ANNUAL FUEL UTILIZATION EFFICIENCY (A.F.U.E.)
a test procedure developed by the U.S. Department of Energy to simulate the efficiencies a consumer may expect during the operation of their heating equipment. This standardized test procedure may be useful in comparing various manufacturers' equipment.

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ATMOSPHERIC BOILER
a boiler on which combustion air/flue gases are drawn through the boiler without a fan.

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COMMERCIAL BOILER
a boiler whose input is 300,000 BTUH or more.

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DAMPER
an electro-mechanical device that prevents heat from escaping through the chimney during off cycle periods.

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DIRECT VENT BOILER
a boiler designed and installed so that all combustion air is obtained directly from outdoors and all flue gases are vented directly outdoors.

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FORCED DRAFT BOILER
a boiler on which the pressure in the chamber is maintained at a positive pressure.

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HEATING CAPACITY
the rate of useful heat output from boiler (less than 300,000 input) operating under steady state conditions, expressed in BTU/hr, and rounded to the nearest 1,000 BTU/hr. "Useful heat" includes jacket losses given up to the building. Heating Capacity is determined by multiplying the specified input by the steady state efficiency, as tested.

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INDUCED DRAFT BOILER
a boiler that uses a blower to draw air into the combustion chamber.

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NET RATING
the recommended amount of installed radiation to be served by the boiler, based on the normal allowance for piping and pickup losses. Net Ratings for water system boilers are determined by dividing the steady state test results by a factor of 1.15. To obtain Net Ratings for steam boilers, the test results are divided by a varying factor to provide for varying piping and pickup losses in larger steam systems.

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RESIDENTIAL BOILER
a boiler whose input is less than 300,000 BTUH.

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SCOTCH MARINE BOILER
a cylindrical, horizontal 3-pass design boiler in which there is equal water volume above and below the center line of the boiler.

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SEALED COMBUSTION
See Direct Vent.

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WET BASE BOILER
a single-pass design boiler in which the fire is surrounded by water and has thermal pins or bars to improve heat exchange.