How To Use Attic Fan: Temperature, Humidity Settings

Attic fans help to lower the temperature inside your attic by exhausting hot air from it and replacing it with the cooler air from outside. Here are the best settings for using it effectively;

Attic fan temperature setting

The majority of the attic fan models come with thermostats. You can set a minimum temperature on these devices above which the fan should start running. For example, if you set the temperature to 100 degrees Fahrenheit, the thermostat will kickstart the attic fan once the attic temperature goes above this value. Once the temperature falls below this threshold the thermostat will turn off the fan.

image of a thermostat for attic fan
An attic fan thermostat

The most recommended temperature setting for an attic fan in summer is from 90 to 110 degrees Fahrenheit. Ideally, the temperature setting should be equal to the outside temperature, and this is the thermostat setting most HVAC technicians recommend.

This temperature set on the thermostat depends on the area you live in, and how low you want the attic temperature to be. If you want to start the attic fan at a lower temperature, change the thermostat setting accordingly, however for best results it should be on par with the outside temperature.

Humidistat setting

Not all attic fans come with a humidistat however some models do. You can also buy a humidistat separately to work with your attic fan.

A humidistat helps to reduce the moisture levels in the attic by running the fan when the attic space exceeds a preset humidity level. High humidity in the attic can cause problems like mold, so using an attic fan to reduce moisture levels can dodge these risks.

A humidistat works just like a thermostat. In most cases, it comes coupled with a thermostat. In this case, the attic fan will start running if either the temperature or the humidity goes above the preset value.

For an attic fan, the best humidity level to be set on the humidistat is 50%. That is, if the relative humidity in the attic exceeds 50% then the fan will turn on. But as mentioned before, this setting depends on the area you live in. If your area has a high relative humidity it is recommended to change the humidistat setting accordingly.

Running the attic fan in winter

Many people are not sure if they can run their attic fans in the winter. Many consider it unnecessary since winters are cold, and keeping the attic vents open can add a load to the heating systems. But HVAC technicians always recommend having enough attic ventilation during winter.

This is because during winter, the attic will be warmer, and this will melt the ice on the roof, but they will freeze again, building ice dams which are dangerous. Running an attic fan during winter will not only reduce the temperature to avoid ice dams but also reduces the moisture levels, which will otherwise cause the growth of bacteria and mold.

Recommended temperature and humidity settings for winter

The recommended attic fan humidistat setting for winter is 40 to 50% relative humidity. This means that if the relative humidity exceeds this value, the fan will start working.

There is no need to set a temperature on the attic fan for winter. Since the temperatures are low during winter, a thermostat doesn’t play any role in attic ventilation; in fact, these thermostats cannot be set for low temperatures. It is the humidistat that controls the attic fan during this season. Running the fan to reduce humidity will also reduce the attic temperature and prevent ice dams.

In case your fan doesn’t have a humidistat, you can bypass the thermostat to run it. Here is a good guide to bypassing attic fan thermostat.

Image of an ice dam on roof
An ice dam on attic eaves

How to use an attic fan in summer?

Summer is the hottest time of the year, and an attic fan plays an important role in reducing your attic temperature and air conditioning bills.

During the summer, the temperature inside your attic is always higher than the temperature outside. This temperature difference can go up to 30 to 50 degrees, depending on where you live. The hotter the day, the more will be this temperature difference.

The best time to use an attic fan in summer is during the hottest time of the day, usually around noon. You can either use a timer or use the appropriate thermostat settings to restrict running the fan only during this peak time.

Recommended attic fan thermostat setting for summer is 90 to 110 degree Fahrenheit, and the humidistat setting is 50% relative humidity.

This is how it works; For example, if the hottest temperature outside your home is 90 degrees F, then the attic temperature will probably be around 130 to 140 degrees F. By replacing the hot attic air with the cooler outside air the fan cools the attic to around 95-100 degrees F. So in effect, the fan reduces your attic temperature by 30 to 40 degrees. But at night this temperature difference will reduce to, maybe, 5 to 10 degrees even though the fan continues to consume the same amount of electricity.

So it is safe to assume that attic fans are most effective during the hottest time of the day.

Here are some practical tips for using an attic fan effectively;

  1. Run the attic fan during the hottest time of the day
  2. Use recommended thermostat settings
  3. Set the humidistat level to 50% for effective removal of moisture
  4. Run the attic fan in the winter too
  5. Make sure that the attic has enough venting area
  6. Use an attic fan of the right size
  7. Service the attic fan once or twice a year

Final thoughts

There is no one size fits all rule when it comes to using an attic fan effectively. The temperature, humidity settings, and other things mentioned in this article are just general guidelines. I strongly suggest that you find the optimal settings by trial and error. It may take a few trips into the attic but it is definitely worth it.

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Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

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