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The following is list of commonly used terms relating to natural gas, heating and water heating systems.

Air Duct: A tube or conduit moving air from one place to another.

Air Vent: Valve, either manual or automatic, to remove air from the highest point of a coil or piping assembly.

Annual Fuel Utilization (AFUE): The ratio of annual output of useful energy or heat to the annual energy input to the furnace or boiler. The higher the AFUE, the more efficient the equipment and the higher the efficiency the more savings on fuel bills.

Balancing: The process of adjusting the flow of air in duct systems or water flow in hot-water heating systems. Proper balancing is performed using accurate instrumentation to deliver the right amount of heating or cooling to each area or room of the house.

British Thermal Unit (BTU): The quantity of heat required to raise the temperature of a pound of water 1 degree from 59 degrees to 60 degrees. BTUH stands for British Thermal Unit Per Hour- this establishes a time reference to BTU input or output rates.

Efficiency: The rate at which a (heating) system maximizes fuel use. This rate is numerically described as a ratio called AFUE. As of January 1992, no furnaces can be manufactured with efficiencies lower that 78% AFUE. Boilers cannot be manufactured with efficiencies lower than 74% AFUE.

Heat Exchanger: A heat exchanger transfers heat from combustion gases to the air blowing through the ductwork. It is vital that none of the combustion gas itself gets into the air stream. The primary heat exchanger handles the hottest gases.

In high-efficiency furnaces, secondary heat exchangers recover heat that used to be vented up the chimney with the exhaust gases. By recovering this heat, the furnace becomes more efficient. Part of the heat recovered causes the water and acid to condense out of the exhaust gas. Because this liquid is corrosive, secondary heat exchangers must be designed to prevent deterioration. Usually this means they are made of stainless steel or some derivative of it.

Heat Loss Calculation: Calculation to determine house btu heat loss; factors include conduction through construction materials, air infiltration losses and the difference between actual or projected outside temperatures and desired temperatures inside the house. Used for determining necessary heat output from the heating appliance.

Indirect Water Heater: An indirect water heater circulates water through a heat exchanger in the boiler. This heated water then flows to an insulated storage tank. Because the boiler does not need to operate frequently, this system is more efficient than the tankless coil. In fact, when an indirect water heater is used with a highly efficient boiler, the combination may provide one of the least expensive methods of water heating.

Infrared Radiant Heating: A gas-fired infrared heating system emulates the efficiency of the sun by generating radiant energy that is converted into heat when absorbed by objects in its path. Once the floors, machinery, stock and people absorb the infrared energy, it is then re-radiated to warm the surrounding air.

This method of heating, as opposed to filling a room with warm air (such as a forced air unit) allows the source of heat to begin at the floor level and not the ceiling. This makes it the most efficient and effective method in which to heat under the diverse conditions present in warehouses, storerooms and even the most immense structures imaginable.

National Fuel Gas Code (ANSI Z23.1, NFPA 54): A standard for the installation of gas appliances, piping and venting.

Radiation: Heat that moves out in waves from a central point and heats objects in its path. The closer you get to a source of radiant heat the more heat you will feel.

Sizing: The procedure a heating contractor goes through to determine how large a system is needed to heat a house efficiently. Too small a system will not deliver enough heating; too large a system increases energy costs and can have an adverse effect on comfort. Sizing depends on the square footage of a home, the amount of ceiling and wall insulation, the window area, use of storm doors and storm windows, etc.

Therm: Another measurement of heat. One therm equals approximately 100,000 BTUH

Zoning: A system in which living areas are divided into separate spaces, with each space's heating/air conditioning controlled independently. This can be accomplished by using either multiple independent systems or a singe system using electronic controls and/or motorized dampers.