Whole-house airflow: ceiling fans, whole-house fans and attic fans

This section covers the equipment that gets installed rather than plugged in: ceiling fans, whole-house fans and attic fans. They have very little in common mechanically, and it is worth separating them clearly, because they get confused constantly and they solve different problems.

• A ceiling fan moves air within a room, so people in that room feel cooler. It does not change the temperature and it does not exchange any air with outside. It is the cheapest cooling available anywhere in a house.
• A whole-house fan sits in the ceiling of your living space and pulls outdoor air in through open windows, through the house, and out via the attic. It genuinely lowers the house temperature — but only toward the outdoor temperature, never below it.
• An attic fan exhausts the attic only. It does not move air through your living space at all. What it does is stop a 140°F attic radiating heat down through your upstairs ceilings.

They are complementary rather than alternatives. A house in the right climate can sensibly have all three.

This is the one category where the useful data is completely public. ENERGY STAR certifies ceiling fans and publishes, for every certified model, the tested CFM per watt, the wattage at high and low speed, and the standby wattage. A manufacturer cannot write its own CFM/W figure.

We pulled every US-market certified fan available from 2023 onward — 987 models — and what comes out is that blade count, blade shape and price tell you almost nothing, while the certified efficiency figure tells you almost everything. Two fans that look identical on a shelf can differ by 35% on it.

A ceiling fan on low costs about 7.9¢ for a month of nights. That is roughly one-fiftieth of a box fan for comparable comfort, and the reason is blade diameter rather than brand: a 52-inch blade turning slowly displaces an enormous volume of air for very little energy.

Efficiency also rises with size — the best figure in the whole certified dataset, 385 CFM per watt, comes from a 72-inch fan. The full ranking, with sizing guidance, is here.

If you already have a ceiling fan and it feels like it is doing nothing, check this before anything else. In summer a ceiling fan should turn counterclockwise viewed from below, pushing air straight down onto you.

Clockwise is the winter setting: it pulls air up the middle of the room and pushes warm ceiling air down the walls without creating a draft. Useful in January, close to useless on a hot day.

Stand directly underneath on high speed. If you feel air on your face and arms, it is right. If you feel almost nothing, it is on the winter setting and there is a small switch on the motor housing that fixes it in ten seconds. Full instructions here.

One more free thing: turn it off when you leave the room. A ceiling fan does not lower the temperature, so in an empty room it is doing nothing at all except adding a little motor heat.

A whole-house fan can exchange the entire air volume of a house every two to three minutes. On the right evening it takes a house that has been baking all day down to outdoor temperature within the hour, and it flushes the attic on the way through — which is often the larger benefit.

But it is a fan, so it can never cool below the outdoor temperature. Which makes the decision a single lookup: your average summer overnight low.

• Below 65°F — it can replace air conditioning for much of the season. Excellent value.
• 65–70°F — worthwhile for dumping the day’s heat; you will still want AC on hot nights.
• Above 75°F — it is an expensive attic fan.

You can test the idea for the price of a fan you keep using: once the outdoor temperature is genuinely below indoor, put a window fan in an upstairs window blowing out and open windows downstairs on the opposite side. If the house cools noticeably over an hour, a whole-house fan will do the same far faster. The full test and the running-cost math are here.

An attic under a dark roof in summer sun gets far hotter than the outdoor air, and that mass sits directly above your upstairs ceilings radiating heat downward all afternoon and evening — continuing long after sunset, because the roof structure holds heat.

Crucially this is radiant heat, not air movement. No fan inside the room addresses it, because you are being warmed by a hot surface above you rather than by hot air around you. It is why an upstairs bedroom can feel oppressive at 11pm with the air temperature reading fine.

The fixes, cheapest first: improve attic ventilation (ridge and soffit vents, entirely passive), add attic insulation (which also pays back in winter), or fit an attic fan to actively exhaust it. Unlike a whole-house fan, an attic fan works regardless of what your overnight lows do.

The upstairs page has a ten-minute diagnosis that tells you whether attic heat is actually your problem — touch the ceiling at 4pm, and if it is warm, it is.