A lot of our customers have been contemplating installing a whole house fan to either replace their AC or as a complement to it. Recently I came across a great article at GreenBuildingAdvisor.com that discussed - Whole House Fans: Do They Help? It was good enough that we wanted to reprint it here. Thanks to GreenBuildingAdvisor.com for a great article!
Whole House Fans are used to cool a house at night when the heat of the day has passed and the outdoor temperature has dropped enough to feel comfortable. When should you turn on a whole-house fan? The answer depends on your climate and your comfort range. The outdoor temperature should certainly be below 80°F — or, better yet, below 70°F.
The main advantage of using a whole-house fan instead of an air conditioner is to save energy. A whole-house fan usually draws between 200 and 700 watts — about 10% to 15% of the power drawn by a central air conditioner (2,000 to 5,000 watts). If evenings are cool enough, it’s fairly easy to lower the temperature of your home and your furniture with a whole-house fan — sometimes in less than an hour.
Whole-house fans are intended to be used in homes that are not air-conditioned. It makes no sense to introduce lots of (potentially humid) exterior air into a house at night if you intend to turn on an air conditioner the next day. If you're using your air conditioner, keep your windows closed, 24 hours a day, so that the air conditioner isn't faced with an increased latent load due to exterior humidity entering the house at night.
In most cases, a whole-house fan is mounted in the attic floor, above a rectangular grille in the ceiling of a central hallway. Once the outdoor temperature cools down — usually in the evening or early morning — the homeowner opens a few downstairs windows, closes the fireplace damper, and turns on the fan. (The wall switch that controls a whole-house fan should be properly labeled so that it isn’t accidentally turned on during the winter.)
The fan pulls air from the hallway and blows it into the attic. Since whole-house fans are relatively powerful — they are usually rated between 2,000 cfm and 6,000 cfm — they quickly exhaust the hot indoor air, allowing cooler outdoor air to enter through the downstairs windows. Once the house has cooled off, the fan can be turned off and the windows closed. Most people who have whole-house fans keep their windows closed from early morning until evening, so that the cool air inside the house doesn’t escape.
You need enough attic vents to let the air escape
Since a whole-house fan blows all of the hot air from the home into the attic, the fan won’t work effectively unless the attic has large openings to exhaust the hot air. Most old-fashioned whole-house fans require more attic venting than the minimum amount required by the building code — anything from a little more to about twice as much, depending on the size of the fan.
Here’s the rule of thumb: you need one square foot of net free vent area for every 750 cfm of fan capacity. The vent area can be made up of a combination of soffit vents, ridge vents, and gable vents. If the vent has insect screening, remember to make the opening 50% larger than the rule of thumb dictates. It’s better to have too much vent area than not enough.
How do you size your whole-house fan? The traditional recommendation is to choose a fan that can move between 15 and 20 air changes per hour (ach). If you’re aiming for 15 ach, that means you need to divide your home’s volume by 4 to obtain the cfm rating of your fan. If your ceiling height is between 8 and 9 feet, just multiply the floor area of your house by 3 to obtain the cfm rating of your fan.
Where does a whole-house fan make sense?
If you live in the right climate, whole-house fans are a great way to keep your house cool. In the U.S., they make more sense in the arid West than in the humid Southeast, since most homeowners don’t want to invite lots of humid air into their homes.
Whole-house fans make sense in areas with cool nights. If you live somewhere where the temperature stays in the 80s all night long, a whole-house fan won’t help you much.
However, even if you need to seal up your house and turn on your air conditioner during the hottest months of summer, a whole-house fan may be useful during the spring and fall seasons, when nights are cool but days remain hot.
A few caveats
Whole-house fans make sense in some, but not all, homes:
- They don’t make sense for homes in neighborhoods where security concerns prevent homeowners from leaving their windows open.
- They don’t make sense for homes with a furnace or water heater in the attic.
- Because they depressurize a home, whole-house fans can cause atmospherically vented appliances located inside a home — for example, a gas-fired water heater — to backdraft. If the homeowner remembers to open plenty of windows before turning on the fan, back-drafting probably won’t occur. But the best way to avoid back-drafting problems in a house with a whole-house fan is to make sure that the house doesn’t have any atmospherically vented combustion appliances.
- Whole-house fans represent a big hole in your ceiling — a hole that is likely to leak a lot of heat during the winter unless it is properly sealed.
Finally, it should be noted that some homeowners complain that whole-house fans are noisy. However, newer models of whole-house fans specifically the QuietCool brand, which Bellows Plumbing, Heating & Air installs, are quieter than traditional whole-house fans with higher cfm ratings.
If you’ve been thinking about installing a new AC or a Whole House Fan, call Bellows Plumbing, Heating & Air today. We’d be happy to help your get the work done befopre it gets much hotter! Bellows Plumbing, Heating & Air is the San Francisco Bay Area plumbing, heating, and air conditioning contractor you can count on!
Knowing how to quickly and properly resolve various cooling and heating is what separates Bellows from other plumbing and HVAC companies. In Santa Cruz, Santa Clara, and Marin Counties, Bellows Plumbing, Heating, & Air is the leader when it comes to speed of service, quality of install, and the ability to take on all your water heater repairs.
Our team is here to serve you 24 hours a day, 7 days a week, 365 days a year. If you’d like to schedule a Summer tune up for your heating or plumbing systems please consider calling Bellows at 877-477-7151. Bellows Plumbing, Heating, & Air services, repairs, and installs plumbing, heating, and air conditioner equipment in: Santa Cruz, Aptos, Capitola, Scotts Valley, Felton, Watsonville, Ben Lomond, Boulder Creek, Santa Clara, Los Gatos, Campbell, Saratoga, San Jose, Sunnyvale, Cupertino, Tiburon, Mill Valley, Corte Madera, Larkspur, San Anselmo, Ross, Fairfax, San Rafael, Marinwood, Terra Linda, and Novato.