Whole House Fan: How It Works and How to Size One
Updated 2026-08-16 · 10 min read
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A whole house fan mounts in the ceiling between the living space and the attic. With windows open, it pulls cool outdoor air in through the windows, across the house, and exhausts hot indoor air into the attic, which vents it outside.
A whole house fan can replace the entire air volume of a house every 1.5 to 2 minutes, which is why a house that felt stifling at 8 p.m. feels comfortable twenty minutes later.
It draws a small fraction of the power of central air conditioning. The catch is that it can only ever deliver outdoor air — so the climate decides whether it is excellent or useless.
Whole house fan vs attic fan vs air conditioning
| Whole house fan | Attic fan | Central AC | |
|---|---|---|---|
| What it cools | The living space | The attic only | The living space |
| How | Flushes the house with outdoor air | Exhausts hot attic air | Refrigeration |
| Windows | Must be open | Irrelevant | Must be closed |
| Typical draw | ~200–700 W | ~150–300 W | 2,000–5,000 W |
| Works when | Outdoor air is cooler | Attic is hot | Any time |
| Removes humidity | No — brings outdoor humidity in | No | Yes |
| Best climate | Dry, cool nights | Any | Any, especially humid |
The row that decides it for most people is humidity. A whole house fan is a ventilation device, not a refrigeration one — it cannot dehumidify, and in a humid climate it imports moisture along with the cooler air.
What size whole house fan do I need?
Two methods, and they should roughly agree.
By floor area — the common rule:
| House size | CFM at 2/sq ft | CFM at 3/sq ft |
|---|---|---|
| 1,200 sq ft | 2,400 | 3,600 |
| 1,600 sq ft | 3,200 | 4,800 |
| 2,000 sq ft | 4,000 | 6,000 |
| 2,500 sq ft | 5,000 | 7,500 |
| 3,000 sq ft | 6,000 | 9,000 |
By air changes: house volume in cubic feet ÷ the minutes you want per full air change (1.5–2). A 2,000 sq ft house with 8 ft ceilings is 16,000 cubic feet; at 2 minutes per change that's 8,000 CFM, which sits above the floor-area rule — so treat the two as a range and pick within it.
Size toward the higher end in hot climates or where you want fast evening cool-down, and toward the lower end where nights drop sharply and noise matters more.
What does a whole house fan require to work?
1. Attic exhaust venting
Everything the fan pushes into the attic has to get out. Passive attic venting sized for stack effect is nowhere near enough for a fan moving thousands of CFM.
Common manufacturer guidance is roughly 1 square foot of net free exhaust area per 750 CFM. A 5,000 CFM fan therefore wants around 6–7 square feet of net free venting — often several times what the attic already has.
Undersized exhaust venting is the most frequent installation mistake. It pressurizes the attic, chokes the fan back below its rating, and in bad cases pushes hot attic air down through ceiling leaks into the house — the exact opposite of the intent.
2. Open window area
The fan needs a path in as much as a path out. A working rule is 2 to 3 square feet of open window per 1,000 CFM, distributed across the rooms you want to cool — opening windows only in one room pulls all the air through that room and leaves the rest of the house untouched.
Running the fan with the house closed is a genuine hazard, not just poor performance. The negative pressure it creates can backdraft an atmospherically vented gas water heater or furnace and draw combustion gases, including carbon monoxide, into the living space. Modern units often include an interlock or a reminder; older ones do not. See carbon monoxide safety.
Does your climate suit one?
| Climate | Verdict |
|---|---|
| Dry, big day-night temperature swing (Mountain West, inland California, high desert) | Excellent — often replaces most AC hours |
| Mediterranean coastal | Very good — cool evenings, low humidity |
| Continental with cool nights (Upper Midwest, Northeast) | Good for shoulder seasons and most summer nights |
| Hot and humid (Southeast, Gulf) | Poor — imports moisture, little relief |
| Hot with warm nights (Desert Southwest in peak summer) | Limited to early morning hours |
The test is simple: does outdoor air actually drop below indoor air at night where you live, and is it dry when it does? If yes, a whole house fan will do real work. If your overnight low is 78°F at 80% relative humidity, it will not.
Types
Traditional belt-drive. Large blade in a ceiling frame, high CFM, low cost per CFM. Loudest option, and the ceiling opening is a significant air-leak and insulation gap in winter unless it seals well.
Insulated damper / ducted. The fan sits remote in the attic and connects through an insulated, motorized damper. Much quieter, seals far better in winter, costs more.
Multi-speed and variable. Lets you run low and quiet overnight, high for a fast flush at dusk. Worth having — most owners run low most of the time.
The winter sealing question matters more than it first appears. An uninsulated fan opening in the ceiling is a permanent hole between conditioned space and a vented attic. Insulated, gasketed doors — motorized or manual — are the difference between a summer asset and a winter liability.
How much does a whole house fan cost to run?
A whole house fan draws roughly 200 to 700 watts depending on size and speed, against 2,000 to 5,000 watts for central air conditioning. Running one for four hours costs a fraction of a dollar in most of the US.
The saving is not the fan's own consumption though — it is the AC hours it displaces. In a suitable climate, a household that runs the fan from dusk until bed and closes the house up in the morning can cut a large share of cooling-season compressor runtime. Work out your own numbers with the electricity cost calculator, and see cost to run a whole house fan.
How do you use a whole house fan?
- Wait until outdoor air is cooler than indoor. A thermometer on a shaded porch beats guessing; running it too early pulls hot air in.
- Open windows first, then start the fan. Never the other way round.
- Open windows in the rooms you want cooled, not just the nearest one.
- Run high for a fast flush, then drop to low for the night.
- Close the house in the morning — windows shut, blinds down on the sun side — to hold the cool you banked.
- Seal and insulate the opening in winter.
Step 6 is the one people skip, and it quietly undoes a season's savings.
Code reference
The requirements behind this guide, for looking up in your adopted edition:
- IRC R806.2 — attic ventilation net free area, the baseline the fan's exhaust requirement sits on top of
- IRC M1503 / M1505 — mechanical ventilation and makeup air provisions, relevant where a large exhaust fan can depressurize a dwelling
- Manufacturer instructions govern the required exhaust area per CFM, and NEC 110.3(B) requires listed equipment to be installed per its listing
Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ, and have combustion-appliance backdrafting evaluated where natural-draft equipment is present.
Where to go next
- Attic fan — the one that only ventilates the attic
- How to cool a house without AC
- How to cool an upstairs room
- How to air seal a house
- Cost to run a whole house fan
- Attic fan installation
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