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A fan is sized either to remove heat or to change air. For heat removal, the sensible load divided by 1.08 times the temperature difference gives the airflow. For ventilation, the target air changes per hour and the room volume set the flow. Take the larger of the two when both apply.
- 24,000 BTU/h sensible, ΔT 20°F.
- CFM = 24,000 ÷ (1.08 × 20) = 1,111 CFM.
Reference tables
CFM per 12,000 BTU/h (1 ton) sensible by ΔT
| ΔT (°F) | CFM |
|---|---|
| 15 | 741 |
| 18 | 617 |
| 20 | 556 |
| 25 | 444 |
A number the tables do not give: at a common 20°F rise, plan on roughly 550–560 CFM per ton of sensible cooling.
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FAQ
Where does 1.08 come from?
It is 60 min/h × 0.075 lb/ft³ air density × 0.24 BTU/lb·°F specific heat ≈ 1.08 at standard air conditions.
Sensible vs total load?
Use the sensible portion for airflow; the latent (moisture) load is handled by the coil, not the fan CFM.
What if ventilation needs more than cooling?
Then ventilation governs — size the fan to the larger CFM and let the coil match the load.
Does altitude change it?
Yes — thinner air lowers density, so high-altitude systems need more CFM for the same heat; correct the 1.08 factor.
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