In most areas of the country, summer means roasting days and the nearly constant hum of the air conditioner. While cooled air feels great, air conditioners use huge amounts of electricity, accounting for more than half of all energy use in some homes during hotter months.
Because air conditioners use so much energy, anything you can do to use them less and still keep cool will have a big impact on total energy use and monthly utility bills.
Whole house fans are a low cost and energy efficient way to cool your home at night. Often even very hot days are followed by pleasantly cool nights, yet homes retain so much heat that air conditioning must run all evening or even all night to keep the house feeling comfortable. A whole house fan pulls cool outside air through the entire home, cooling the house in just minutes while using just a fraction of the energy of an air conditioner.
Whole house fans are about volume. Installed between the ceiling and the attic, a good whole house fan will draw all the air from the house and eject it into the attic in a matter of minutes. Cooler outside air then floods in through open windows, absorbing heat from walls, furniture, and floors, and creating a constant, cooling breeze throughout the home.
A typical whole house fan uses a few hundred watts of power, compared with the several thousand watts used by an air conditioner. Any time you use the whole house fan instead of an air conditioner, you will cut energy used for cooling by 65 to 90 percent.
How they work
Whole house fans install in the attic, and pull air in through a ceiling vent in the home. Air from the entire house is drawn into the vent and exhausted into the attic. The hot air is pushed out through existing attic vents, while cooler air enters through open windows in the house.
Air in the house must have an easy, direct pathway to the vent. The vent should be centrally located (often the top of the stairwell works well), and interior doors should be left open to leave the airflow unobstructed. If there is not a clear pathway for air, some whole house fans can pull in air from multiple vents.
Types
The simplest type of whole house fan uses a large diameter fan mounted on top of the joists on the floor of the attic. These fans are typically 20 to 36 inches in diameter, and can pull a lot of air out of the home very quickly. Direct drive models connect the motor directly to the back of the fan body, which is reliable but tends to send a lot of vibration and noise through the fan. Belt drive models are quieter, but take up more room and may require occasional maintenance on the belt.
Because the size and shape of large diameter fans makes them difficult to seal and insulate, several manufacturers offer narrower fans that can fit between joists and come equipped with motorized doors to seal the opening when not in use. These models typically use two or more smaller fans, around 10 inches in diameter, so the unit will fit entirely between attic joists. Models with motorized doors are more expensive than large diameter models, but seal off the ceiling vent from the cold attic during winter months.
A third design is a ducted fan. These use attic ductwork to carry air from the vent to the fan. The fan can be mounted or hung almost anywhere in the attic. Some ducted systems use multiple ducts, allowing air to reach the fan from multiple locations in a large home, or from bedrooms with closed doors. Ducted systems are more expensive to buy and to install, but can be very quiet, since fan noise is partially absorbed and insulated by the ductwork.
Usage
Whole house fans can be used two different ways, depending on climate. In desert or mountain areas, evenings are typically cool. A whole house fan may only need to be used for a short time to quickly cool down the house, and then can be shut off for the night. In this type of scenario, the ideal fan is large and capable of moving a lot of air. A powerful fan may be able to completely exhaust interior air in as little as three minutes, quickly cooling your home once the sun sets. Noise is typically less of a concern, since the fan is running for a fairly short amount of time.
In most other areas, hot days are typically followed by warm nights. In these cases, the whole house fan is used all night to keep a gentle, comfortable flow of outside air moving throughout the house. For this type of use, fan speed is important, but so are sound levels, since the fan will be going all night. The ideal fan can exchange air every 10 to 15 minutes, but can do so with minimal noise and very modest energy usage.
Whole house fans should not be used to rid homes of odors or to improve indoor air quality. Although whole house fans are much more efficient than air conditioning, they are hugely oversized for general air exhaust, and waste lots of energy compared to bathroom exhaust fans, kitchen hoods, or even a whole-home air exchanger.
Sizing
Fans are sized by the number of cubic feet of air (CFM) they can move per minute. You can calculate the volume of your home by multiplying the home’s square footage by the average ceiling height. This total, divided by a fan’s CFM, tells you how many minutes it will take for a fan to complete a full exchange of interior air.
The right CFM for your home depends on how the fan will be used (see the previous section). For quick air exchange, look for a CFM high enough to exchange all the air in the home in 3-6 minutes. For all-night cooling, you can accept a lower CFM—but make sure it moves enough air to complete an exchange every 10-15 minutes.
Some fans offer two speed operation. Two speed fans can be set on high to initially cool the house, then left on the quieter low setting to keep air flowing all night. Ideally the high speed CFM figure will provide an air exchange ever few minutes, while the low speed rate will be enough to exchange household air several times an hour.
Airflow
Whole house fans require openings to both push out old air and draw in new air.
In the attic, a fan requires a square foot of open venting for each 750 CFM of capacity. A 1500 CFM fan would require 2 square feet of unobstructed soffit, ridge or gable venting. If the venting is screened, you generally need about 1.5 to 2 times as much, or 3 to 4 square feet in this case.
Whole house fans also require adequate airflow into the house to feed the fan. As with attic venting, you should have at least a 1 square foot opening for each 750 CFM of capacity, ideally in a location that is far from the intake vent.
Whole house fans do depressurize a home and can create a dangerous backdraft if there are gas burning appliances in use. Fireplaces and heaters are not normally operating when you are trying to cool a home, but gas hot water heaters may be. Make sure the water heater is not in a location where the fan will affect its exhaust.
Efficiency
Most of the efficiency of a whole house fan comes from using the fan instead of an air conditioning system. Central air conditioning systems use 2,000 to 5,000 watts of power in a typical home. Whole house fans use 100 to 700 watts, providing at least a 65% savings.
Still, a more efficient whole house fan will use less energy, especially if run all night. Fan efficiency is measured in terms of CFM per watt. Efficient models get about 11 CFM per watt, while older designs attain only about 2 or 3 CFM per watt. A higher efficiency fan can save hundreds of kilowatt-hours over the summer.
Insulation
While whole house fans can be very effective at reducing cooling costs, they can cause substantial heat loss in winter if not properly insulated. An uncovered whole house fan offers a direct path for heat to escape from warm living areas to the unheated attic, and can also contribute to ice damming and other winter attic problems. Attic floors are typically the most heavily insulated part of the entire house, so an uninsulated or underinsulated whole house fan can significantly impact heating costs.
Large diameter fans are difficult to seal and insulate because of their size and exposed construction. One option is a magnetic cover for the ceiling vent, but this typically provides a very small amount of insulation. A better solution is an insulated box that covers the entire fan during the winter. The box should also seal against the floor to prevent air leaks.
Fans equipped with motorized doors provide some insulation, although the amount varies by model. R values range as high as R38 on the thickest doors. Insulated doors are electrically controlled, opening and closing when the fan is turned on and off.
Ducted fans are relatively difficult to insulate. Some offer inserts that block each duct, but these have questionable R values and may be difficult to install. Another option is to disconnect the ductwork from the fan and seal it in winter.
The other key factor in reducing heat losses is to make sure the fan and insulation is well sealed to the attic floor. Fans should be airtight to avoid heat leaking around the edges of the vent into the attic.
Sound
Whole house fans range from quiet to loud. The loudest models are often the most powerful floor-mounted models, while smaller floor mounts and most ducted units are quiet.
If you expect to run the fan all night, look for a unit that can run at less than 65 decibels. Fans with two speed settings offer the option of a faster setting for evening and a quieter setting for nighttime use.
Controls
Whole house fans are typically controlled by either a remote control or a wall switch. The remote allows you to avoid running wires into the wall.
Negatives
Whole house fans are not perfect for everyone. Here are a few reasons to not get a whole house fan:
- You suffer from allergies that could be exacerbated by outside air.
- Attic venting is inadequate to exhaust air from the fan
- Nights are too hot for outside air to provide cooling (although a whole house fan may still work in spring and fall)
- Nights are too humid without air conditioning
- Opening windows adequately cool the house at night without a fan
Cost
A basic, uninsulated floor mounted model can be as little as $150. Models with insulated doors are $500 to $800, and multi-room ducted models can be over $1500. Installation will generally be a few hundred dollars at most, although installing ductwork can be more expensive.
Belt drive. A fan that uses a belt to connect the motor to the fan. A belt absorbs a good deal of the vibration of the motor, making the fan quieter.
CFM. Cubic feet per minute. The amount of air a fan can move.
CFM/Watt. A measure of the energy efficiency of a fan. Indicates how much air the fan will move for each watt of power used. CFM/Watt varies from under 3 to over 10.
Direct drive. A fan that has a motor that directly drives the fan. Direct drives can be noisy than belt drives, although there are many other ways to deaden vibration and sound.
Ducted fan. A fan that uses ducts to bring air from ceiling vents to the fan.
Joists. The supporting members on the floor of an attic. Many whole house fans mount on top of or between joists. Joists are typically 16 or 24 inches apart, so between-joist units are usually 16 inches wide.
Sealed door fan. A fan that uses motorized doors to seal the opening when the fan is not in use.
