How Long Should You Run An Attic Fan?

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Attic fans are relatively common additions to homes with large attics. These fans serve to draw hot or humid air out of the attic. Whichever of these is your goal; you may be wondering how long you should run your attic fan for. In this post, we leverage industry professional knowledge and research to provide a thorough answer.

Generally speaking, an attic fan should run as long as it takes to replace all the attic air with outside air. Usually, this takes roughly 15 minutes. However, you can calculate a much more accurate time by dividing the strength of your fan by the volume of your attic space.

Keep reading the rest of this post for a detailed rundown on how to determine how long to run your attic fan. We follow this with information on how frequently to run your attic fan and a quick introduction to the main reasons for using an attic fan. In addition, we answer several questions related to the topic of this post.

Wooden framing interior of an attic without insulation foam, How Long Should You Run An Attic Fan?

Deciding How Long to Run your Attic Fan

It is only necessary to run your fan as long as it needs to replace the attic air with outside air. Running your fan any longer than this is just a waste of electricity. This is because you are pulling air outside your attic that is the same as the air you pull into the attic.

A light blue colored asymmetric gable type roofing

To calculate how long to run your attic fan for, all you need to do is calculate and measure your attic’s volume and understand the strength of your fan. Read the following three sub-sections for directions on accomplishing this task.

How to Calculate the Volume of your Attic

Calculating the volume of your attic is a process that varies depending on the type of attic you have. However, no matter your attic, the goal is to understand the cubic feet of the attic. This involves measuring the attic space using a tape measure and then using basic geometry to calculate the volume.

Let us take the example of a simple peaked attic with a length of 40-feet, a width of 20-feet, and a height of 10-feet. First, multiple the width (20) times the height (10) to get 200 square feet. Then, because the gable end is a triangle, divide this by 2 to get 100 square feet. Now multiple this number (100) by the length of your attic (40) to get a grand total volume of 4,000 cubic feet.

Most attic spaces are unfortunately not this simple. Common techniques required to calculate volume include the area of a triangle, the area of a rectangle, volume calculations, and the Pythagorean theorem. It helps to sketch the space out and break it into manageable sections as you perform the math.

Understanding the Strength of Your Fan

It is usually a matter of checking your fan owner’s manual or perhaps your fan housing to understand the cubic feet per minute that it blows. Often, this is written in the format of a number followed by CFM (cubic feet per minute).

Sometimes you will not be able to find the cubic feet per minute of your fan written anywhere. In this case, you can always hire a weatherization professional to test your fan’s strength. This might be a good idea even if you can find your fan’s strength, as it is rare that a fan pulls the CFM as advertised.

Calculating How Long to Run your Fan

Now divide the cubic volume of your attic by the CFM of your fam to get the number of minutes needed for your running fan to replace the air in your attic. For example, a 200 CFM fan divided by a 4,000 cubic foot attic results in 20 minutes of fan run time.

If the math involved in this process is not an option, you can test the time needed in person. Wearing the proper personal protective equipment, head up into your attic. Now, find an out-of-the-way area on the far end of the attic. Turn the fan on, start a timer, and wait for the cooler air to hit you. Use this as an estimate of how long it takes your fan to replace your attic air.

Deciding How Frequently To Run Your Attic Fan With a Timer

Often, attic fans are equipped with thermometers and hygrometer switches (a hygrometer measures humidity). This allows you to set humidity or temperature at which the fan automatically turns on.

At What Temperature Should An Attic Fan Be Set?

Generally, set your attic fan to anywhere between 95 degrees and 115 degrees. However, it is important to set the fan hotter than the ambient outside temperature to avoid burning out the fan’s motor.

At What Humidity Should An Attic Fan be Set

Usually, set your fan to turn on around 70% relative humidity. However, like for temperature, if you live in a very humid area, you do not want to set the fan, so it runs all the time.

Deciding How Frequently To Run Your Attic Fan Without a Timer

The timers take the guesswork out of deciding how frequently to run your attic fan. However, if your fan is switch operated, you have some decisions to make. Read the following sub-sections to learn how to decide the approximate frequency you should run your attic fan.

Cooling Your Home

An attic heats up fast in the summer. Often you can feel the heat if you touch the ceiling that is adjacent to the attic. If you feel that heat radiating into the room near your attic, it is usually time to turn your fan on.

Turn the fan on, let it run the correct amount of time, and then turn it off again. Wait a few hours and then repeat. If you take notes on how often you run your fan and how hot the attic-adjacent ceiling gets, you will develop a sense of how often to run your fan. For a more accurate understanding, install a thermometer that is visible from your attic hatch and check it now and then.

Click here for a wall mount thermometer from Amazon.

Once you have identified an appropriate frequency, you might have to adjust based on outside temperature and day length. You will know you have succeeded in understanding how often to use your attic fan when you notice that your attic-adjacent rooms are cooler and more comfortable.

Preventing Ice Damming

Ice damming occurs on roofs because the heated air from inside the home escapes into the attic melting the roof’s snow. That newly formed ice creates a dam that blocks other snowmelt from flowing off the roof. This water then causes roof leaks. By using a fan to pull the hot air out of your attic and replace it with cold outside air, the ice damming problem is solved.

Ice damming occurring on the roof of a house

For ice damming, run your fan as often as it takes to prevent ice build-up. Unfortunately, this is also a bit of a guess and check process. Ideally, you should run the fan before the ice starts to build up. As this is not always feasible, begin to run your fan every few hours or so to stop ice damming once you notice it. Installing a thermometer near the attic hatch will also help with this issue.

Removing Harmful Humid Air

If highly humid air is escaping from your home into your attic, it can cause wood deterioration, pests, and more. An attic fan helps suck that humid air out. As with cooling your home and ice damming, the frequency of removing that humid air depends on how much humidity is in your attic and the humidity outside.

Generally, cycle your attic air every few hours to ensure that the humidity does not get too high. If you notice any mold or pest issues, it is good to start cycling the air even more frequently or perhaps call a professional.

Is It Okay To Run An Attic Fan All Night?

No. Running your attic fan for too long runs the risk of blowing out the fan motor. Further, nighttime is usually cooler, so you do not need to run your attic fan as often. It is ok to run the fan for a few hours at a time, but then you should let it cool off before firing it up again. However, and as stated above, running your fan longer than it takes to replace the attic air is a waste of electricity.

Should You Run Your Attic Fan In The Summer?

Yes, run your attic fan in the summer if your goal is to cool off your house by removing superheated attic air. Be careful; if you run your attic fan for too long in one go, you run the risk of harming the fan motor. This often happens when a fan thermometer is set lower than the outside air temperature.

Should You Run Your Attic Fan In The Winter?

Yes, if you are combating ice damming, run your attic fan in the winter. If ice damming is not an issue for you (which it should not be in a well-insulated attic), then there is less of a reason to run your attic fan in the winter. You still might want to run your attic fan in the winter if the humidity in your attic is your issue.

Can You Run An Attic Fan With The AC On?

Yes, you can. However, if your attic plain is poorly air-sealed, it means that your attic fan might pull the cooled air through your ceiling into the attic. Because of this, it is essential to air seal your attic before frequently using an attic fan. For more information on this topic, read this great article: Can You Run An Attic Fan With The AC On? [And Should You?].

Are Attic Fans Appropriate for all Homes?

No, many homes do not require attic fans. Most other home issues that might be solved with attic fans are better solved with better insulation and air sealing. Also, having too many vents in your attic can reduce the effectiveness of attic fans. Read this article to learn more: Is An Attic Fan Necessary (Even With Ridge Vents)?

In Closing

In this post, we cover how long to run your attic fan. Next are included tricks to understanding the frequency you should run your attic fan. To close, we answered several questions related to this post. Good luck!

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