CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A duct system must deliver a sensible cooling load of 24,000 Btu/hr with a supply air temperature drop (ΔT) of 20°F. Using the standard sensible heat formula, what CFM is required?
- A1000 CFM
- B1200 CFM
- C1333 CFM
- D1111 CFM
Show answer & explanationAnswer & explanation
Correct answer: D. 1111 CFM
The sensible heat formula is CFM = Btu/hr ÷ (1.08 × ΔT). Substituting: 24,000 ÷ (1.08 × 20) = 24,000 ÷ 21.6 = 1,111 CFM.
Why the other options are wrong
- A. This results from using ΔT of 22.2 instead of 20.
- B. This overestimates by ignoring the 1.08 constant properly.
- C. This results from dividing by 18 instead of 20 ΔT.
Sensible Heat CFM Formula
CFM = Btu/hr ÷ (1.08 × ΔT) is used to calculate the airflow needed to deliver a given sensible heating or cooling load.
- 1.08 is the standard air sensible heat constant at sea level
- ΔT is the temperature difference between supply and return air
- Formula is foundational to duct design (Manual D)
Memory trick: 1.08 times Delta T tells you how much air you need to see.