CSLB C-20 HVAC ContractorHeating SystemsMedium
A technician is performing a heat loss calculation for a home to properly size a replacement furnace. The calculation determines the home loses 60,000 Btu/h at the design outdoor temperature. The new furnace has an Annual Fuel Utilization Efficiency (AFUE) of 90%. What is the minimum required input rating for the new furnace to meet the heat loss?
- A60,000 Btu/h.
- B66,667 Btu/h.
- C72,000 Btu/h.
- D54,000 Btu/h.
Show answer & explanationAnswer & explanation
Correct answer: B. 66,667 Btu/h.
To determine the required input rating, divide the heat loss (output needed) by the furnace's efficiency (AFUE). Required Input = Heat Loss / AFUE. So, 60,000 Btu/h / 0.90 = 66,666.67 Btu/h. Therefore, a furnace with an input rating of at least 66,667 Btu/h is needed.
Why the other options are wrong
- A. 60,000 Btu/h would only be sufficient if the furnace was 100% efficient, which is not the case.
- C. 72,000 Btu/h would be oversized for this application, leading to short cycling and reduced efficiency.
- D. 54,000 Btu/h would be the output if the input was 60,000 Btu/h and efficiency was 90%, which is incorrect; we need to find the input.
Furnace Sizing (Input vs. Output)
Furnace sizing involves matching the appliance's heat output to the building's heat loss, accounting for the furnace's efficiency (AFUE) to determine the necessary input rating.
- Heat loss calculation determines required output.
- AFUE is the efficiency rating.
- Input = Output / AFUE.
Memory trick: What you lose, divided by what you use, gives the input you choose.