Example Data Table
This example uses a single-level home and a moderate ventilation target.
| Input | Example value | Why it matters |
|---|---|---|
| Floor area | 2,200 ft² | Sets the base served area. |
| Ceiling height | 8 ft | Converts area into air volume. |
| Target airflow | 20 ACH | Sets how quickly indoor air changes. |
| Duct loss | 8% | Raises fan capacity for resistance. |
| Safety margin | 12% | Allows for real-world uncertainty. |
| Fan power | 550 W | Estimates electricity use and cost. |
Formula Used
Base CFM = house volume × target ACH ÷ 60
Recommended CFM = [base CFM × climate multiplier ÷ (1 − duct loss)] × (1 + safety margin)
Duct loss and safety margin are entered as decimal percentages in the formula. The result rounds upward to the next 50 CFM. This helps compare practical fan sizes.
How to Use This Calculator
- Enter the total conditioned floor area the fan will serve.
- Add average ceiling height and the number of connected levels.
- Choose an air-change target that matches your comfort goal.
- Adjust for duct resistance, climate needs, and a safety margin.
- Enter attic exhaust capacity and a candidate fan rating.
- Add power, runtime, and electricity price for cost estimates.
- Calculate, then compare the airflow result with fan specifications.
Choosing Whole House Fan Airflow
A whole house fan moves indoor air outdoors through the attic. It works best during cool, dry evenings. Open windows create the intake path. Attic vents create the discharge path. Both paths matter. A powerful fan cannot perform well through restricted openings.
Start with the home volume. Floor area alone is not enough. Taller ceilings hold more air. Multiple connected levels also increase the volume. The calculator multiplies these dimensions. It then applies your target air changes per hour. This produces a baseline airflow requirement.
Air changes per hour describe flushing speed. A higher number replaces indoor air more quickly. It may improve comfort after a hot day. It can also increase noise and power use. Choose a target that suits the home, climate, and evening temperature. Test a moderate setting first.
Ducts, dampers, shutters, and grilles create resistance. This resistance reduces delivered airflow. The duct-loss field increases the required fan rating. A safety margin provides room for imperfect assumptions. It also helps when airflow paths become less open than expected.
Attic exhaust deserves careful attention. The fan pushes a large air volume into the attic. That air needs a clear exit. Compare calculated airflow with evaluated attic discharge capacity. A deficit can raise attic pressure. It may reduce airflow into rooms. It may also create unwanted leakage paths.
Windows are the other half of the system. Open several windows around occupied rooms. Spread openings across the home. This reduces drafts at one window. It also helps air travel through more rooms. Close windows in unused spaces when that improves comfort.
Energy estimates support operating decisions. Enter the fan's rated watts. Add expected runtime and the electricity price. The calculator estimates daily, monthly, and seasonal costs. These figures are planning estimates. Motor loading and control settings can change actual consumption.
Quiet controls improve night use. Insulated ducts reduce sound transfer. Timers prevent unnecessary operation. Variable-speed equipment gives finer control. Consider maintenance access for shutters, belts, filters, and motors. Record installation settings for seasonal adjustments.
Use the result to shortlist fan models. Compare rated airflow, sound level, controls, and installation needs. Confirm structural support and electrical requirements. Review local rules before work begins. Use qualified installers when the project needs them. A balanced installation delivers better comfort and dependable nightly cooling.
Frequently Asked Questions
1. What does CFM mean?
CFM means cubic feet per minute. It measures how much air a fan moves each minute. Higher CFM usually means faster air replacement, but only when windows and attic outlets provide enough open airflow paths.
2. Why does the calculator use home volume?
Airflow changes air volume, not floor area alone. Ceiling height and connected levels determine that volume. A tall home requires more airflow than a same-size home with lower ceilings.
3. What is a reasonable ACH target?
The best target depends on outdoor conditions, comfort expectations, noise tolerance, and fan design. Start with a moderate value. Then compare it with manufacturer guidance and observe room comfort during suitable evening weather.
4. Why add duct loss?
Ducts, grilles, shutters, and turns reduce delivered airflow. The loss field increases the fan rating needed to meet the target. Use a conservative estimate when the installation has long or restrictive air paths.
5. Does attic ventilation affect fan performance?
Yes. The attic needs a clear route for the air pushed into it. Limited attic discharge can reduce room airflow and raise attic pressure. Verify the ventilation approach before selecting a high-capacity fan.
6. Why should windows be opened?
Open windows provide the intake air that replaces indoor air. More distributed openings often reduce uncomfortable drafts. Follow the fan manufacturer's instructions for the minimum opening area and proper operating sequence.
7. Can I use a selected fan CFM?
Yes. Enter a candidate model's published CFM. The calculator compares it with the recommended airflow and estimates its air changes per hour. Treat published ratings as reference figures because installed performance can differ.
8. How is electricity cost estimated?
Daily energy equals fan watts multiplied by daily runtime, divided by 1,000. The result is multiplied by your electricity rate. Actual use can change with motor speed, controls, weather, and electrical supply.
9. Should I run the fan during smoky weather?
Avoid operating it when outdoor air quality is poor or unsafe for open-window ventilation. Check local air-quality guidance. A whole house fan brings outdoor air inside, so outdoor conditions remain essential.
10. Is this result an installation approval?
No. This page provides a planning estimate. It does not replace a site review, structural assessment, electrical evaluation, code check, or manufacturer installation instructions. Verify those details before buying or installing equipment.
11. Why round recommended airflow upward?
Fans are commonly sold in capacity steps. Rounding upward avoids selecting a unit slightly below the calculated target. It also gives a small buffer for normal system losses and changing operating conditions.