CSLB C-20 HVAC ContractorPlanning and LayoutHard
A Manual J calculation shows a total cooling load of 36,000 Btu/hr with a sensible heat ratio (SHR) of 0.75. If the design supply-to-return air temperature difference (ΔT) is 18°F, what CFM is required to satisfy the sensible portion of the load?
- A1389 CFM
- B1500 CFM
- C1667 CFM
- D1250 CFM
Show answer & explanationAnswer & explanation
Correct answer: A. 1389 CFM
First find sensible load: 36,000 × 0.75 = 27,000 Btu/hr. Then apply the sensible heat formula: CFM = 27,000 ÷ (1.08 × 18) = 27,000 ÷ 19.44 ≈ 1,389 CFM.
Why the other options are wrong
- B. This results from rounding errors and using ΔT of 16 instead of 18.
- C. This results from using the full load without applying the SHR at all.
- D. This results from using the full 36,000 Btu/hr load instead of the sensible portion.
Sensible Heat Ratio (SHR) Application
SHR is the fraction of total heat load that is sensible (temperature) versus latent (humidity). Multiplying total load by SHR isolates the sensible load for CFM calculations.
- SHR = sensible load ÷ total load
- Sensible CFM formula uses only the sensible portion
- Typical residential SHR ranges from 0.70 to 0.80
Memory trick: Split the load first, then send it through the sensible formula door.