CLOSED-LOOP HEAT PUMP SYSTEM: a heat pump system that uses a loop of buried plastic pipe as a heat exchanger. Loops can be horizontal or vertical.

COP (COEFFICIENT OF PERFORMANCE): The ratio of heating provided by a heat pump (or other refrigeration machine) to the energy consumed by the system under designated operating conditions. The higher the COP, the more effective the system.

COMPRESSOR: The central part of a heat pump system. The compressor increases the pressure and temperature of the refrigerant and simultaneously reduces the volume while causing the refrigerant to move through the system.

CYCLING LOSSES: The actual efficiency of a heating or cooling system is reduced due to start-up and shut-down losses. Over sizing a heating or cooling system increases cycling losses.

EER (ENERGY EFFICIENCY RATIO): The ratio of cooling provided by a heat pump (or other refrigeration machine) to the energy consumed by the system under designated operation conditions. The higher the EER, the more efficient the system.

FOSSIL FUEL: Any of several types of combustible fuels formed from the decomposition of organic matter. Examples are natural gas, propane, fuel oil, and coal.

GEOTHERMAL HEAT PUMP: A heat pump that uses the earth as a heat source and heat sink.

HEAT EXCHANGER: A device designed to transfer heat between two physically separated fluids or mediums of different temperatures.

HEAT PUMP: A mechanical device used for heating and cooling which operates by moving heat from one location to another. Heat pumps can extract heat from air, water, or the earth. They are classified as either air-source or ground-source (geothermal) units.

HEAT SINK: The medium - air, water, or earth - which receives heat rejected from a heat pump.

HEAT SOURCE: The medium - air, water, or earth - from which heat is extracted by a heat pump.

HOT WATER GENERATOR: A device for recovering superheat from the compressor discharge gas of a heat pump or central air conditioner for use in heating or preheating potable water.

OPEN-LOOP HEAT PUMP SYSTEM: A heat pump system that uses groundwater from a well. The water is returned to the environment.

PAYBACK: A method of calculating how long it will take to recover the difference in costs of two different heating and cooling systems by using the energy and maintenance costs savings from the more efficient system.

SUPPLEMENTAL HEATING: A heating system used during extremely cold weather when additional heat is needed to moderate indoor temperatures. May be in the form of electric resistance or fossil fuel.