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The interior of your house is hot, the exterior cool. What would the most efficient orientation be for a box fan? Pushing hot air out of a window or pulling cool air in through it? (Solved)*

Secondarily, should the fan be outside or inside the window?

Asking for a friend. (Me, help. I can’t remember physics, it’s too goddamn hot)

Edit: I’ve opened all the windows and set up a fan a short distance from the window pointing out and it’s reduced the temperature to near-tolerable levels. Unfortunately, venting through the attic was a no-go but moving things aside might make it an option next time. Thank you all, I appreciate the help!

Dudewitbow ,

if you have one and only one opening into your room, id believe setting the fan to exhale is optimal, as the logic would be similar to radial fans in low airflow situations, where in that situation, its ideal to move hot air out.

if you have 2 windows, I think it was one intake. if you have a window and a door (to the rest if the house), placement depends if youre okay with dunping heat into the other part of the house or not.

theres a lot of factors though, as it pertains to the rooms internal pressure and the condition of the air/sunlight outside of the window, as well as how high the window is

HubertManne ,

It depends on the windows placements that are open and temperature of the building sides vs the location which can be time of day dependent. It is not always good to open all windows as it can be valuable to create an airflow that does what you want. Air pressure goes down as you go up in height so its good to have windows open lower on the cooler side and higher on the higher side. Its possible the whole heat risese effect helps this to im not sure. This should create its own airflow that is slight compared to a fan but very consistent and can help a lot on its own. From there I do think pulling out seems to work better than pushing in but it is way better if it covers the whole opening as much as possible. So like if you can use foam insulation to make a form fitting collar for the fan that sizes it to sit in the center of the window opening hole and seals it to the edges. That will work better.

BaroqueInMind ,

I love how a simple small A/C is essentially a more efficient version of what you just did with less wasted power, and yet every idiot here is cheering for your success.

Electric_Druid ,

You sound cool and fun

BaroqueInMind ,

No I’m actually ugly and boring. Wanna hang out?

Randomgal ,

Ew. Fuck no.

BaroqueInMind ,


riodoro1 ,

If he had AC he’d use it. Who’s the idiot now, idiot?

BaroqueInMind ,

I’m stupid for having the mental faculty to easily purchase a cheap Chinese A/C from Aliexpress or Amazon?

riodoro1 ,

No, that makes you albert fucking einstein.

Everyone look at this genius right here. He bought a cheap AC unit yesterday and now he is willing to shine upon all of us the light of his intelligence.

BaroqueInMind ,

Thanks bro I’m hella smart.

NeatNit ,

Happy to hear you solved it for now. For a more long-term solution, consider investing in an air conditioner. And make sure it also has a heating mode (a.k.a a heat pump), it should cost pretty much the same and is the most efficient way to heat a home in winter.

rand_alpha19 ,

The most efficient way is to force cold air into the space while also forcing hot air out. The best way to do this is to have maximum airflow from one end of the house to the other, often by opening all of your windows and using multiple fans to direct the air. For example, an intake fan pulling air in at the north end and an exhaust fan pushing it out at the south end.

lemmefixdat4u ,

I’ve tried it many ways. The most efficient method using a fan is to open the windows in a room on the leeward side of the house, then place a fan a few feet in front of the doorway, blowing air into the room. It’s more efficient because the impelled air will entrain surrounding air moving a greater volume through the doorway and out all the windows in that room. Then open windows in the farthest rooms to get a cross breeze.

Also look into an attic fan. They exhaust hot air from the attic, and it makes a world of difference in keeping your interior cool. Another trick is to mount shade cloth 6 inches away from the sunny walls of your home. It absorbs most of the solar radiation and keeps your walls cooler.

ordellrb ,

Have an fan pushing out on the ceiling in the middle of the house would work good, it will get the hottest air out. But also keep in mind: the air has to come from somewhere, if the house is sealed (new windows) this will not work well.

litchralee ,

I’m not a physicist. But I did live in an apartment at university, and shortness of funds plus hot weather meant experimenting with various box fan configurations.

What I found most optimal was to open two windows on opposite sides of the dwelling, with the one box fan aimed outward, using cardboard to block the openings around the fan. In my case, the choice of egress (ie air flowing out) window was based on: the window which best fit the box fan’s shape, proximity of noise to the bedroom, and the quality of the window screen.

As for why the fan points outward, this sends the heat of the motor (60-100 W) out of the dwelling, rather than drawing it in. Also, if facing inward, the high airflow at the tips of the fan blades would tend to draw small flies into the dwelling. But if the fan is at the egress window, then the ingress airflow will average out over the full surface area of the ingress window, producing a lower peak airflow rate, akin to a gentle breeze.

If you have a multi-floor unit or house, it would be optimal to place the fan at the highest egress window, to take advantage of heat naturally rising. Opening multiple ingress windows will quickly cool those rooms, while also reducing the peak ingress airflow and resulting drafts (eg blowing papers off tables). Of course, it’s necessary to open all the doors to form a path between the ingress and egress windows.

HeyThisIsntTheYMCA , avatar

We put ours in the ceiling between the attic and the top floor. Pull the air out of the house, open the windows at night. I’ve heard it called a “whole house fan”. I would occasionally turn it on with the windows closed while the air conditioning was running to, uh, make it go faster, but I’m not sure you’re supposed to do that. Cooled the place down in seconds tho.

etchinghillside ,

Exhaust air out from the ceiling if possible.

Bell ,
TropicalDingdong ,
Fondots ,
FuglyDuck , avatar

No, but they can drive a geothermal heat pump. Just saying.

franglais , is the best way to get what you want.

mwproductions ,

I’m glad to see someone linked to this video. No one ever believes me when I tell them this is the way.

massive_bereavement ,

This is the way.

ChronosTriggerWarning ,

I don’t believe @mwproductions.

morrowind , avatar

This guy seems to have a different goal then OP. He just wants the max airspeed in other parts of the house

bitwaba ,

The goal is the same.

Scenario: cold air outside, hot air inside.

Goal: replace hot air inside with cold air outside.

Method: open one intake window and one exhaust window. Use a fan to blow hot air out the exhaust window while the low pressure vacuumed created inside will cause cold air to be sucked in through the intake window.

Maximize efficiency of method: follow video posted.

kemsat ,


cerement , avatar
  • reversible fan
    • blow hot air out during the day
    • bring cool air in a night (night flushing)
ShepherdPie ,

Any box fan is reversible if you turn it around.

bizarroland ,

Who are you who is so wise in the ways of science?

Brkdncr ,

The make up air is coming from outside. The fan direction doesn’t matter.

BombOmOm , avatar

Any fan pushing out is also causing air to pull in somewhere else. The inverse is also true. Setup all your fans in the same orientation (ex: all blowing out or all blowing in) and have the other windows in the house open as well.

For maximum effect, have the cool air first coming into a room (or rooms) you are in or most want to cool down. As that will give you the most pronounced, direct effect.

It is also generally helpful to make sure the air has to travel through several rooms in its journey. If it is coming in a window, and leaving from another window in the same room, most of your house will get nothing.

You can also run your home’s HVAC on fan mode to distribute air more, if you are having troubles with certain areas of the house. Also remember bathroom vents can help out, too.

ShepherdPie ,

I wouldn’t point all my fans in or out unless they’re all on one side of the building. You want to create a ‘wind tunnel’ effect to pull the air through the house and this is best achieved by keeping all the air flowing in one direction (north to south, east to west, etc).

Using the HVAC fan definitely helps a ton. Our vents are all below windows and this really causes the air to get moving.

TropicalDingdong ,

What are you trying to get someone diviorced?

The key to happiness in life is to know what questions can’t be answered and then to not ask them.

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