The attic is a space that many people tend to neglect. Since you don’t have a reason to go up there very often, you probably don’t find yourself giving a lot of thought to the fan in your attic. However, if you want to do a better job of managing your home and your energy costs, you should definitely learn a little bit about attic fans, how they work, and what types of them exist. This article is meant to help you do exactly that. 

First, let’s talk about the two main types of attic ventilation systems: Whole-house fans and powered attic ventilators.

THE TWO MAIN TYPES OF ATTIC FANS

Whole-house fans are primarily designed to provide fresh air for the occupants of a house. By constantly moving the air around, the fan keeps the air in your home from “going stale” and smelling musty. Whole-house fans also serve the purpose of lowering the temperature in a home. To fulfill this purpose, they must be combined with a ventilation system that sends the hotter air out of the attic at night. 

This sort of thing is necessary because of natural law. Specifically, the fact that hot air tends to rise and cool air tends to descend. This natural law is the reason that your attic gets so hot in the summertime while the basement tends to stay cooler. This is because the hot air, as it rises, tends to get caught in the attic. Here, it can build up to ridiculous levels if the heat is not properly ventilated. This can be a major problem for anything sensitive that may be stored up there. 

The other kind of attic fan is called a “Powered Attic Ventilator,” and it works in a completely different way. Instead of trying to regulate temperature and control the flow of fresh air, these types of ventilation fans work as air exchangers. They use fans to expel all the hot air that has built up in the attic, and other fans to draw in fresh air from the outside. By exchanging the hot air from inside the attic with the cooler air outside the house, a reduction in temperature is achieved. This kind of system is meant for high-use attics that need to be kept somewhat comfortable. 

WHOLE-HOUSE FANS

Whole house fans are mainly intended to be used in the evening or at night. Although they do not totally exchange the air in your home with the outside air, they do involve circulating air from outside the house as part of the cooling system. It should be noted that homes equipped with air conditioning don’t really need this kind of thing. It would be kind of pointless to equip your home with both a whole-house fan and an air conditioner. 

So why should you replace your air conditioner with this great big fan? The answer to that question is efficiency. A whole-house fan uses only 10%-15% of the electricity that is needed for a standard air conditioner. Since it offers a cooling factor that is almost as good (and in some cases, a little bit better), whole-house fans give you a more efficient way to cool your home. 

To operate at full efficiency, a whole-house fan must be properly used. Never turn on the fan during the winter, as there is simply no point. There is no sense in wasting electricity. You also shouldn’t use a fan of this sort during the hottest part of the day because you will only succeed in bringing more hot air into your home. The result would be like sitting in front of a hot dryer vent. 

To use a whole-house fan, you wait until the temperature has cooled down, and then you open a few downstairs windows. Turn on the fan, and you’re done. Here is what will happen: the fan will pull the air from your home up into the attic. From there, the hot air will be pushed out of the ventilation openings that all attics must have. As the fan continues to pull, it will end up pulling the cool evening air through the windows, making your home comfortable. 

When the home reaches a comfortable temperature, you shut the windows and turn off the fan. Keep the windows shut until it’s time to repeat the process tomorrow. By doing this, you hold the cool air that you gathered the night before, keeping you relatively cool and comfortable during the heat of the day. 

A whole-house fan must have sufficient ventilation in order to do its work. As you can tell by reading the previous section, the fan will draw hot air into the attic. After it arrives, the hot air must have someplace to go. If not, it will build up to ridiculous levels and will eventually begin creeping downward. Yes, warm air doesn’t normally descend, but there is one way to make it descend, and that is by creating a situation where the hot air has nowhere else to go. 

Building codes are usually not written with this kind of cooling system in mind. For this reason, most building codes do not require the kind of ventilation that you need for a fan-based cooling system. To figure out how much ventilation space you really need, just use this simple process:

First, find out how many CFM (cubic feet per minute) your fan is able to push. Any whole-house fan that you buy should have this figure listed somewhere on the packaging. Now take that number and divide it by 750. This is done because you need one square foot of ventilation area for every 750 CFM that your fan can push. So, if your fan is rated at 1500 CFM, you will need about two feet of totally open space in your attic. Note that these two feet would not need to be one large opening. It doesn’t matter if the ventilation openings are spread out, as long as there is sufficient space for all the air to escape.

Remember; that means two feet of open space. If you are using grates, make sure to take the surface area of the grate into account. If you need a simple rule of thumb, just use 50% more open area when using grates or screening.

You also need to look at the ACH rating when you are buying a whole-house fan. ACH stands for “air changes per hour.” If you paid attention when we described the way that these fans work, you understand what this means. If not, go back and read again. 

It is recommended to get a fan that is rated for 15-20 air changes per hour. The simplest way is to take the total volume of your home and divide it by four. This will tell you the CFM rating that your fan should have. There is a different rule for houses with particularly high ceilings. If your home has ceilings that are at least eight and a half feet high, you will need to use a different process. Just divide the floor space area of your home by three. 

Whole-house fans are better suited for some environments over others. For instance, the American southeast tends to be pretty humid. For that reason, it isn’t really a good idea to bring all that humid air into your home. Homes in the southwest can end up reaching ridiculously high levels of in-home humidity if they use one of these systems. 

The American southeast, by comparison, is one of the dryest and hottest places on the planet. Humidity certainly isn’t a concern here, but cooling is a much bigger concern if you live in a place like this. However, a whole-house fan does have its limits. Because of the way it works, you have to stick to areas that have relatively cool nights. If you live in a place where the night-time temperatures don’t get lower than 70, a whole-house fan probably won’t help you too much. These kinds of fans are also not very useful for those who live in extremely cold climates. 

In a sub-polar climate like that of, say, northern Canada, keeping your home sealed is of the utmost importance. A whole-house fan means that there will be a big hole in your roof, and this can compromise your home’s ability to hold heat. Some homeowners have been able to solve this problem through the use of an insulated box that is placed over the opening. 

Whole-house fans can also be a security risk since their proper use requires you to open your windows for fairly long time periods. It may also be inconvenient for those who have pets that are prone to escaping. 

1. 2023 NEC - NFPA 70, NATIONAL ELECTRICAL CODE EBOOK
2.  2020 NEC,NFPA 70,NATIONAL ELECTRICAL CODE EBOOK

POWERED ATTIC VENTILATORS

We have already discussed the basic way in which a powered attic ventilator works. It exchanges air between the exterior and the interior of your home. It does this by way of various fans and pipes that direct the air as desired. These systems are generally hooked to your home’s thermostat so that they will automatically engage when the temperature gets above a certain predetermined level. 

There are several problems with this kind of system. First of all, they are complicated systems that require the precise movement of air in precise ways in order to do an effective job of cooling your home. Since every home is at least a little bit unique, this presents a special problem. All it takes is one little non-standard feature or one little flaw to your home, and this system will fail to live up to its expected results. 

By far the biggest problem with a powered ventilator is the fact that this kind of system is not very efficient in terms of power usage.

The powered attic ventilator was intended to be a way of reducing electric bills. They hoped to achieve this by using an efficient system to do some of the work that an air conditioner would normally do. The idea is to let this system take some of the strain off the AC unit, which would allow it to do the job more efficiently, adding up to fewer dollars on your electric bill.

Unfortunately, this theory doesn’t work out in practice. When you install one of these systems, you will need to put a number of openings in the attic so that the air can be efficiently sucked in and blown out. However, this depressurizes the attic, changing the way that air behaves within. 

When you use one of these systems, the attic will be kept at the same temperature as the rest of the house. This is the intended result, but it comes at a price. Think about it this way: when you take hot water and cold water and mix them, what do you get? You get warm water. Likewise, when you take super-hot attic air and mix it with normal-temperature air, you get something that is a little hotter than room temperature without being blistering hot. This is not conducive to the comfort of those within your home. 

As the powered ventilator removes the hottest air from the attic, it is replaced with air from the inside of your home. The air conditioner is then forced to work harder in order to correct the temperature changes that result. In the end, this kind of ventilator will actually raise your electric bill rather than making it lower.