System Performance and Efficiency
Domain focus
This domain tests whether you can prove a heat pump is delivering rated capacity efficiently: airflow, static pressure, charge, duct losses, defrost, balance point, and auxiliary heat. Questions often ask which measurement explains a low delta-T or high energy use.
Core rules
- Nominal airflow is about 400 cfm per ton; a 3-ton system needs roughly 1,200 cfm.
- Total external static pressure includes supply and return; typical residential target is 0.5 in. w.c., with filter and coil pressure drops included.
- Sensible capacity = 1.08 × cfm × ΔT for dry air; total capacity also depends on latent load and wet-bulb.
- A heat pump's balance point is where capacity equals building load; below it, auxiliary heat makes up the difference.
Worked example
A 3-ton heat pump moves 1,000 cfm, not 1,200, because of a restrictive filter. Return air is 70°F and supply is 55°F, so ΔT = 15°F. Sensible output ≈ 1.08 × 1,000 × 15 = 16,200 Btu/h. At design airflow the same ΔT would give 19,440 Btu/h. The low airflow raises delta-T but reduces capacity and can freeze the coil; correct the filter or duct restriction before charging.
Traps
Using dry-bulb ΔT alone to judge charge, measuring static pressure only after the filter, assuming nameplate cfm without blower-table correction, and ignoring auxiliary heat during defrost.