Ah the attic fan. Personally I have lovely memories of going to sleep on cool summer nights listening to the attic fan and feeling all that fresh air blowing across my face. I loved it. To this day I am much more comfortable sleeping in a room that has air circulating than not. However, as a child I was blissfully unaware of possible problems that could arise from running an attic fan. This article gives you all the information you may not have about attic fans before you make the decision to use or install an attic fan in your home.
How An Attic Fan Works
Attic fans (also called whole house fans) are specifically designed to cool your home using the outside air. The fan is attached to the floor of the attic, usually above a main hallway or living space. When you turn on the fan and open the windows in the house, outside air is drawn in through the windows and up through the attic where the hot inside air is vented out of the home.
Attic fans work best at night when the outside temperature cools and that air is drawn into your warm home to cool it off.
Negative Pressure
Think of an attic fan as a huge 30″-36″ fan that puts a negative pressure on your house. People love attic fans primarily for two reasons:
1. They think of it as free cooling for their home
2. Bringing fresh air into the house feels nice
The Disadvantages For an Attic Fan Begin With The Two Advantages
1. It is not really free cooling. An attic fan uses 700 to 1000 watts of electricity. That is like using 10 or more 65 watt light bulbs. So think of it this way: when you turn on your attic fan before you go to bed it is like going around the house and turning on 10 light bulbs before going to bed. Its is not going to be terribly expensive, but it is certainly not free.
2. When we are bringing in outdoor air, the temperature feels cooler at the time, however it is heavily laden with moisture. Everything in your home acts like a sponge. Your mattresses, drapes ,carpets, everything is soaking up that moisture. At night the air feels nice however in the morning your air conditioner will need to work overtime and spend a good part of the day getting things dried out. NOTE: If you live in a drier climate or there is low humidity, this would not be as much of a problem as in high humidity areas.
Bringing in Outside Air Also Means Bringing In Allergens
When you pull the outside air into your house you are also inviting in all the allergens from the air. Ragweed, cottonwood seeds all come streaming in and anyone who is sensitive to allergens will feel the difference immediately. If you have air cleaners, you don’t want to pull in all the pollen, ragweed, cottonwood seeds, etc. After you get used to really clean air you wouldn’t think of turning on an attic fan and letting in all those irritants.
Consequences of Negative Pressure
Here’s something most people don’t think about when running their attic fan: the consequence of putting negative pressure on your house. The negative pressure produced by the attic fan is relieved by any opening from the inside of your house to the outside, such as opening windows.
Windows are not the only openings in your home.
You also need to think about the openings from dryer vents, sewer vents, and almost most importantly: your chimney and furnace flue vents.
Presumably you are not using your furnace when you are using your attic fan, but you are certainly using your hot water heater. Your hot water heater is hooked to a pipe from inside the furnace room all the way up to the roof outside. The attic fan is creating a massive down draft from the flue and if your water heater turns on and starts burning natural gas, it is not being vented properly out your flue. Ramon Blachly from One Hour Heating and Air Conditioning says, “We are so sensitive about building codes and safety, yet we throw that all out vent your water heater into your house.”
Alternatives
If you love your attic fan primarily because, like me, you enjoy lots of air circulation in your home, Ramon recommends including a variable drive motor on your furnace coupled with a high efficiency air cleaner. It creates circulation in your home where the air never feels stale and also cleans allergens and irritants from your indoor air making everything more comfortable.
