CSLB C-20 HVAC ContractorPlanning and LayoutHard
A gas furnace has an input rating of 80,000 Btu/hr and an AFUE rating of 92%. A Manual J calculation shows the home's design heating load is 68,000 Btu/hr. What is the furnace's actual heating output capacity, and is it adequately sized for the load?
- A86,960 Btu/hr output; oversized excessively for the load
- B73,600 Btu/hr output; adequately sized for the load
- C92,000 Btu/hr output; adequately sized but overrated for the load
- D68,000 Btu/hr output; exactly matched to the load
Show answer & explanationAnswer & explanation
Correct answer: B. 73,600 Btu/hr output; adequately sized for the load
Output capacity = Input × AFUE = 80,000 × 0.92 = 73,600 Btu/hr. Since 73,600 Btu/hr exceeds the 68,000 Btu/hr design heating load by a modest margin (about 8%), the furnace is adequately sized without significant oversizing, consistent with Manual S heating equipment selection guidance.
Why the other options are wrong
- A. 86,960 Btu/hr incorrectly divides instead of multiplying by AFUE.
- C. 92,000 Btu/hr incorrectly reverses input and output relationship.
- D. 68,000 Btu/hr ignores the AFUE conversion and just repeats the load value.
AFUE Output Conversion
A furnace's usable heating output is found by multiplying its input Btu/hr rating by its AFUE efficiency percentage; this output is compared to the Manual J heating load for proper sizing.
- Output = Input × AFUE
- Output should meet or slightly exceed the design heating load
- Manual S limits acceptable oversizing to avoid short-cycling
Memory trick: Input times efficiency equals what actually warms the house.