People mix these two up all the time. Some folks even call a whole house fan an "attic fan." But they do very different jobs. After 45 years on roofs, we've seen people buy one thinking it's the other. Here's the difference, and how to make either one work with your attic.
Whole house fan vs. attic fan: the short answer
The U.S. Department of Energy's Building America Solution Center spells it out: "Whole-house fans are designed to exhaust and cool the living space, while attic ventilation fans are designed to exhaust and cool the unconditioned attic space."
- Whole house fan: cools the rooms you live in, using cooler outside air.
- Attic fan: moves hot air out of the attic above those rooms.
How a whole house fan works
A whole house fan usually sits in a ceiling near the center of the house. You open some windows, turn it on, and it pulls outside air in through the windows, across the rooms, and up into the attic. The Department of Energy's Energy Saver guide puts it this way: "Whole-house fans pull cool air through the house and exhaust warm air through the attic."
The timing matters. The Building America whole-house fan guide says these fans run "when the outside air is cooler than the inside air" and "should not be used at the same time as air conditioning." So it's an evening and early morning tool.
Why a whole house fan needs plenty of attic exhaust area
Unless the fan is ducted straight outside, all that air gets dumped into your attic. Then it has to get out through the attic vents. If the vents are too small, the air has nowhere to go.
The same guide says, "Most whole-house fans blow air from the house into the attic." It recommends at least 1 square foot of net-free attic vent area for every 750 CFM of fan airflow, and notes that screens and louvers cut a vent's net-free area by about half.
- Check the fan maker's vent area requirement first.
- Count your ridge, gable and roof vents, and use net-free area, not the size of the opening.
- If the attic can't breathe out, add exhaust before you add the fan.
How a powered attic fan works (and where it goes wrong)
A powered attic fan sits on the roof or in a gable. It pulls outside air in through the soffit vents and pushes hot attic air out. That's fine with plenty of intake. Trouble starts without it.
ENERGY STAR warns that "if your attic has blocked soffit vents and is not well-sealed from the rest of the house, attic fans will suck cool conditioned air up out of the house and into the attic." The Building America attic fan page explains why: the fan creates negative pressure in the attic, and "This negative pressure draws conditioned air from the living space into the attic through gaps in the ceiling."
Not sure your intake is big enough? See How Many Soffit Vents Do I Need?
Why bigger is not better
A high-CFM attic fan that's short on intake starves for air. It pulls from wherever it can, including gaps around ceiling lights, the attic hatch, and the house itself.
The building code points the other way, toward balance. IRC R806.2 says, "The minimum net free ventilating area shall be 1 / 150 of the area of the vented space." It allows 1/300 when conditions are met, including putting 40 to 50 percent of the vent area in the upper part of the attic and the rest in the bottom third. In low, out high, and roughly even. More in Why Solar Blaster Bets on Balance, Not Brute Force.
Where a small solar attic fan fits
A small solar fan isn't trying to replace passive ventilation. It helps it. It runs on sunny days, when the attic is hottest, and adds a modest push to a balanced soffit-and-ridge system. Solar Blaster's product pages use a 10'x10'x10' space, or 1,000 cubic feet, as an example:
- The Solar Mega RoofBlaster moves 96 cfm, about one air exchange every 10.4 minutes with proper intake ventilation.
- The Solar RoofBlaster moves 35.2 cfm, about every 28.4 minutes with proper intake ventilation.
Notice "with proper intake ventilation" again. Building America lists a lower flow rate and solar power, plus air sealing and ample intake, as ways to reduce attic fan risks. Wondering about pairing a fan with a ridge vent? Read Ridge Vent and Attic Fan Together.
Can you have both?
Yes. Think of them as working different shifts: the attic fan on hot, sunny afternoons, the whole house fan in the evening once it's cooler outside. Either way, the attic needs real exhaust area and clear intake.
The Arizona angle
Here in Fountain Hills, the attic bakes all afternoon, and that heat lingers into the evening. Building America says whole house fans are best suited where cooling-season nights drop below about 67°F and the day-to-night swing is more than about 25°F. Check your forecast low against that guideline before you open the windows. The warmest midsummer nights may not qualify. A solar attic fan, meanwhile, slows down as the sun sets.
Being honest about the limits
- An attic fan doesn't cool your rooms directly. It moves attic air. It's not air conditioning.
- A whole house fan only helps when it's cooler outside. On a hot night, it just pulls in hot air.
- Some experts don't recommend powered attic fans at all. Building America notes ENERGY STAR doesn't recommend them. If you use one, seal the ceiling and size the intake first.
- No fan fixes a leaky ceiling, blocked soffits or a roof leak. Those need their own repairs.
- Your local building official and the fan maker's instructions have the final say on vent area and installation.
The bottom line
A whole house fan cools the house and dumps air into the attic, so the attic needs plenty of exhaust. An attic fan cools the attic, so it needs plenty of intake. Balance intake low and exhaust high, seal the ceiling, and pick a right-sized fan instead of the biggest one.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.
