CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A Manual J calculation shows a design cooling load of 34,000 Btu/hr sensible and 6,000 Btu/hr latent (40,000 Btu/hr total). Per Manual S equipment selection guidelines, which criterion should the contractor prioritize when comparing available air conditioner models?
- AMatching only the nominal tonnage rounded up to the nearest half-ton
- BSelecting the largest available unit to provide a comfort safety margin
- CSelecting based solely on the lowest SEER2 rating available
- DSelecting equipment whose total capacity falls within about 90%-130% of the calculated total load at design conditions, with sensible capacity closely matching the sensible load
Show answer & explanationAnswer & explanation
Correct answer: D. Selecting equipment whose total capacity falls within about 90%-130% of the calculated total load at design conditions, with sensible capacity closely matching the sensible load
Manual S provides matching criteria that go beyond simple tonnage rounding; equipment total capacity should generally fall within 90%-130% of the calculated total load at design conditions, and sensible capacity should closely match the sensible load to ensure proper dehumidification and comfort.
Why the other options are wrong
- A. Rounding tonnage without checking sensible/latent split ignores Manual S guidance.
- B. Oversizing beyond Manual S tolerances causes short-cycling and poor humidity control.
- C. SEER2 relates to efficiency, not capacity matching.
Manual S Equipment Selection
ACCA methodology for matching HVAC equipment capacity (sensible and total) to the Manual J load within specified tolerances, rather than simply rounding nominal tonnage.
- Total capacity typically 90%-130% of design load
- Sensible capacity should closely match sensible load
- Prevents oversizing and poor humidity control
Memory trick: Match the sensible split, not just the ton.