How Long Can a Bathroom Exhaust Fan Run?

The main goal of a bathroom exhaust fan is to remove excess steam from the room to prevent mold growth, clear the mirror, and make a bathroom more comfortable. But, it’s important to know how long you should run a bathroom exhaust fan, so below I will cover how long a bathroom exhaust fan should run.

As a general rule, a bathroom exhaust fan should run for 10 to 30 minutes after a shower to reduce the humidity to a comfortable level. A comfortable humidity level is around 45% to 50% humidity and stops mold growth.

If a bathroom exhaust fan is left on it doesn’t use too much power, so there is a good margin for error. The temperature also affects how effective a bathroom exhaust fan can reduce the humidity in a bathroom. 

Below, I will cover how much power an exhaust fan uses, how the length of time a bathroom exhaust fan runs affects humidity levels, and how much it costs to run a bathroom exhaust fan.

How Fast Does a Bathroom Exhaust Fan Reduce Humidity

a bathroom that is humid and full of stem

Studies have shown that after a shower or bath the humidity in a bathroom is 100%. This is as humid as a room can get. A bathroom exhaust fan, opening the door, and opening any windows will reduce the humidity very quickly. But, here’s how fast exactly an exhaust fan reduces humidity. 

On average, it takes 10 minutes for a bathroom exhaust fan to reduce the humidity to a comfortable level. An average-sized bathroom is 240 cubic feet, and an average exhaust fan moves 25 cubic feet per minute. So after 10 minutes all of the air in a bathroom has been sucked out by the exhaust fan.

It’s important to note that an exhaust fan won’t reduce the humidity in a room. Instead, it sucks air from the rest of the house into the bathroom. 

As air is blown out of the exhaust fan, it creates a vacuum that sucks in air through the door of the bathroom, and through any open windows. But, if there is a slight breeze or wind blowing past an open window it will also suck any air in the bathroom out the window.

The average air humidity in your home will also be the average humidity in the bathroom after about 10 minutes. It’s widely established that a humidity of 45% to 50% is comfortable (source). Once, the exhaust fan has sucked out all the air in a bathroom. So, if the humidity in your home is about 50%, the bathroom will also have a humidity of 50%. 

Or, if you keep the windows in a bathroom open. The humidity in the bathroom will be roughly equal to the humidity outside. 

How Long to Leave the Bathroom Fan on After a Shower

Modern exhaust fan in the bathroom with remote control for blade speed and power. Airing the room, domestic

After a shower, there is typically a small amount of water on the walls and fixtures in a bathroom. This happens when the moisture content in the air settles onto surfaces. 

If the surfaces are cold such as the toilet bowl, the water will partially freeze to them, and then melt. As you may know, this is called condensation. Hot air goes up, so the high humidity air after a shower can form droplets on the ceiling if a bathroom is kept completely sealed during a shower.

It’s best to start running a bathroom exhaust fan before you get in the shower. This will reduce how much water gets onto the walls, ceiling, and fixtures in a bathroom. 

It will also reduce the total time before the exhaust fan has done its job, and removed as much of the humidity from a room as possible. A bathroom exhaust fan is ineffective at reducing the average humidity in the air. 

If your bathroom is particularly damp, or you want to really dry it out it’s best to use a dehumidifier for about 30 minutes a day. This will slowly draw out all the moisture in the fixtures and walls, over a week or two, and make your bathroom far drier overall.

How Much Electricity Does a Bathroom Exhaust Fan Use

A bathroom fan does need to run for quite some time before it will remove all of the high humidity air from a bathroom. There can be a bit of a concern about how much power an exhaust fan in a bathroom uses, so here’s how much it costs to run one.

On average, a bathroom exhaust fan costs $0.12 per month to run. Based on an average exhaust fan size which is 36 watts, and running it for the recommended time, 10 minutes to 30 minutes, 6 times per day.

This is assuming that there are 3 people in a household and they each take a shower once a twice per day. Also, when they turn it on they keep it on for the maximum amount of time 30 minutes. So, you can see that running an exhaust fan in a bathroom is incredibly cheap.

What Happens if You Leave a Bathroom Exhaust Fan On Too Long

A bathroom exhaust fan can easily be left on because a bathroom is not used that often and you can easily forget about it. But, here’s what happens if a bathroom exhaust fan is left on too long.

Overall, nothing happens if you leave a bathroom exhaust fan on too long. Bathroom exhaust fans use about 1/10th of 1 cent of electricity per hour. So, if you run it overnight for 12 hours it will use about $0.01 of electricity. 

But, about 10 to 30 minutes is the maximum a shower exhaust fan needs to be left on. Any more than this and the humidity in a bathroom will be balanced with the outside air or the air in your home. So, keeping it on won’t help reduce the moisture in the bathroom.

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