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AFUE: the Annual Fuel Utilization Efficiency (AFUE) is a thermal efficiency measure of combustion equipment like furnaces, boilers, and water heaters. The AFUE differs from the true 'thermal efficiency' in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment, including the operating transients.

Air Source Heat Pump: it is one of the types of heat pump. It gets renewable energy (heat) from the outdoor air and moves it into the building. According to the distribution system, two different models are available: air-to-air (where the energy is transferred directly to the indoor and air) and water-to-water heat pumps (where the energy is transferred to the building water loop. An air-source heat pump may also run in cooling mode if necessary.

Boiler: a boiler is a closed case in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.

Btu: short for British Thermal Unit. The amount of heat required to raise or lower the temperature of one pound of water by one degree Fahrenheit.

Btu/h: the heat transfer rate of HVAC equipment is measured in British Thermal Units per Hour.

Condensing boiler: a condensing boiler removes the heat from the fuel it is burning and also cools the products of combustion, which are normally wasted up the flue, so much so that the water vapour in them turns into liquid. Therefore condensing boilers are more efficient than regular boilers (approx. 10% higher if the fuel is natural gas).

COP: the Coefficient Of Performance (COP) is the ratio of useful output to the amount of energy input, used generally as a measure of the energy-efficiency of heat pumps, air conditioners, space heaters and other HVAC devices. COP equals heat output in Btu per hour divided by the heat equivalent of the electric energy input. Higher the COP, higher the efficiency of the equipment. The COP rates a heat pump's ability to efficiently use electricity in its operation.

EER: the Energy Efficiency Ratio (EER) is the ratio of cooling in Btu/h provided by a heat pump to the energy consumed by the system in Watts . The higher the EER, the more efficient the system.

Electric heat pump: see comparison between eletric heat pump and gas absorption heat pump

Furnace: a household furnace is a major appliance that is permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water. The most common fuel source for modern furnaces is natural gas; other common fuel sources include LPG (liquefied petroleum gas), fuel oil, coal or wood.

Gas Absorption Heat Pump: see comparison between gas absorption heat pump and eletric heat pump

Geothermal energy: it is defined as heat from the Earth. It is a clean, renewable resource that provides energy in a variety of applications and resources. The heat of the earth is available everywhere and it is considered a renewable resource because the heat emanating from the interior of the Earth is essentially limitless.

Geothermal Heat Pump: see Ground Source Heat Pump.

Ground Source Heat Pump: a ground source heat pump (GSHP) is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems.

GUE: the Gas Utilization Efficiency (GUE) is the ratio of the energy supplied by a gas-fired heat pump or boiler to the energy consumed by the burner.

Heat pump: a mechanical device used for heating (and cooling if necessary) which operates by moving heat from a lower temperature location to a higher temperature location. Heat pumps are able to extract heat from air, water or ground. In the cooling mode a heat pump works the same as an ordinary air conditioner. A heat pump uses an intermediate fluid called a refrigerant which absorbs heat as it vaporizes and releases the heat when it is condensed. It uses an evaporator to absorb heat from inside an occupied space and rejects this heat to the outside through the condenser.

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.

HSPF: the Heating Seasonal Performance Factor (HSPF) is the efficiency of air source heat pumps. The higher the HSPF rating of a unit, the more energy efficient it is. HSPF is a ratio of Btu heat output over the heating season to watt-hours of electricity used.

HVAC: Heating, Ventilating and Air Conditioning. Used to refer to the industry of heating and air conditioning equipment.

Renewable energy: it is energy which comes from natural resources such as air, water, geothermal heat, sunlight, wind, etc. which are renewable. Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth.

SEER: the Seasonal Energy Efficiency Ratio (SEER) is the cooling output in Btu during a typical cooling-season divided by the total electric energy input in watt-hours during the same period. The higher the unit's SEER rating the more energy efficient it is. It represents the expected overall performance for a typical year's weather in a given location.

Water Source Heat Pump: a ground-water heat pump is a central heating and/or cooling system that pumps heat to or from surface or underground water source such as a lake, river, or well.