How a whole house fan works
A whole house fan is mounted in the ceiling between the living space and the attic. In the evening, when outdoor air is cooler than indoors, you open some windows and switch it on: the fan pulls cool air through the house and pushes warm air out through the attic vents. In a few minutes it replaces all the air in the house and also flushes heat out of the attic. It uses a fraction of the electricity of air conditioning.
CFM = living area × ceiling height × air changes per hour ÷ 60
Sizing guide (8 ft ceilings)
| Home size | Airflow (20 air changes/h) | Attic net free vent area |
|---|
| 1,000 sq ft | ≈ 2,700 CFM | ≈ 3.6 sq ft |
| 1,500 sq ft | ≈ 4,000 CFM | ≈ 5.3 sq ft |
| 2,000 sq ft | ≈ 5,300 CFM | ≈ 7.1 sq ft |
| 3,000 sq ft | ≈ 8,000 CFM | ≈ 10.7 sq ft |
Attic venting is critical
The fan can only move as much air as can leave the attic. Manufacturers typically require about one square foot of net free area (the actual open area of vents after screens and louvers) for every 750 CFM. Many homes need extra gable, soffit or ridge vents when a whole house fan is installed. Insufficient venting reduces airflow, makes the fan noisier and can force dust and hot air back into the living space.
Whole house fan vs attic fan
An attic fan only exhausts hot air from the attic to reduce heat transferred through the ceiling. A whole house fan ventilates the living space itself and cools the people and the house's thermal mass. They solve different problems; in hot-dry climates with cool nights, a whole house fan has the bigger effect on comfort and AC bills.
Choosing a fan
- Ducted, insulated models are much quieter and seal tightly in winter.
- Multi-speed or variable-speed lets you run quietly overnight.
- Insulated, automatic damper doors prevent heat loss when the fan is off.
- Timers and wall controls with safety interlocks.
- Check the CFM rating at the speed you'll actually use, not just on high.
Safety
Always open windows before running the fan. Running it with the house closed can draw combustion gases down chimneys and flues of gas appliances. Follow local codes and the manufacturer's instructions, and install a carbon monoxide alarm.
Frequently asked questions
What size whole house fan do I need?
Multiply living area by ceiling height and 20 air changes, then divide by 60. A 2,000 sq ft home with 8 ft ceilings needs about 5,300 CFM.
How much attic venting does a whole house fan need?
About 1 sq ft of net free vent area per 750 CFM of fan capacity.
Can a whole house fan replace air conditioning?
In dry climates with cool nights it can replace much of it; in hot, humid climates it helps mainly in spring and autumn.
How much electricity does a whole house fan use?
Typically 100 – 600 W, a small fraction of a central air conditioner's 2,500 – 4,000 W.
Is a whole house fan the same as an attic fan?
No. An attic fan only exhausts the attic; a whole house fan ventilates the living space through open windows.
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