Cool Your House With Negative Pressure Ventilation - Not A/C




Let's face it, when it's hot and you don't have air conditioning it can be borderline homicidal. What's worse is when the temperature begins to dip outside and your home feels like an oven. Most of us turn on a fan to get some air moving across our skin - which is a great idea.

But moving hot air around feels pretty futile, doesn't it?

What if you could replace the air in your house with new, cooler air, quickly and efficiently? You can! All you need is a fan and a cooling plan.

When I was a volunteer firefighter we had to replace the air in people's homes all the time. We would get called to a home because someone forgot to take the bread from the oven and the smoke emanating from the stove would begin to fill the entire home.

Often time we could completely exchange the air in a house with one fan in just a few minutes by using Positive Pressure Ventilation (PPV) or Negative Pressure Ventilation (NPV), depending on the situation.

Here's how it works.

If a home is full of smoke, all of the windows would be opened and a fan would be placed about 6' from the front door on the exterior of the house. The air blowing into the house would increase air pressure and force the smoke out of the rooms.

Conversely, if the smoke was localized then we would place the fan about 6' from the door on the inside of the house facing out, thus creating a negative pressure in the house. This would cause air to rush into the open windows throughout the house, pushing the smoke out the front door.

Granted, we used a fan that had a wooden prop you might find on a small aircraft, powered by a 10 horsepower engine.

Don't despair! The principals of PPV and NPV can still work for you even if you have a regular box fan - it just takes some planning.

Spending all kinds of time and energy cooling your living room with a fan in the window does nothing for that EZ Bake Bedroom at the end of the hallway when it's time to hit the hay. What we really want to do with that futile fan is put it to work replacing the hot air in the bedroom before bedtime.

Step 1: Close Up the House

Yes, close all the doors and windows. Our goal is to systematically cool the house room by room and we need to seal off areas that don't need immediate cooling and provide a single place for cool air to enter the house. A box fan can only do so much so we want to be as effective with it as we can. Trying to empty the entire house with one box fan is a waste of time.

Step 2: Make an Exit for the Hot Air

Open a door or window at the other end of the house from the bedroom. We'll use this as our exit hole to dump hot air from the house.

Step 3: Placing Your Fan - Critical Step

Place the box fan inside the house about 3' to 6' from the window, facing outdoors and turn it up as high as it will go.

The placement of the fan is critical to the success of our plan. By placing the fan near the opening and not *in* the opening we increase the effectiveness of the fan considerably by creating a bit of a vortex.

Here's how that works. When air moves quickly from one place to another it creates a low pressure around the column of fast moving air. To see this in action, take two strips of paper, hold them from the top about 3 inches apart. Blow downward between the two strips of paper. You'll see that the paper strips pull together at the bottom of the air stream. The fast moving air has created a low pressure and the air on the other sides of the paper is trying to move in to fill that low pressure space.

What that means for our fan plan is this. Our fan is pulling double duty because, in addition to the air being moved from the house by the fan blades we're also picking up a significant amount of ambient air on the other side of the fan that rushing in to fill the low pressure area around the column of air coming from the fan.

Pretty cool huh? (pun intended)

Step 4: Cool Your First Room

Pick a room and cool it.

Now that we've got a good stream of air leaving the house it's time to decide where we want the new air to come to replace it. I choose the bedroom. I can deal with a hot living room for an hour, but I can't abide a hot bedroom for 5 hours of torture before trying to leave for work.

Open a grand total of 1 windows in the bedroom and prop the door open. You'll begin to feel a breeze of outside air coming in through the window. If all your other doors and windows are closed, the amount of air coming through the window will be similar to the combined air steam leaving the room at the other end of the house.

Step 5: Caveats

I don't cool the entire house. I stick to cooling the bedrooms and call it good. I just want to sleep well.

Work with the prevailing winds in your area. Don't point your exit fan at a door that's got wind blowing in. Use that wind in your favor.

Cooling a two story home can be a bit tricky. You may need to implement a fan plan on each level of your home. If it's impossible, cool the downstairs and sleep on the couch if you can. It's only temporary.

Step 6: Tips and Tricks

Fan Placement
You know your fan placement is spot on when the stream of air coming from the fan completely fills the opening of the exit window or door. When I was on the fire department we would place the fan and put our hand at each corner of the opening we were using. You should feel air movement in all corners of the opening. You may need to put the fan on a table or tilt it back to get the best coverage.

Fan Stacking

This is one of my favorites. If you really want to get the job done, try placing two fans in series. The first fan will create a column of air blowing into the back of the second fan that's pointing to your exit opening. Place the first fan in such a way as to capitalize on the vortex effect, thus increasing the second fan's capacity. We would do this on large houses filled with smoke and it worked amazingly well.

Be Patient

Look, you don't have an air conditioner. This technique isn't going to turn your house into a place where you can leave milk on the counter and come back the next day and have it be cool. We're talking about exchanging the air inside the house for air outside the house. If it's hot outside, it's not going to be magically cooler when it passes through your window.

Enjoy the breeze

Once the bedrooms are cool I leave the fan in place and open all the bedroom windows. Once you get the majority of the hot air out it's nice to enjoy the little bit of breeze that the fan plan affords during the night.

Draw the blinds

Keeping your house cool during the day is always a good idea. We've always lived in old houses and this is no small task. In addition to our fan plan we had to have a shade plan during the day, blocking out sun from windows that face south.

For the record, the Voranado fan is the best dang fan for the job. We bought one from our local power cooperative about 10 years ago and it's amazing. It wasn't cheap, but it'll blow like a banshee.

Best of luck! Stay cool.



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    15 Discussions


    11 months ago

    Thanks for this excellent writeup! I found this post looking for advice to help cool a garage where I plan to setup a home office. The house is a rental so I want to limit modifications. The garage (19x19) does get hot and stuffy during the day, but where I live (coastal California), the outside temperature is generally pleasant year round with low humidity.

    Besides a typical garage door, the garage also has a side patio door with a screen. In addition, it has a rear wall exhaust vent that looks to have been connected to some kind of ductwork in the past. There is no ductwork now, just the bare open duct inside the garage and the vent on the exterior.

    Ideally I'd like to keep the main garage door closed and create circulation using the screen door and this old duct exhaust. The screen door and the old exhaust are about 10 feet apart.

    If I were to setup fans both at the screen door and at the exhaust vent, which fan should blow in which direction? The screen door would be the larger "hole" (I could build a blank to fill in the gap above the fan blower, if that would help.) The old exhaust vent is the smaller hole (12 inch circumference) and is low to the ground.

    I'm unsure whether the larger hole should be assigned to exhaust hot air or to pull in the cooler outside air, and vice versa for the smaller hole.

    Thanks for your wisdom!


    11 months ago

    Ah, this will be useful- It will cool my garage. Although I installed garage door instillation, it is still several degrees hotter than outside. And with the warm summer nights coming in, This will work! Cricket ambience included. (Also, there's a lot of electronics in the garage)


    1 year ago

    Hi, thanks for this article. My google searches around this topic haven't succeeded at all. My question is this. I want to air out my bedroom in the morning to reduce moisture in the air. What is most effective? Window open, door closed? Or window open, door open, another window in an opposite room open to create cross-flow? How does temperature outside vs inside affect it? Thanks.


    Good explanation of a simple concept that's not always obvious when trying to cool a hot house. It seems its almost counterintuitive until you think it through a little bit. Nice work!

    2 replies

    True story. Closing up the house to cool it seems insane. It works really well though. Additionally, I hate listening to an A/C unit. Tis way, the fan is at the other end of the house and I still get a nice breeze coming through the window. A triple bonus... Cooler air, moving air and minimal noise. Thanks for the compliment. Sorry the drawings were so rough. I did this in a few airports while trying to get home from a business trip.


    Reply 1 year ago

    "Sorry the drawings were so rough."

    I can't draw to save my life, and (I swear) my first thought was: "If I could draw that good, I would draw all my projects!" (Currently, I am replacing and rerouting some a.c. ducts, and I'm using the "Paint" program for my Blueprints, and yes, they suck.) lol. I am so amazed by how artists like yourself, can capture the depth of the door threshold, or make the slivers of paper come together at the bottom, and how some arrows clearly show the varying air velocities. :)


    1 year ago

    Great article. Not only are the readers homes cooler but the
    planet is too….great work!

    My question is about accomplishing this cooling effect using
    existing vent fans (kitchen range hood and bathroom).

    I asked my builder to install larger than average fans in these
    locations and to do so inline (in a remote location for less noise).

    The builder, appliance sales, and my wife are all seemingly
    perplexed at my design initiatives.

    I would appreciate any dialogue about using vent fans to
    accomplish the cooling described in the article. Thanks.


    2 years ago

    Would stacking two mini vornados work as well as stacking a box fan and a vornado as you described?

    1 reply

    Reply 2 years ago

    It will definitely help. The goal is to get about 50% to 75% of the opening you're pointing at to be filled with air from the fan. The other 25% to 50% will be air that is carried along by the low pressure created by the stream of fan-driven air passing through the opening. As I read this, I'm not sure I'm making good sense. I can see it in my head but the words are failing me.


    2 years ago

    Absolutely fabulous instruction. It works!!! Thank you so very much for taking the time to share this.


    3 years ago on Step 3

    Looks like a good idea. This lseems a bit like the same method Dyson use with their fans.


    4 years ago

    we do the same with a small plastic 6 inch desk fan. place it on the windowsill next to an open window and it sucks cool fresh air in. much more effective than just blowing hot sweaty air around.


    4 years ago on Introduction

    Some 13 years ago I did a ~40 cm hole in the highest part of the attic's wall, and attached a cheap chinese box fan. In the heat summer evenings, I turn it on after the sunset, when the outer air is a bit cooler than the inner. It is very effective.

    1 reply

    Reply 4 years ago on Introduction

    The fan is aiming to outdoors. The cool air enters through the windows or doors of the ground floor.

    Thanks for explaining this process so well! I bet it works really great, and you're right there is nothing worse than a hot day with no air conditioning. Thanks for sharing!