CSLB C-20 HVAC ContractorPlanning and LayoutEasy
A residential split-system air conditioner is rated at 3 tons of cooling capacity. Using the standard rule-of-thumb airflow requirement commonly applied for typical comfort cooling systems, what design supply airflow should the duct system be sized to deliver?
- A1,500 CFM
- B900 CFM
- C1,200 CFM
- D1,050 CFM
Show answer & explanationAnswer & explanation
Correct answer: C. 1,200 CFM
The standard rule-of-thumb airflow for residential comfort cooling is approximately 400 CFM per ton of cooling capacity. For a 3-ton system: 3 tons × 400 CFM/ton = 1,200 CFM.
Why the other options are wrong
- A. Corresponds to 500 CFM/ton, more typical of high-latitude dry climates, not standard design.
- B. Corresponds to only 300 CFM/ton, too low for standard comfort cooling.
- D. Not a standard rule-of-thumb multiple for tonnage.
CFM per Ton Rule of Thumb
A quick estimating guideline stating that residential cooling systems typically require about 400 CFM of airflow per ton of cooling capacity.
- 1 ton = 12,000 Btu/hr
- 400 CFM/ton is standard for comfort cooling
- Actual design should still be verified with Manual D
Memory trick: Four hundred per ton keeps the airflow right where it belongs.