Are whole house fans worth it?

A whole-house fan is a large fan mounted in the ceiling between your living space and your attic. Open some windows, switch it on, and it pulls outdoor air in through those windows, through the house, up through the fan, and out through the attic vents.

In the right climate it is one of the most effective and cheapest things you can do to a house. In the wrong climate it is an expensive hole in your ceiling. The variable that decides which is your overnight low, and nothing else comes close in importance.

A whole-house fan is an air exchanger, not a cooler. There is no refrigeration cycle anywhere in it. The best it can ever do is bring your indoor temperature down toward the outdoor temperature, quickly.

So look up your average overnight low in July and August:

Roughly: much of the interior West, the Mountain states, inland California, the Pacific Northwest and parts of the Northeast have the nights for this. The Southeast, the Gulf Coast and Florida largely do not — a Houston August night sitting at 79°F gives a whole-house fan nothing to work with.

Humidity is the second check. Even where nights are cool, pulling in air at 90% relative humidity brings that moisture into the house, and it lands on your walls and furnishings. In a humid climate with cool nights this is a real trade-off rather than a free win.

Two separate things, and the second is the one people underestimate.

One: it dumps the day’s heat fast. A house that has been baking all day holds heat in its structure — drywall, furniture, floors. Opening windows alone relies on whatever breeze exists, which is often nothing. A whole-house fan can exchange the entire air volume of a house every two to three minutes, so the house reaches outdoor temperature in an hour rather than overnight.

Two: it flushes the attic. An attic in summer sun can reach 140°F, and that heat radiates down through your ceilings all evening. A whole-house fan pushes house air up through the attic and out the vents, and in doing so it drags the attic temperature down toward outdoor temperature. That alone changes how an upstairs feels — which is often the actual complaint that led people here.

Whole-house fans generally draw between 100 W and 600 W depending on size and speed. At 18.34¢/kWh:

Against central air conditioning drawing several thousand watts, the saving on a night when the fan can do the job instead is substantial. And in practice most people run a whole-house fan for an hour or two in the evening to flush the house, not all night, which makes the real cost lower still.

You can approximate the effect before committing to an installation, and we would suggest doing so for a couple of weeks.

1. Wait until the outdoor temperature drops below your indoor temperature. Usually mid to late evening. Check with a thermometer rather than by feel — this is the whole premise and it is easy to get wrong.
2. Put a window fan in an upstairs window blowing out. That exhausts the hottest air in the house.
3. Open windows downstairs on the opposite side to let cool air in.
4. Watch what happens over an hour or two.

If the house cools noticeably, a whole-house fan will do the same thing far faster and across the whole building, and it is very likely worth installing. If two hours of exhaust fan makes almost no difference, a whole-house fan will not either — and you have learned that for the price of a fan you can keep using.

A Lasko twin window fan draws 22–31 W and moves up to 560 CFM, which costs about $1.36 for a month of nights. That is a cheap experiment.

Yes, if your summer nights drop below about 65°F and your air is not very humid. In that case it is one of the highest-value cooling investments available to a homeowner: it can displace air conditioning for a large part of the season, it costs a fraction of a compressor to run, and it fixes the attic heat problem as a side effect.

No, if your nights stay above about 75°F. The physics does not care how good the unit is. It can only move outdoor air, and if that air is 79°F you are running a large fan to fill your house with 79°F air.

In between, test it first with the window fan experiment above. Two weeks of evidence about your own house beats any general recommendation, including this one.

And if the answer is no, the alternatives are a correctly sized window unit for the rooms that matter, or ceiling fans at 14 W for the rooms that only need air moving.