The best window fans for exhaust, and the opening nobody mentions

An intake fan and an exhaust fan do the same mechanical work in opposite directions, but they fail in completely different ways.

Blow air into a room and the room leaks it back out everywhere at once — under the door, around the sash, through the gap where the accordion panels do not quite meet. The fan wins anyway, because it is pushing air at a target and every leak still carries outdoor air inward.

Pull air out of that same room and those leaks become the problem. The fan will move its rated volume only if an equal volume can get back into the house from outside. If the room is sealed, the easiest path back in is the gap around the fan itself, and the fan spends the night recycling the same hot air past its own blades. Nothing gets cooler and the meter still turns.

Airflow is not the first question here. Whether the fan can be reversed without being taken out of the window, and whether the opening it sits in can be closed around it, both matter more — because an exhaust setup that is awkward to switch simply does not get switched. In order:

1. How it reverses. From a control, or by lifting the fan out and turning it around. Only the first one survives contact with a real night.
2. How well the opening seals. Storm Guard housings, accordion panels and foam blocks all do this to different standards, and every square inch left open is air the fan pulls back in.
3. Whether airflow is published at all. Two of the four manufacturers here publish a CFM figure. Two publish nothing, and we say so on their cards rather than quoting a number from a retail listing.
4. CFM per watt, our own division on published airflow and published wattage, because no certification program covers window fans.
5. Minimum opening. Height, almost always — this is what rules fans out, not width.
6. Control. A thermostat and a remote decide whether the fan is still doing the right thing at 3 a.m.

Ventilation is sized on air changes: room volume divided by airflow gives the time for one complete change. The formula the rest of this site uses is CFM needed = (volume × air changes per hour) ÷ 60, and it rearranges to minutes per change = volume ÷ CFM.

Room volume is floor area times ceiling height. At a standard 8-foot ceiling, the two fans that publish airflow land here:

Two things follow from that table, and they are the whole argument of this page.

For a single room, fan size is not your constraint. A bedroom is theoretically flushed in under half a minute by either fan. Nobody experiences that, because the house cannot feed air in that fast — which is another way of saying the inlet, not the fan, sets the real rate.

For a whole floor, size is the constraint. Once you are trying to move 9,600 cubic feet, the difference between four minutes and six and a half is the difference between finishing before bed and still running at midnight.

These are free-air ratings divided by room volume. A real install, with a partly blocked grille, panels that do not seal and an inlet smaller than the fan, moves less. How much less is not something we can put a number on: no certification program measures window fans, so every figure on this page is a manufacturer claim or our arithmetic on one.

Buying a second fan is the usual instinct when exhaust underperforms. An open window is free and almost always the better fix.

1. Open one window on the coolest side of the house, shaded, and as far from the exhaust fan as the layout allows. Air takes the shortest path between two openings, so the distance between them is what decides how much of the house it crosses.
2. Open the inlet wider than the fan. A restricted inlet throttles the fan more than a restricted outlet does, because the fan is trying to pull against it.
3. Open the doors in between, or at least leave a real gap under them. A closed bedroom door reduces a whole-house exhaust to a one-room exhaust.
4. Run it when outside is cooler than inside, not before. In most of the country that is after sunset and again at dawn; the daily temperature curve decides it, not the clock.
5. Turn it off if the air conditioner is running. They are opposite machines: one spends money cooling air and the other throws it outside.

One safety note that applies to any strong exhaust fan in a closed house. Pulling air out lowers indoor pressure slightly, and a depressurized house can draw combustion gases back down the flue of a gas water heater, furnace or fireplace. An open inlet window prevents that as well — it is the same fix, which is the best argument for never skipping it.

Only two fans on this page publish wattage, so only two have a running cost. At 18.34¢/kWh:

Even the thirstiest fan here runs a full night for less than the daily standing charge on most utility bills. The comparison that matters is not between these fans; it is between any of them and the air conditioner they let you leave off, which is worked through in how much electricity a fan actually uses.

• The clear opening height, with the window fully open. Under 22 inches rules out both Air Kings; under 10 rules out everything here.
• The clear opening width, which sets whether the panels or foam blocks can close the gap either side of the fan.
• Where the second opening is going to be. If the only other window is in the same room, you have a room fan, not a house fan, and you should buy accordingly.
• Whether the sash can close onto or behind the fan, which decides both weather and security for anything left running overnight.