Two Fans, Two Different Jobs
The confusion is understandable: both are large fans installed near the top of a house, both are typically hardwired, and both get marketed around cutting cooling costs. But they move air through completely different paths.
Whole-House Fans Cool People
A whole-house fan is mounted in a central hallway ceiling, right at the boundary between the living space and the attic above. To use it, you open a few windows around the house and switch the fan on. It pulls outside air in through those open windows, sweeps it across every room in the house, and pushes it up through the fan into the attic, where it escapes through the attic's own vents (soffit, gable, or ridge). The effect is a strong, whole-house cross-breeze — similar in principle to opening windows on a breezy day, but powerful enough to fully exchange the air in an average home in two to three minutes.
Because it works by bringing in outside air, a whole-house fan only helps when outside air is actually cooler than inside air — typically mornings and evenings in climates with a meaningful day-night temperature swing. Run one during a humid, still summer afternoon and you're just pulling in more heat and moisture.
Attic Fans Cool the Attic, Not the House
An attic fan (power attic ventilator) sits inside the attic space itself, usually mounted on the roof deck or in a gable end. It draws hot air out of the attic and exhausts it outside, drawing replacement air in through soffit vents lower on the roofline. It has no duct or opening into the living space below — its entire purpose is to keep the attic from overheating.
Why does attic temperature matter if the fan never touches your living space air directly? Because a hot attic radiates heat down through the ceiling insulation and drywall into the rooms below, adding to your air conditioning load, and because sustained high attic temperatures shorten the life of roofing shingles from underneath. Bringing attic temperatures down closer to outdoor ambient reduces both of those effects.
Sizing Works Differently Too
Whole-house fans are sized to the entire house's cubic footage — square footage multiplied by ceiling height — so the whole volume of air can be exchanged in a couple of minutes. Attic fans are sized to the attic floor area alone, using a rough guideline of about 0.7 CFM per square foot of attic space. Buying the wrong type, or applying house-sizing math to an attic fan (or vice versa), will leave you with a fan that's badly mismatched to the space it's actually supposed to move air through.
Can They Work Against Each Other?
Yes, potentially. A whole-house fan is actively pushing warm indoor air up into the attic while it runs, and an attic fan is simultaneously trying to exhaust attic air outward. Running both at the same time isn't dangerous, but it isn't especially efficient either — most homes get better results choosing the fan type that matches the actual problem (hot rooms vs. a hot attic) rather than installing both by default.
The Bottom Line
If your complaint is "the house feels hot at night even with windows open," you want a whole-house fan. If your complaint is "the attic is unbearably hot" or "our shingles seem to be wearing out fast," you want an attic fan. They're complementary technologies that happen to look similar, not interchangeable options for the same problem.