CSLB C-20 HVAC ContractorPlanning and LayoutHard
A duct system is routed entirely through an unconditioned attic, resulting in an estimated 15% loss of delivered cooling capacity due to duct heat gain. If the conditioned space requires 36,000 Btu/hr of delivered cooling, what equipment output capacity should be selected to compensate for this duct loss?
- A41,400 Btu/hr
- B39,600 Btu/hr
- C38,000 Btu/hr
- D42,350 Btu/hr
Show answer & explanationAnswer & explanation
Correct answer: D. 42,350 Btu/hr
To compensate for a 15% delivery loss, the equipment output must satisfy: Output × (1 − 0.15) = 36,000. Solving gives Output = 36,000 ÷ 0.85 ≈ 42,353 Btu/hr, matching the closest option.
Why the other options are wrong
- A. This adds 15% directly rather than dividing by 0.85.
- B. This adds 10% instead of correctly dividing by (1 − 0.15).
- C. This adds only a flat 2,000 Btu/hr rather than dividing by the loss factor.
Duct Loss Compensation
When ducts run through unconditioned spaces, delivered capacity is reduced by heat gain/loss through duct walls; equipment must be sized larger to compensate, calculated as required load ÷ (1 − loss fraction).
- Attic ducts commonly lose 10-25% of capacity without proper insulation
- Formula: Equipment Output = Delivered Load ÷ (1 − loss %)
- Properly insulating/sealing ducts reduces this compensation need
Memory trick: What the attic steals, the equipment must reveal — divide by what remains.