How Much Fan Do You Need When Riding Indoors?

January 15, 2019

LAST UPDATED April 20, 2020

I’m still amazed when I hear from Zwifters who don’t yet use a fan. How do you survive? Granted, I run pretty hot, and perhaps you don’t need as much cooling as I do. But you do need a fan for Zwifting, unless you are specifically training for heat adaptation.

WHY YOU NEED A FAN

Your body only runs at around 25% efficiency, which means that every watt of pedal power comes with three “wasted” watts which are turned into heat. Cruising along at 200 watts? You’re generating 600 watts of heat. Race at 400 watts and you’re producing enough waste heat (1200 watts) to power a toaster oven!

Without a fan, that waste heat creates a superheated pocket of air around your body. Soon enough your body temperature will rise, your RPE will increase, and your power output will decline as your body kicks in its “governors” to keep you from overheating and damaging internal organs. Without a fan, you simply can’t hold your maximum wattage! This is probably the biggest cause for new Zwifters complaining that they can’t hold the same power indoors as they can outdoors.

Adequate cooling also helps to reduce cardiac drift. Here’s what coach Hunter Allen has to say in this article:

Staying cool is very important while riding indoors, a small change in body temperature can create a large change in the amount of power you can produce. As your body becomes hotter and hotter, you will perspire more and more to help cool the skin. Losing that fluid creates thicker blood and therefore more resistance internally in the cardiovascular system. The result of this is that your heart has to pump faster in order to get the same amount of blood to your working muscles. This is called cardiovascular drift and while you are stressing the heart more, you are not increasing your absolute FTP.

By keeping yourself cool and hydrated, you will be able to produce more wattage for a given heart rate and thus increase your overall absolute fitness. Overheating can easily cause a reduction in indoor power by 20 to 30 watts alone, so it’s critical that you have a large fan blowing on you during your workout and if you can do it in a cool room, that will make a difference as well.

HOW STRONG IS MOTHER NATURE’S FAN?

One way to choose how much air you need blowing on you indoors is trial and error. But this is Zwift Insider, so let’s use some math to come up with another solution by calculating how much air cooling we receive when riding outdoors. Wind generated by fans is typically quoted in CFM, or cubit feet per minute. What CFM do we experience when riding outside? That is, how much air hits us every minute? We start by figuring out our frontal area. This obviously varies depending on rider size and position, but typical frontal areas taken from this page are:

Bar Tops: 0.632 square meters, or 6.8 square feet
Hoods: 0.40 square meters, or 4.3 square feet
Drops: 0.32 square meters, or 3.4 square feet

Then we multiply that frontal area by how far we travel in a minute in order to get CFM. At 20 miles per hour you are traveling 20*5280=106,600 feet per hour. Multiply that by our frontal area–let’s use the hoods for a best guess estimate–and we get 20*5280*4.3=454,080 cubit feet of air per hour. Divide by 60 and you get 7568 cubic feet per minute. This is how much air is hitting you when riding outdoors on a windless day on the hoods at 20mph (32.2kmph). What if we tweak our variables a bit?

20mph on the hoods = 7568 cfm
15mph on your bar tops = 8976 cfm
25mph in the drops = 7480 cfm

So you can see that at a minimum you need around 7500 cfm of fan power to replicate outdoor air movement in your pain cave.

HOW MUCH DO YOU REALLY NEED?

There are other factors to consider, of course:

Some of the fan’s output will miss your body, so you’ll need more than 7500 cfm of air movement in order to replicate the outdoor feel
Your ambient pain cave temperature may be higher or lower than outdoors, requiring more or less air movement
You may want more or less air movement than you receive outside

Cooling needs vary from person to person and setup to setup, but for me I like having two powerful fans so I can turn them both on or off independently when desired. I would say if you find yourself dripping sweat during anything other than maximal efforts, you need to improve your fan setup.

FAN RECOMMENDATIONS

Here are three good Zwifting fans with consistently good reviews on Amazon:

Stanley Blower Fan
350CFM

If you want a lot of air in a narrow window, “blower” fans are the ticket. They don’t move as much air as a good box fan, but they move a narrow column of air very quickly, great for keeping sweat off your face.

Lasko 20″ High Velocity Fan w/Remote
3460CFM

Lasko’s 20″ fans move a lot of fair, even though I doubt their quoted CFM number. Not a bad price, either, considering it includes a remote.

Vornado Mid-Size Air Circulator
331CFM

Kind of between a “blower” fan and the Wind Machine in terms of tightness of the air column, the Vornado packs a punch in a smaller footprint.

THE PREMIUM SOLUTION

Want the very best? Check out Wahoo’s Kickr Headwind. It blows a very tight column of air right where you need it, and wind speed can be automatically modulated based on your virtual speed (to simulate a headwind) or heart rate (to cool you when you need it most). I’ll be posting a review on this soon, but everything I’ve read and seen on this has folks very pleased with its functionality.

Buy on Wahoo.com >
Buy on Amazon >

Pro tip: get a wifi-connected power strip so you can easily turn your fan(s) on using your phone!