CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A room has a calculated sensible heat gain of 24,000 BTU/hr. The design temperature split (supply air to room air) is 20°F. Using the standard sensible heat formula, how much supply airflow (CFM) is required to offset this load?
- A1,440 CFM
- B800 CFM
- C960 CFM
- D1,111 CFM
Show answer & explanationAnswer & explanation
Correct answer: D. 1,111 CFM
CFM = BTU/hr ÷ (1.08 × ΔT) = 24,000 ÷ (1.08 × 20) = 24,000 ÷ 21.6 ≈ 1,111 CFM. The constant 1.08 accounts for air density and specific heat at standard conditions.
Why the other options are wrong
- A. This overstates CFM, likely from omitting the 1.08 constant.
- B. This results from dividing by too large a denominator, understating airflow.
- C. Close but results from using an incorrect constant instead of 1.08.
Sensible Heat CFM Formula
CFM = BTU/hr ÷ (1.08 × ΔT), used to determine airflow needed to offset a sensible heat load.
- 1.08 is the standard air density/specific heat constant
- ΔT is temperature difference between supply and room air
- Used for both heating and cooling load airflow sizing
Memory trick: BTU over 1.08 times Delta-T gives you the CFM.