Jim Winkle - whole-house fan

Beat the Heat with a Whole-House Fan

by Jim Winkle (updated 18-Jul-2024; original 26-May-2022)

When the outdoor temps heat up -- especially when it reaches 90 in May(!) -- people start thinking warm thoughts about their air conditioners. Unfortunately, A/C uses a lot of electricity -- including the more efficient mini-splits -- but here are some ways to keep your living space relatively cool using much less electricity by using fans.

Box window fans

Some folks use inexpensive box window fans in exhaust mode to draw in the cool night air from other windows and then close up the house during the day; this uses far less electricity than A/C. However, it's a bit of a pain to lug fans in and out of windows every day, especially since you'll need at least two of them to get adequate air flow through even a modestly-sized house. My dad did this when I was growing up and I've done it for three decades. It's not 100% efficient unless your box fan fits exactly into your window (or you build a frame), since the fan will draw back in a bit of the air from around the sides of the fan.

Whole-house fan

The best solution using fans is to use what's called a whole-house fan. A whole-house fan is more powerful than a box fan and is typically installed in the ceiling of the top story of your house and blows into the attic. It's often called an "attic fan". Louvers open automatically when the fan is on and close up when it's off. Note that a whole-house fan is noisy and you might not want this installed near your bedrooms. In our house, there's simply no good place to install one.

Whole-house window fan

Fortunately, there's another option these days: a whole-house window fan. We bought the Air King 9166F in 2021 and really like it. It mounts semi-permanently on a window and exhausts to the outside. The window closes even with the fan mounted, so there's no need to put it in and take it out of the window every day.

This fan moves 3560 cubic feet per minute (cfm) of air on its highest speed. Your average $20-$40 box window fan moves air at the rate of 1800 - 2500 cfm. So the 9166 fan is 42% more powerful than a high-end box fan. It's also more efficient since it comes in an enclosure which adjusts to the width of many windows for much less air leakage. And it seems very well built (metal blades!) so will hopefully last.

The one downside is that it's a little harder to open the window since you have to reach over the fan to pull the window up, as opposed to pushing it up. If your window opens easily, this shouldn't be much of a problem.

If it starts to rain and it's just a light rain with little wind, you can leave the window open since the fan is inside the house.

Will an exhaust fan always be enough?

In the Madison area, there's generally a time or two when the nighttime temperature doesn't dip below 80 for several nights in a row. Your home will likely be uncomfortably warm starting on the second day. I highly recommend a trip to the library, movies, or the beach on those days. :) We have ceiling fans in our bedrooms and those are the only nights we use them. We don't have any trouble sleeping, but your mileage may vary.

Purchase and installation costs

Here are very rough extimates to purchase and install A/C in 2022.

A window A/C unit costs around $300, more if you need electricial work done.
A mini-split system costs between $1500 - $4000.
Central air costs between $3000 - $7000.

The 9166F fan costs around $240, and you or a friend can probably install it; it takes just four screws which go into the side of the window frame.

Electricity usage and CO2 emitted

As of 2015, 17% of residential electricity usage is from A/C. This figure is almost certainly higher today since A/C installation is on the rise.

Here is the power used while cooling the air with A/C.

A window A/C unit uses 500-1400 watts per hour, which will cool a bedroom (up to 1000 square feet).
A typical mini-split system uses 600 - 2000 watts per hour depending on how many areas you cool.
Central air uses an average of 3500 watts per hour.

On its highest setting, the 9166F fan uses only 170 watts per hour. If you happen to subscribe to time-of-use electricity, it costs considerably less during the nighttime hours (about half of the regular rate in Madison), which is when you'd run the exhaust fan.

On average, 0.85 pounds of CO2 is emitted per kilowatt-hour of electricity used (for example, 1 kWh is used while using 500 watts for 2 hours). Cooling a whole house all day with A/C might use 24 kWh (2000 watts from 10 am - 10 pm). That generates about 20 pounds of CO2. On the other hand, cooling a whole house all night with the Air King fan would use 1.4 kWh (170 watts from 9 pm - 7 am), or generates about 1 pound of CO2. A ceiling fan on low uses very little electricity (about 4 watts).

Conclusion

Overall, using a whole-house fan costs far less to install and run than any kind of A/C. A couple of friends have also installed a whole-house window fan and are happy with it. No matter what you go with, consider using renewable electricity to power your home.

Thanks for reading; please let me know if it was helpful! Also, one final note: I haven't double-checked some of the figures in here, but my engineer gut sense tells me they're pretty accurate.

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