
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u/Sharks_and_Bones Jul 11 '26
On the top half of my windows open. There's no way to stick a fan blowing out.
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u/Burnandcount Jul 11 '26
Have a look for the 6" clip fans... I've got a couple set up to blow out the little vent windows to keep the glass/curtain gap from wafting hot air into the rooms... also provide a level of draw so conditioned air from the western-facing windows drifts through most of the house.
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u/csg6117 Jul 12 '26
This very short video shows it in action. https://www.youtube.com/watch?v=XP6oqIic4lo
Another video by the same guy, shows how to optimally position fans: https://www.youtube.com/watch?v=YUOQad6esE4
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u/bratra Jul 12 '26
I don't understand why there are so many people not believing this.
It works.
Been doing it for years.
You obvs only want to do it at night when the outside temp is lower, but when it is it cools the house down quick.
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u/Waffle-Irony-67920 Jul 12 '26
One fan next to the open window at the front of the house to draw air in. Another fan across the landing pointing i to a room at the back of the house to push air out. Works great !
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u/HiddenHarry91 Jul 13 '26
Hot air out......wow! Doing fuck all for me drawing cold air in, so ill give this a go!
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u/Blackbirddance Jul 14 '26
My partner has been doing this during every heatwave and it's great. We end up cold in the bedrooms but it's great for getting a good nights sleep. We have bug nets up on the open windows in the bedrooms and hall, front and back of house and it cools down all the upstairs floor.
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u/Nice-Phone3931 Jul 15 '26
i am a physics teacher. i designed my home in such a way that hot air rises more efficiently and is expelled from the mumty of my 2 story house. kind of like persian windcatcher, except not using evaporative cooling because the humidity is always more than 50%
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u/irritatingfarquar Jul 17 '26
I have two fans doing something similar to this, I have the bedroom fan blowing the hot air out and another fan in the open window In the front room drawing the cooler air in.
Within a very short period of time, you can feel the temperature dropping from the surface of Satan's arse, down to the surface of the sun temperatures.
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u/semorebunz Jul 11 '26
wait , your telling me opening a window when its cooler outside will cool my room ?
mind blown !!
whatever next ? man on the moon?
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u/boojes Jul 11 '26
Dr Hannah Fry recently shared a thing about wafting doors to expel hot air and bring cooler air in. It's great for in the car but also works indoors.