These terms are important to understand and will be used often when determining the efficiency of a heating and cooling system.

1. AFUE: Annual Fuel Utilization Efficiency; seasonal heating efficiency of furnace or boiler in %. Condensing Systems
2. Steady State: % measures how efficiently a furnace converts fuel to heat, once the furnace has warmed up and is running steadily. - furnaces and boilers, forced air, natural aspirating system.
3. EF or UEF: Energy Factor or Unified Energy Factor; water heater overall energy efficiency. Hot Water Heater
4. COP: Coefficient of Performance; determined by dividing the energy output of a heat pump or AC by the electrical energy needed to run at a specific temperature. The higher the COP, the greater the efficiency of the appliance.
5. SEER: Seasonal Energy Efficiency Ratio; annual cooling efficiency of AC or heat pump in BTU/watt-hr
6. HSPF: Heating Seasonal Performance Factor; seasonal heating efficiency of heat pump in BTU/watt-hr; weather data characteristics of long-term climatic conditions are used to represent the heating season in calculating the HSPF.
7. SRE: Sensible Heat-Recovery Efficiency; efficiency rating of HRV or ERV
8. ASE: Apparent Sensible Effectiveness; For HRV/ERV, the measured temperature rise of the supply air stream divided by the difference between the supply temperature and exhaust temperature and multiplied by the ratio of mass flow rate of the supply divided by the minimum of the mass flow rate of the supply or exhaust streams. This value is useful principally to predict final delivered air temperature at a given flow rate.