CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A home has a conditioned volume of 12,000 cubic feet. A Manual J infiltration estimate uses a natural air change rate of 0.35 ACH. What is the estimated infiltration airflow in CFM?
- A70 CFM
- B140 CFM
- C210 CFM
- D35 CFM
Show answer & explanationAnswer & explanation
Correct answer: A. 70 CFM
Infiltration CFM is calculated as (Volume × ACH) ÷ 60. Using the given values: (12,000 × 0.35) ÷ 60 = 4,200 ÷ 60 = 70 CFM.
Why the other options are wrong
- B. This doubles the correct answer, likely from a calculation error.
- C. This triples the correct answer and does not match the formula.
- D. This value results from an incorrect division, understating the airflow.
Infiltration CFM Formula
Infiltration airflow is estimated from the building volume and air changes per hour (ACH), converted to CFM using CFM = (Volume × ACH) ÷ 60.
- ACH represents how many times the house air volume is replaced per hour
- Divide by 60 to convert hourly volume exchange to CFM
- Used as input to sensible/latent infiltration load formulas
Memory trick: Volume times ACH, then divide by the hour's minutes (60).