CSLB C-20 HVAC ContractorPlanning and LayoutHard
A Manual S equipment selection process for a home in a high-desert California climate zone uses a 1% cooling design temperature of 105°F. The condensing unit's nameplate capacity is rated at standard AHRI test conditions of 95°F outdoor temperature. What must the contractor do to correctly verify equipment capacity at design conditions?
- AIncrease equipment sensible capacity but ignore latent capacity changes at higher temperatures
- BUse the AHRI-rated capacity directly since nameplate ratings apply at all outdoor temperatures
- CAssume capacity increases at higher outdoor temperatures due to greater ΔT across the coil
- DApply the manufacturer's extended performance data table to find actual capacity at 105°F
Show answer & explanationAnswer & explanation
Correct answer: D. Apply the manufacturer's extended performance data table to find actual capacity at 105°F
AHRI nameplate ratings are only valid at the standard 95°F test condition. Manual S requires contractors to consult manufacturer extended performance data (capacity tables at various outdoor temperatures) to determine true sensible and latent capacity at the actual 1% design condition, since capacity typically decreases as outdoor temperature rises above 95°F.
Why the other options are wrong
- A. Both sensible and latent capacity are affected and must be checked together.
- B. Nameplate ratings are condition-specific and cannot be applied universally.
- C. Capacity actually decreases, not increases, as outdoor temperature rises above rating conditions.
Manual S Extended Performance Data
Manual S requires verifying equipment capacity using manufacturer extended performance tables when the site's design temperature differs from the standard AHRI 95°F rating condition.
- AHRI nameplate ratings are only valid at 95°F outdoor condition
- Capacity generally decreases as outdoor temperature rises above 95°F
- Extended performance tables list capacity across a range of temperatures/CFM
Memory trick: Don't trust the nameplate past its test conditions—check the extended chart.