How Does An Attic Fan Work? (*2024 Ultimate Guide*)

Want to know the lowdown on how attic fans work?

Attic fans can help keep your attic cool and dry during the warm season, and can also ventilate your attic during winter (here is my review on the best gable attic fans).

In this guide, I will go over...

  • How attic fans work to cool your attic
  • The basics of how the thermostat works to turn the fan on & off
  • How attic fans bring in fresh outdoor air into your attic
  • Things that can help or hinder attic fan operation
  • And why an attic fan can help your HVAC system
how does an attic fan work

How Does An Attic Fan Work?

Attic fans are electrically powered fans that are installed on roofs or gable walls in order to exhaust hot attic air to the outside. 

Most attic fans operate by using a simple thermostat which looks like a small metal box usually attached to a rafter or truss within a foot or two from the attic fan.

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This thermostat has a simple temperature dial so you can adjust it to the desired temperature where you want the attic fan to automatically turn on. 

Then, when the attic temperature drops below that temperature, the fan will turn off.

Wall Switch

In addition to an automatic thermostat, you can also wire the attic fan to a wall switch so you can turn it on or off manually.

This wall switch can be installed in the attic such as next to the attic hatch, or it can be installed in an interior hallway.


Similar to a bathroom exhaust fan, an attic fan can also be wired to a timer so that it will turn on for a certain amount of hours throughout the day --- such as between 10am and 5pm.

Programmable Timer For Attic Fan

Installing a timer can be useful if you want to operate the fan during the cold weather season to help prevent mold growth or hazardous ice dams.

Read Also: How Do You Vent Attic Without Soffits?

Where Do Attic Fans Pull in Fresh Air?

Attic fans work by pulling in fresh outdoor air and at the same time exhausting the stagnant attic air.

The attic fan pulls in fresh outdoor air by the passive vents already existing in the attic such as...

#1. Soffit Vents

These are the vents along the soffits or eaves of the home. The most common soffit venting is continuous (no breaks) and the vent holes are pretty small, and looks like a white plastic mesh.

#2. Turtle Vents

These are the circular or rectangular vents on the roof.

If a home has these smaller passive vents, there are frequently more than one, spaced out over the roof.

#3. Gable Vents

Homes usually have two gable vents, one on each side of the home, the so-called gable walls. These vents are commonly rectangular in shape and are around 2 to 5 square feet in size.

Read Also: What Are The Best Roof Mounted Attic Fans?

How Do You Inspect Attic Fans?

#1. Will It Turn On?

Locate the thermostat (small metal box) usually mounted on a rafter or truss next to the fan in the attic. Turn the temperature knob so it is below the current attic temperature. You can also use a heat gun, hair dryer, or lamp to heat up the thermostat if the attic is cold.

#2. Turn The Blades

If the attic fan doesn't turn on, try turning the fan blades. If the blades won't turn, you may have a seized motor.

#3. Operation Quality

If the the attic fan turns on, are their any weird noises? Does it run smoothly? Can you feel air flow or is it very weak? The fan screen or louvers should not be blocked with debris impeding air flow.

#4. Water Stains

A common problem with attic fans are leaks. Are there any signs of water leaks around the fan on the roof sheathing?

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What Attic Fans Can Do?

The main point of having an attic fan is to keep the attic cool and dry. Here are a few of the top things that attic fans do...

#1. Increase HVAC Efficiency

The most common reason is to reduce the load on your air conditioner and to help cool down the home.

If your attic gets very hot due to improper ventilation, this heat can radiate towards the interior of the home. This is why with many homes the upper level of the house is noticeably warmer than the lower level or basement.

This temperature gradient is also because cold air is heavier than warm air and sinks.  Hot air rises. 

In addition, when the cold interior sinks towards the basement or lower level, it can actually create a suction effect and pull in hot attic air towards the interior.

Read Also: What Are The Seven Pros & Cons of Attic Vent Fans?

#2. Roof Protection

One reason to do what you can to cool down your attic is to protect an asphalt roof.

If you have asphalt shingles, the most common type of roof covering in the world, then you may not realize that a hot attic can prematurely age the shingles. Asphalt shingles is an oil-based product, and if these shingles get exposed to extreme heat, then will become more brittle and crack.

Keeping your attic can help prevent any premature aging.

Read More: How Can Attic Fans Protect And Prolong Your Roof?

#3. Mold And Mildew Prevention

Attic fans can also help prevent mold growth. I have seen numerous attics during my time as a home inspector with signs of mold growth on the roof sheathing and rafters.

Mold can only grow when there is an excessive amount of moisture: whether it is a bathroom, attic, or basement.

Mold only needs moisture, darkness, and a food source to grow --- and a dark humid attic with a lot of wood checks off all three boxes.

Thermostat & Humidistat

Some attic fans include both a thermostat and a humidistat.

This means that when you can't use the thermostat during winter, you can rely on the humidistat which will turn on the attic fan when the humidity rises.

And then, when the humidity drops, the attic fan will turn off.

#4. Ice Dam Prevention

In colder parts of the country, ice damming is a common problem during the cold season. It works like this, hot air from the interior home goes into the attic.

Remember how hot air rises? So when this warm interior air goes into the attic, it warms up heavy snow on the roof, which causes the snow to melt. And then when this melted snow moves towards the edges of the roof, it re-freezes.

It basically creates an ice dam, holding back water. Ice damming can lead to significant wood rot and and roof leaks.

I hope you enjoyed my quick rundown on how attic fans work.

Read Also: What Are The Best Solar Attic Fans?

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