Best Whole House Fan by Home Size: CFM, Attic Venting and Savings vs AC

CFM = square feet x ceiling height x air changes per hour / 60. The calculator sizes to a minimum of 15 air changes (the DOE's low end, and how QuietCool rates its fans) and a target of 20. At 8 ft ceilings: 1,000 sq ft needs 2,000 to 2,667 CFM, 1,500 sq ft needs 3,000 to 4,000, 2,000 sq ft needs 4,000 to 5,333, and 2,500 sq ft needs 5,000 to 6,667. Taller ceilings scale it up.

Bigger is not free: more CFM means more attic vent area and more watts. Two-speed fans let you run high to flush the heat in the evening, then low overnight. Past about 3,000 sq ft, two fans (one at each end, or one per floor) beat one huge one; at 3,500 sq ft the calculator pairs two 5,024 CFM fans for about 22 air changes.

The fan pushes house air into the attic, and it has to leave through the attic vents. DOE and the makers ask for about 1 sq ft of net free vent area per 750 CFM; QuietCool publishes the exact figure for each model (5.89 sq ft for the CL-4700). Too little and the fan moves less air and pushes hot attic air back into the house.

Net free area is printed on each vent or its spec sheet, and screened vents have about half their gross opening. Count what you have at the soffits, gables and ridge first. If you are short, a box of 24 Air Vent soffit vents at 56 sq in each adds about 9.3 sq ft, enough for a fan up to about 7,000 CFM on its own.

Ducted fans (QuietCool, Centric Air) hang the motor in the attic and run a flexible duct to the ceiling grille, so the motor is away from the opening. The grille fits between 16 or 24 in joists with no framing, and R5 damper doors close when the fan is off.

The traditional louvered fan sits on the joists right over a big ceiling shutter. It moves the most air per dollar, but it is louder, the 25 1/2 x 27 3/4 in rough opening means cutting and heading a joist, and the metal louvers leak heat into the attic all winter unless you cover them.

Tamarack's HV1000 is the other approach: a small fan with R38 doors that seal the ceiling, for a bedroom wing or a cold climate where the winter heat loss through the opening matters most. At about 1,000 CFM each, most homes need two or more.

A whole house fan only cools when the outside air is cooler than inside. It pays where summer nights drop below 65 F and the air is dry: most of the West, the Mountain states and the northern tier, including California, Colorado, Oregon, Washington, Utah, Michigan, Minnesota and northern New England.

Where nights run 65 to 75 F it still works most evenings: run it once the outside drops below the indoor temperature, then close up in the morning. Where nights stay above 75 F or are muggy (Florida, the Gulf Coast, Arizona's low deserts), it runs few hours and pulls humid air in that the AC then has to dry out. There, a smart thermostat is the bigger lever.

Season savings = AC watts x the AC hours the fan replaces, minus fan watts x the hours the fan runs (it runs longer, all evening and into the night). The calculator's defaults: a 3,300 W central AC, and 400 AC hours replaced with 600 fan hours where nights are cool, 250 and 450 where they are mild, 75 and 150 where they are warm.

For a 1,500 sq ft home with the CL-4700 on high at 17.3 cents per kWh: cool nights save about $228 of AC for $57 of fan power, $171 a season. Mild nights: $143 against $43, about $100. Warm nights: $43 against $14, about $29, which never pays back the fan. An ECM fan run on low cuts the fan side further: the ES-4700 at 75 W for 450 hours is about $6.

For a 2,000 sq ft home in a cool-night climate the CL-7000 saves about $109 a season, because its 1,147 W on high eats into the AC savings. The ES-7000 does the same airflow at 727 W on high and 164 W on low.

With a flat or cathedral roof there is no place for a ceiling fan. A window fan blowing out, with windows open on the far side of the house, does the same job. The Air King 9166 at about 3,560 CFM gives about 22 air changes an hour in a 1,200 sq ft home, for about $129 a season of net savings on mild nights.

A gable or roof attic fan only exhausts hot attic air. It does not bring night air through the rooms. Size it at about 0.7 CFM per sq ft of attic floor with 1 sq ft of intake vent per 300 CFM: a 1,500 sq ft attic needs about 1,050 CFM and 3.5 sq ft of soffit intake. Without that intake it pulls cooled air up from the house through ceiling gaps.