BTU (British Thermal Unit): The amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit. BTU is used to signify the heating and cooling capacity of a system and the heat losses and gains of buildings and homes.

BTUH: The number of BTUs produced in one hour.

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 or cooling 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 efficient the system.

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

Condenser: A heat exchanger in which hot, pressurized (gaseous) refrigerant is condensed by transferring heat to cooler surrounding air, water or earth.

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

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

Fossil fuel: Any of several types of combustible fuels formed from the decomposition of organic matter. Examples are natural gas, propane, fuel oil, 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 pumping heat from a cooler to a warmer location. Heat pumps can extract heat from air, water, or the earth. They are classified as either air-source or 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.

Open-loop heat pump system: A heat pump system that uses groundwater from a well or surface water from a lake, pond, or river as a heat source. 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 cost 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 fossil fuel or electric resistance.