CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A Manual J cooling load calculation for a west-facing window uses a peak solar heat gain factor of 200 Btu/hr per square foot at design conditions. The window has a net area of 40 square feet and a solar heat gain coefficient (SHGC) of 0.30. What is the estimated peak solar heat gain through this window?
- A1,800 Btu/hr
- B3,000 Btu/hr
- C1,200 Btu/hr
- D2,400 Btu/hr
Show answer & explanationAnswer & explanation
Correct answer: D. 2,400 Btu/hr
Solar gain = Area × SHGC × peak solar factor = 40 × 0.30 × 200 = 2,400 Btu/hr. This calculation reflects how glazing area, glass performance rating, and orientation-specific solar intensity combine to determine cooling load contribution from windows.
Why the other options are wrong
- A. Uses an incorrect window area of 30 sq ft instead of 40.
- B. Uses an inflated SHGC of 0.375 instead of 0.30.
- C. Uses half the peak solar factor (100 instead of 200).
Window Solar Heat Gain
Solar heat gain through glazing is calculated by multiplying window area by its SHGC and an orientation-specific peak solar factor from Manual J tables.
- Gain = Area × SHGC × Solar Factor
- Lower SHGC glass reduces solar heat gain
- Solar factor varies by window orientation and latitude
Memory trick: Area times shading coefficient times sun strength equals glare's heat.