HVACR System Efficiency Analysis
What this domain tests
This domain tests delivered efficiency of the installed system, not the box label. NATE expects you to separate equipment ratings from field performance and to explain every loss between the two.
Core rules
- AHRI ratings use fixed conditions: 80°F dry-bulb/67°F wet-bulb return air and 95°F outdoor air for EER. SEER/SEER2 and IEER/IEER2 are seasonal or part-load metrics, so a field EER is not a SEER.
- Verify capacity at the coil, not at the nameplate. Total capacity = 4.5 × CFM × Δh (Btu/lb); sensible capacity = 1.08 × CFM × ΔT.
- Installed efficiency falls with low airflow, incorrect charge, duct leakage, excessive static pressure, oversizing and short cycling.
- Sensible heat ratio (SHR) links total and sensible capacity. A low measured SHR at design conditions often signals low airflow or a wet coil, not a better system.
- Compare the AHRI reference number, model, coil and furnace or air handler as a matched set.
Worked example
A 3-ton split system is rated 36,000 Btu/h at 11.0 EER under AHRI conditions. Field data: return 80°F DB/67°F WB, outdoor 95°F, airflow 1,050 CFM, Δh 7.2 Btu/lb, and compressor plus outdoor fan draw 3.55 kW. Total capacity = 4.5 × 1,050 × 7.2 = 34,020 Btu/h. Field EER = 34,020 / 3,550 = 9.58, about 13% below rating. Airflow is 350 CFM/ton, below the 400 CFM/ton nominal, so correct airflow and charge before blaming the equipment.
Traps
- Comparing field EER directly with SEER or IEER.
- Treating nominal 400 CFM/ton as a measured value.
- Taking capacity from the nameplate rather than calculating it from airflow and enthalpy.
- Ignoring fan heat and the indoor/outdoor conditions at the time of test.