A Few Basic Terms
AFUE (Annual Fuel Utilization Efficiency): The percent (efficiency) of fossil fuel furnaces. Includes cycling and flue losses and other factors.
A British thermal unit (Btu) is a standard measure of heat energy. One Btu is the amount of heat energy required to raise the temperature of a pound of water by one degree Fahrenheit. Its metric thermal equivalent is 252 calories per hour. As a unit of power, one Btu/h equals 0.2929 watts (W). Manufacturers classify the size, or capacity, of an air-conditioning unit in terms of Btu/h.
Closed Loop System: A heat pump that system that uses a loop of buried plastic pipe as a heat exchanger. Loops can be horizontal or vertical.
Coefficient of Performance (COP) of a heat pump is the ratio of the change in heat at the "output" (the heat reservoir of interest) to the supplied work. For example, a geothermal heat pump operating at COPheating 3.5 provides 3.5 units of heat for each unit of energy consumed (e.g. 1 kW consumed would provide 3.5 kW of output heat). The output heat comes from both the heat source and 1 kW of input energy, so the heat-source is cooled by 2.5 kW, not 3.5 kW.
Compressor: The central part of a refrigeration 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 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.
Cooling capacity, measured in British thermal units per hour (Btu/h), indicates the quantity of heat a room air conditioner can remove in one hour.
Cooling load, also expressed in Btu/h, refers to the maximum amount of heat that can build up in a space without a cooling system.
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.
Energy efficiency ratio (EER) is a comparative measure of how much cooling an air conditioner provides for each unit of electrical energy that it consumes under standard operating conditions. A unit's EER is calculated by dividing its cooling capacity by its electrical power input at a specific temperature. In general, the higher the EER, the more efficient the unit.
Evaporator: A heat exchanger in which cold, liquid refrigerant absorbs heat from the low-temperature source.
Fossil Fuel: 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 seperated 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 (Geothermal): The medium - air, water or earth - which receives heat rejected from a heat pump.
Heat Source (Geothermal): The medium - air, water or earth - from which heat is extracted by a heat pump.
Open Loop 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 between two different heating and cooling systems by using the energy and operating 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.
A watt (W) is the standard unit of power; one kilowatt (kW) equals 1000 watts. You purchase electricity from your utility by the kilowatt hour (kWh), equivalent to the amount of power required to operate one 100-W light bulb for 10 hours. To estimate how much electricity an appliance uses, multiply the wattage of the machine by the number of hours it will run.