How NOT to use fans to cool a room

Ian! D. Allen –

July 2019

1 Executive SummaryIndexup to index

  1. Fans do not cool a room. Turn off a fan when you leave the room. Leaving a fan running in an empty room makes the room hotter (because the fan uses energy).

  2. Use fans to send warm air out of a room; do not use them to pull cool air into a room (because you also send the heat of the fan into the room).

  3. Don’t let hot air come in the windows on the sunny (hot) side of the room; let cool air come in the cool (shaded) side of the room.

2 ExplanationsIndexup to index

Here are the longer explanations of the above points.

2.1 Fans do not cool a roomIndexup to index

Fans do not cool a room. Fans move air from one place to another, and moving air feels cooler than still air.

To move air, fans use motors that require electricity. That electricity energy (around 100-500 watts, depending on the size of the fan) moves the air, and the moving air eventually loses its energy in the form of heat. An unwanted by-product of the moving air is heat. The fan motor is also not perfectly efficient and itself produces waste heat.

A fan is a 100-500 watt electric heater that also moves air. A big fan (500 watts) heats a room faster than a small fan (100 watts). The moving air feels cooler if it blows past you, but don’t forget that it’s heating up the room as it goes.

The room will stay cooler if you do not use the fan. Decide if the moving air is worth the hundreds of watts of extra heat that the fan is adding to the room as it runs.

If there is nobody in the room to benefit from feeling the air move, a fan is just an electric heater, making the room hotter as it runs.

Turn off the fan when you leave the room!

2.2 Use fans to send warm air out of a roomIndexup to index

Fans consume energy and thus generate heat. A fan’s heat goes in the direction of the moving air. If you use a fan in a window or door to blow cool air into the room from outside the room, you also send the hundreds of watts of heat from the fan into the room along with the cool air, making the cooling less effective, possibly much less effective. Don’t send fan heat into the room.

If you turn the fan around so that it blows the warm air in the room out the window, allowing cool air to flow in another open window, then all the heat of the fan also goes out the window and only cool air comes in the other window. If the fan blows out, none of the fan heat stays in the room and the room gets maximum cooling; this is good.

Use a fan to send warm air out of a room!

2.3 Let air come in the cool (shaded) side of a roomIndexup to index

If you can, don’t let air enter a room from the sunny (hot) side. Use a fan to blow air out of the sunny-side window; never let the hot air come in that way.

| Ian! D. Allen, BA-Psych, MMath-CompSci Ottawa CANADA
| Home:  Contact Improvisation Dance:
| Former college professor of Free/Libre GNU+Linux @
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