CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A Manual J heat loss calculation includes an exterior wall segment with a net area of 250 square feet and an assembly U-factor of 0.32 Btu/hr·ft²·°F. The design temperature difference (indoor design temp minus the 99% heating design temp) is 45°F. What is the wall's heat loss contribution to the total heating load?
- A2,880 Btu/hr
- B4,000 Btu/hr
- C3,240 Btu/hr
- D3,600 Btu/hr
Show answer & explanationAnswer & explanation
Correct answer: D. 3,600 Btu/hr
Heat loss is calculated as Q = U × A × ΔT = 0.32 × 250 × 45 = 3,600 Btu/hr. This formula converts the wall assembly's thermal resistance, area, and temperature difference into an hourly heat loss value used in Manual J.
Why the other options are wrong
- A. Results from using 200 sq ft instead of the given 250 sq ft.
- B. Rounds too high and does not match the exact multiplication.
- C. Results from using 225 sq ft instead of 250 sq ft.
Envelope Heat Loss Formula
Manual J calculates conductive heat loss through building assemblies using Q = U × A × ΔT, where U is the assembly's U-factor, A is area, and ΔT is the design temperature difference.
- Q = U × A × ΔT
- U-factor is the inverse of total R-value
- ΔT uses 99% heating design temperature, not average winter temp
Memory trick: U times Area times Delta-T equals heat that flees.