Heating, Ventilation & Air Conditioning Systems
One metric family, many labels
Convert fluently: COP = EER ÷ 3.412; kW/ton = 12 ÷ EER; so 0.60 kW/ton equals 20 EER and about 5.9 COP. Know SEER/IPLV for part-load, AFUE and combustion efficiency for boilers, and HSPF for heat pumps. Combustion efficiency is stack-loss driven: excess air, flue temperature, and the dew point of the flue gas determine whether a condensing boiler can actually condense - which requires return water below roughly 130°F.
System knowledge to expect: VAV versus CAV, economizer sequences and their high-limit lockouts, supply air temperature and duct static reset, chilled water supply temperature reset, condenser water reset and tower approach, variable primary flow, ASHRAE 62.1 ventilation and demand-controlled ventilation, heat recovery, and reheat penalties.
Traps. Using full-load efficiency to estimate part-load energy; ignoring fan and pump energy in VAV and hydronic systems; claiming economizer savings without verifying damper stroke and actuator calibration; applying condensing-boiler efficiency to a high-return-temperature loop; and forgetting that reducing airflow can starve ventilation.
Worked example
A 400-ton chiller operates at 0.60 kW/ton for 2,000 equivalent full-load hours. Demand = 400 × 0.60 = 240 kW; energy = 240 × 2,000 = 480,000 kWh. A retrofit to 0.50 kW/ton cuts 40 kW, saving 40 × 2,000 = 80,000 kWh - about $8,000/yr at $0.10/kWh, plus demand savings on the peak. Note the exam habit: express the improvement in kW/ton and EER (0.50 kW/ton = 24 EER).