How to efficiently cool my house
September 2, 2012 8:54 AM   Subscribe

Looking for best practices for a whole house fan - the loud one that has louvers that open up and blows air into the attic. Best in areas with warm dry daytime temperatures that cool off at night.

I have an inside/outside thermometer. I never get much better than within 10 degrees of outside temps. I wander the house early in the morning opening and closing various windows in an attempt to get the whole house cool.

In the back of my mind is the idea that 10 minutes to cool the attic would be most efficient.

Comments? Suggestions? My google-fu is lacking in this area I can't even find a link to show you a whole house fan.
posted by notned to Home & Garden (20 answers total) 4 users marked this as a favorite
Here's how we use ours:

At night when it cools down, open ALL the windows in the house and run the fan for awhile. You can feel the cool air coming in and filling the house as the fan pulls it in by sucking the hot air into the attic . You'll know when to turn the fan off because the house will feel cool.

Leave the windows open and enjoy your cool house.

In the morning, before it gets hot, go around and close all the windows. If you have blinds or shades on the sunny side of the house, close those too to keep from heating the house with the usn. This will keep the cool air in and your house will be cooler than the outside.

In the evening when it cools down, repeat from the beginning.
posted by not that girl at 8:57 AM on September 2, 2012 [1 favorite]

I never get much better than within 10 degrees of outside temps.

What does this mean? Your inside temperature is always 10 degrees (or more) hotter than outside? 10 degrees cooler than outside? Inside is always within 10 degrees (either direction) of outside? Is this differential at night, or middle of the day?
posted by kiltedtaco at 8:59 AM on September 2, 2012

Response by poster: It was 61 outside this morning. I might as well quit at 71 it's not going to get any cooler. I'll take this chance to mention that we run the air some during the day so we're never really much cooler outside than in at night. That's why I don't run at night.
posted by notned at 9:04 AM on September 2, 2012

I can't even find a link to show you a whole house fan

This is what you mean, right? Video of one; photos and wikipedia article.
posted by Houstonian at 9:10 AM on September 2, 2012 [1 favorite]

I turn mine on at night after sunset, opening a window in every room. For the first few hours, the cooling effect is simply from having moving air. I leave it on all night, with fewer windows open, so the pull of cool air is stronger in the bedroom, living room, and kitchen. I either set the timer to go off just before sunrise, or shut it off when I get up. After that, blinds closed to the west and south makes the biggest difference keeping the coolness contained.
posted by donnagirl at 9:11 AM on September 2, 2012

Ditto to all the people who say open the windows in the evening and run the fan all night, then close windows/blinds and shut off the fan in the morning. By mid-afternoon our house has heated up again, but for much of the day it keeps it noticeably cooler and it's definitely cool overnight, which makes for great summer sleeping.
posted by warble at 9:21 AM on September 2, 2012

(1) It's always going to be warmer inside than outside in an occupied building containing heat-producing bodies and electrical equipment.

(2) all the thermal masses inside your house are at around 76 or 78 or whatever you set your day-time AC temperature, and they are contributing to heating up all that 61-degree air you're pulling in. Those thermal masses can take hours to cool down.

If you started running your whole-house fan at 3 or 4 am with all your windows open and let it run until sunrise, you'd get closer to the outdoor minimum, but you'd still be a few degrees warmer because of (1).
posted by drlith at 9:22 AM on September 2, 2012 [1 favorite]

In the back of my mind is the idea that 10 minutes to cool the attic would be most efficient.

With a good fan you can change out the air in 10 minutes, but keep in mind that most of the heat in the house isn't in the air; it's in the plaster, the floors, the furniture -- all the solid, massive stuff. If you just quickly swap the warm air for cool, then the coolness won't last.

On preview, what drlith said.
posted by jon1270 at 9:26 AM on September 2, 2012 [1 favorite]

Looking for best practices for a whole house fan - the loud one that has louvers that open up and blows air into the attic. Best in areas with warm dry daytime temperatures that cool off at night.

That seems counter productive to me. Blowing air INTO the attic. Shouldn't it be blowing air OUT OF the attic. Cooling just the attic that way surely has little effect on the in-house temperature. But then maybe I am missing something.

What - not that girl - says seems the right way.
posted by JayRwv at 9:45 AM on September 2, 2012

JavRwv, that's how whole-house fans work. They draw air from the house into the attic, which in turn draws cool air into the house through the windows.
posted by donnagirl at 9:55 AM on September 2, 2012

Right, agreeing that if you only run it for 10 minutes, the coolness won't last. Our attic fan is the only way we have to cool our house. It gets up into the 80s/90s on summer days, then down into the 40s/50s at night. Once the sun dips low enough for the temperature to drop outside, then we turn the fan on and leave it on for at least half an hour, often more. You have to cool off all the hot furniture, walls, etc. It stays nice and cool until midmorning the next day, when I close up the house again to keep the cool in until evening.
posted by HotToddy at 10:23 AM on September 2, 2012

To clarify, our setup is a giant fan in the roof of the house, then a door in the ceiling that opens to the attic. You open the door, flip the switch, and the fan sucks all the hot air out of the house AND attic and the cool air comes rushing in all the open windows. It's super loud, too loud to leave running all night unless you're totally desperate. But wonderfully effective.
posted by HotToddy at 10:27 AM on September 2, 2012

Here's how we use ours.

We turn the fan ON in the evening, opening windows to produce crossflow in the house, when the outside temperature (on its way down) is just a couple degrees warmer than the inside temperature, because moving air is cooler than static air.

We turn the fan OFF in the morning, and shut the windows, when the outside temperature (on its way up) is a couple degrees cooler than the inside of the house, because we want to keep the house cool.

This policy changes a little when it is very humid outside (e.g. after a rain) so we keep the house dryer inside even if it is cooler outside. I'd rather have it be hot and dry inside than warm and moist, because ick. It also changes a little if the day won't be super hot; we might leave one or two windows open upstairs just to get a little breeze, but if it is going to be a super hot day we'll just shut the house up and block the sun-side windows.

In our older 2 1/2 story brick house this takes care of our cooling needs for 95% of the summer.
posted by seanmpuckett at 10:29 AM on September 2, 2012

JayRwv : That seems counter productive to me. Blowing air INTO the attic. Shouldn't it be blowing air OUT OF the attic.

Well, you do blow it out of the attic - Or perhaps more accurately, through the attic (on preview, HotToddy has a different setup than I do, probably more in line with what you had in mind; Mine has the fan mounted just above the upstairs ceiling, rather than directly in the roof).

You basically want to remove as much heat from the entire house as possible, and heat rises. So, you want a fan that takes air from the highest possible point of the living-space in the house (usually the second-floor ceiling), and moves it out of the house. By blowing it up into the attic, you get the bonus of cycling air through the attic as well; From there, it just goes out your soffit/eaves/ridge vents.

During the day, you may also want a smaller eaves-vent fan moving air out of the attic - But in that case, you don't want to take the air from the living space of the house, because it would just get replaced by warmer air from outside.

One side note on this topic, for those who leave their whole-house fan on all night... Those suckers can draw a LOT of power - Often 300-500W. You may find it more energy-efficient (and thus, cost-efficient) to run the whole-houser for just 15 minutes or so once it gets cool outside, then either turn it down to its lowest setting or switch to a simple box-fan in an upstairs window.
posted by pla at 10:33 AM on September 2, 2012

I don't see a rationale for ever blowing air from the house into the attic if the goal is to keep the house as cool as possible.

If the outside air is cooler than the air in the house or the attic, you should blow outside air into the house and the attic.

If the outside air is hotter than both you should just sit tight, because blowing air from house to attic would fill the house with the hottest air from outside.

If the outside air is between house and air with the attic hotter than the house, you should again blow air from the outside into the attic, because that will reduce the heating the attic does to the house, but blowing air from house to attic would put hotter air into the house.

If the outside air is between house and attic with the house hotter than the attic, you should blow air from the outside into the house and leave the attic alone, because blowing air into the attic from the house would heat the attic, which would then be warming the house (blowing air from attic to house is bad for other reasons, I assume).
posted by jamjam at 10:41 AM on September 2, 2012

The fan blows out, creating a negative air pressure in the house, which the outside air replaces as it heads towards the negative air pressure. You are blowing cool air into the house by blowing the hottest air (at the top) out.
posted by Ironmouth at 10:58 AM on September 2, 2012 [1 favorite]

From the local building codes in my part of the world (pacific northwest) and from all the home remodeling articles that I've read, blowing air into your attic always a no-no.

The classic "whole house fan" is basically an oversized bathroom vent fan, ducted through your roof or wall to the outdoors, with a timer (and/or switch) to keep the air in your house fresh. Your confusion may come from these fans being located in the attic. But a properly installed fan will vent outside, no into the attic. Older house/poor construction may be venting this air into your attic, but that's wrong (and inefficient).

There's an entirely separate issue about venting your attic, through either passive ridge/soffit vents or through active attic fans. This is designed to cool your attic by allowing free air circulation. Venting your attic can help reduce the temperature of your home, along with having the proper amount of attic insulation installed.

A hot attic (heated all day by the sun cooking your shingles) will transmit heat downward into your house. Having good insulation and good ventilation will reduce this effect. Good attic insulation will also reduce your home heating costs in the winter to stop the transmission of heat into the attic.

So, to summarize:

1. Install a proper whole-house fan, sized for your house and it may include multiple inlets from different rooms in your house, and it will be vented outside.
2. Make sure your attic has good ventilation so that it doesn't overheat.
3. Check that you have adequate insulation in your attic.
posted by jpeacock at 11:12 AM on September 2, 2012

I LOVE mine. I had it installed two years ago and it is amazing! While people are concerned above about blowing hot air into an attic, attics are vented to the outside, so the air goes up to the attic, and put the vents. With it running for a while, eventually you cool the attic as well as fresh air is drawn into the house and thus into the attic. I tried fans in the windows to draw air into the house when it was hot inside and cool outside and ceiling fans, but that never worked very well. With the whole house fan (different than an attic fan if you are researching), it takes less than an hour to fully cool the house using outside air and it starts feeling better inside in a few minutes.

Since you are asking about best practices, here's what I know to be true:

1) you must have at least two windows open in the house when you run it.
2) if you don't open windows and turn it on, you will suck the air in any opening, including an ash filled fireplace, as a friend's unfortunately did in their house spreading ashes EVERYWHERE.
3) To cool the upstairs of a two story house, you have to have the downstairs windows closed and the upstairs windows open. If only downstairs windows are open, only the downstairs gets cool.
4) Running it costs next to nothing a month.

For what it's worth, I live in southern California. I wish I'd done it a lot sooner...
posted by cecic at 11:18 AM on September 2, 2012 [1 favorite]

My parents have theirs on a timer, so they turn it on in the evening when it's cool outside, and then it shuts off after a couple of hours and doesn't run all night.

It's a true attic fan, venting out of the attic, so they open windows, open the hatch to the attic, turn on the fan on the timer, and that's it. In the morning, close the attic hatch and close the windows. Works well in humid Virginia summers.
posted by gingerbeer at 11:48 AM on September 2, 2012

If you check the hourly temp graphs at you'll likely see that the coolest part of the day is around 6 AM.
posted by StickyCarpet at 11:57 AM on September 2, 2012

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