CSLB C-20 HVAC ContractorHeating SystemsMedium
A furnace has an input rating of 100,000 Btu/h and is rated at 80% AFUE. The blower delivers 1,200 CFM across the heat exchanger. Using the standard formula Btu/h = 1.08 × CFM × ΔT, what is the approximate temperature rise across the heat exchanger?
- A46°F
- B62°F
- C74°F
- D88°F
Show answer & explanationAnswer & explanation
Correct answer: B. 62°F
Output Btu/h = Input × AFUE = 100,000 × 0.80 = 80,000 Btu/h. Solving ΔT = Output ÷ (1.08 × CFM) = 80,000 ÷ (1.08 × 1,200) = 80,000 ÷ 1,296 ≈ 61.7°F, which rounds to 62°F. This falls within the furnace's typical rated temperature rise range and confirms proper airflow.
Why the other options are wrong
- A. 46°F would result from using input Btu/h with much higher airflow than given.
- C. 74°F would result from an arithmetic error dividing by a smaller CFM.
- D. 88°F is too high and would suggest inadequate airflow, not the calculated value here.
Temperature Rise Formula
The formula Btu/h = 1.08 × CFM × ΔT relates furnace heat output, airflow, and air temperature rise across the heat exchanger.
- Always use OUTPUT Btu/h (Input × AFUE), not input rating
- Manufacturer's data plate specifies an acceptable temp rise range
- Rise outside the rated range indicates airflow or firing rate problems
Memory trick: Output over airflow times 1.08 tells the rise, no.