Airflow (Sensible)
Air volume flow needed to carry a sensible heat load (imperial units).
Formula
Variables
Example
Q=21,600, ΔT=20: CFM = 1000
How it works
This formula tells you how much air you need to move to remove a given sensible heat load, meaning heat that changes air temperature but not moisture content. The constant 1.08 combines the density and specific heat of air under typical HVAC conditions, so dividing the heat load by 1.08 times the temperature rise gives the airflow needed to carry that heat away. A larger temperature difference means each cubic foot of air can remove more heat, so the required CFM goes down.
When to use it
Use this when sizing supply airflow for cooling or heating problems where the load is mainly sensible, such as duct design, room cooling calculations, or estimating fan air volume. It is especially useful when you know the heat load in BTU/h and the allowed supply-to-room temperature difference in °F.
Worked example, step by step
Given: Q = 12,960 BTU/h and ΔT = 12°F Step 1: Write the formula: CFM = Q / (1.08 × ΔT) Step 2: Substitute the values: CFM = 12,960 / (1.08 × 12) Step 3: Multiply the denominator: 1.08 × 12 = 12.96 Step 4: Divide: 12,960 / 12.96 = 1,000 Final answer: CFM = 1,000 ft³/min
Common mistakes to avoid
- Using total load including latent heat instead of only sensible load
- Mixing imperial and metric units, which invalidates the 1.08 constant
- Using the wrong temperature difference, such as outdoor-to-indoor instead of supply-air-to-space air
Did You Know?
The 1.08 factor bundles air density and specific heat at standard conditions.
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