Glossay of terms from the HVAC industry

Glossary of terms

Air Cleaner:
    Device used to remove air borne impurities, such as dust, fumes and smoke.

Air Conditioning
    Control of humidity, air motion, temperature, and air distribution within an enclosure.

    A closed vessel in which steam is generated or in which water is heated by fire.

Boiler Heating Surface
    The area of the heat transmitting surfaces in contact with water (or steam) in the boiler on one side and the fire or hot gases on the other.

Boiler Horsepower
    The equivalent evaporation of 34.5 lbs. of water per hour at 212F. to steam at 212F. This is equal to a heat output of 33,475 Btu per hour, which is equal to approximately 140 sq. ft. steam radiation (EDR)

British Thermal Unit (Btu)
    The quantity of heat required to raise the temperature of 1 lb. of water 1F.

Column Radiator
    A type of direct heater.

    In steam heating, water formed by cooling steam as in a radiator. The capacity of traps, pumps, etc., is sometimes expressed in lbs. of condensate they will handle per hour. One lb. of condensate per hour is equal to approximately 4 sq. ft. of steam heating surface (240) Btu per hour per sq. ft.).

    A piece of equipment for heating water with steam without mixing the two.

    (Standard) A unit which is the difference between 65' F. and the daily average temperature when the latter is below 65' F. The "degree days" in any one day is equal to the number of degrees F. that the average temperature for that day is below 65* F.

Direct-Return System (Hot Water):
    A two-pipe hot water system in which the water, after it has passed through a heating unit, is returned to the boiler along a direct path so that the total distance traveled by the water from each radiator is the shortest possible. There is, therefore, a considerable difference in the lengths of the several circuits composing the system.

Domestic Hot Water:
    Hot water used for purposes other than for house heating such as for laundering, dishwashing, bathing, etc.

Dry-Bulb Temperature:
    The temperature of the air as determined by an ordinary thermometer.

Dry Return (Steam):
    A return pipe in a steam heating system which carries both water of condensation and air.

Equivalent Direct Radiation (E.D.R.):
    See Square Foot of Heating Surface.

Float and Thermostatic Trap:
    A float trap with a thermostatic element for allowing the escape of air
 into the return line.

Float Trap:
    A steam trap which is operated by a float. When enough condensate has drained (by gravity) into the trap body the float is lifted which in turn lifts the pin off its seat and allows the condensate to flow into the return until the float has been sufficiently lowered to close the port. Temperature does not affect the operation of a float trap.

    A perforated covering for an air inlet or outlet usually made of wire screen, pressed steel, cast-iron or other material.

    That form of energy into which all other forms may be changed. Heat always flows from a body of higher temperature to a body of lower temperature. See also: Latent Heat, Sensible Heat, Specific Heat, Total Heat, Heat of the Liquid.

Heating Medium:
    A substance such as water, steam, or air used to convey heat from the boiler, furnace, or other source of heat to the heating units from which the heat is dissipated.

Heating Surface:
    The exterior surface of a heating unit.

Heating Unit:
    Radiators, convectors, base boards, finned tubing, coils embedded in floor, wall, or ceiling, or any device which transmits the heat from the heating system to the room and its occupants.

    A unit to indicate the time rate of doing work equal to 550 ft.-lb. per second, or 33,000 ft.-lb. per minute. One horsepower equals 2545 Btu per hour or 746 watts.

Hot Water Heating Systems:
    A heating system in which water is used as the medium by which heat is carried through pipes, from the boiler to the heating units.

    An instrument which controls the relative humidity of the air in a room.

    The water vapor mixed with air.

Latent Heat of Evaporation:
    The heat (Btu per pound) necessary to change 1 pound of liquid into vapor without raising its temperature. In round numbers this is equal to 960 Btu per pound of water.

One-Pipe System (Hot Water):
    A hot water heating system in which one-pipe serves both as a supply main and also as a return main. The heating units have separate supply and return pipes but both are connected to the same main.

One-Pipe System (Steam):
    A steam heating system consisting of a main circuit in which the steam and condensate flow in the same pipe. There is but one connection to each heating unit which must serve as both the supply and the return.

Outdoor Reset Control (ODR):

A control which senses outdoor temperature and adjusts the water temperature in the heating system to the minimum water temperature to heat the home and reduce fuel consumption.

Panel Heating:
    A method of heating involving the installation of the heating units (pipe coils) within the wall, floor or ceiling of the room.

Panel Radiator:
    A heating unit placed on, or flush with, a flat wall surface and intended to function essentially as a radiator. Do not confuse with panel heating system.

Plenum Chamber:
    An air compartment maintained under pressure and connected to one or more distributing ducts.

Pressure Reducing Valve:
    A piece of equipment for changing the pressure of a gas or liquid from a higher to a lower one.

Radiant Heating:
    A heating system in which the heating is by radiation only. Sometimes applied to Panel Heating System.

    The transmission of heat is a straight line through space.

    A heating unit located within the room to be heated and exposed to view. A radiator transfers heat by radiation to objects "it can see' and by conduction to the surrounding air which in turn is circulated by natural convection.

Recessed Radiator:
    A heating unit set back into a wall recess but not enclosed.

    A grille with a built-in damper or shutter.

Relative Humidity:
    The amount of moisture in a given quantity of air compared with the maximum amount of moisture the same quantity of air could hold at the same temperature. It is expressed as a percentage.

Return Mains:
    The pipes which return the heating medium from the heating units to the source of heat supply.

Sensible Heat:
    Heat which only increases the temperature of objects as opposed to latent heat.

Split System:
    A system in which the heating is accomplished by radiators or convectors and ventilation by separate apparatus.

Square Foot of Heating Surface:
    Equivalent direct radiation (EDR)., By definition, that amount of heating surface which will give off 240 Btu per hour when filled with a heating medium at 215* F. and surrounded by air at 70* F. The equivalent square foot of heating surface may have no direct relation to the actual surface area.

Static Pressure:
    The pressure which tends to burst a pipe. It is used to overcome the frictional resistance to flow through the pipe. It is expressed as a unit pressure and may be either in absolute or gauge pressure. It is frequently expressed in feet of water column or (in the case of pipe friction) in mil-inches of water column per ft. of pipe.

    Water in the vapor phase. The vapor formed when water has been heated to its boiling point, corresponding to the pressure it is under.

Steam Heating System:
    A heating system in which the heating units give up their heat to the room by condensing the steam furnishing to them by a boiler or other source.

Steam Trap:
    A device for allowing the passage of condensate and air but preventing the passage of steam. See Thermostatic, Float, Bucket Trap.

Supply Mains:
    The pipes through which the heating medium flows from the boiler or source of supply to the runouts and risers leading to the heating units.

    An instrument which responds to changes in temperature, and which directly or indirectly controls the room temperature.

Thermostatic Trap:
    A steam trap which opens by a drop in temperature such as when cold condensate of air reaches it and closes it when steam reaches it. The temperature sensitive element is usually a sealed bellows or series of diaphragm chambers containing a small quantity of volatile liquid.

Ton of Refrigeration:
    The heat which must be extracted from one ton (2,000 lbs.) of water at 32* F. to change it into ice at 32* F. in 24 hours. It is equal to 288,000 Btu/24 hours, 12,000 Btu/hour, or 200 Btu/minute.

Two-Pipe System (Steam or Water):
    A heating system in which one pipe is used for the supply main and another for the return main. The essential feature of a two-pipe hot water system is that each heating unit receives a direct supply of the heating medium which cannot have served a preceding heating unit.

Unit Heater:
    A heating unit consisting of a heat transfer element, a housing, a fan with driving motor, and outlet, deflectors or diffusers. It is usually suspended from the ceiling and its heat output is, controlled by starting and stopping the fan by a room thermostat. The circulation of the heating medium (steam or hot water) is usually continuous. It is used mostly for industrial heating.

Vacuum Heating System (Steam):
    A one- or two-pipe heating system equipped with the necessary accessory apparatus to permit the pressure in the system to go below atmospheric.

Vapor Heating System (Steam):
    A two-pipe heating system which operates under pressure at or near atmospheric and which returns the condensation to the boiler or receiver by gravity.

Vent Valve (Steam):
    A device for permitting air to be forced out of a heating unit or pipe and which closes against water and steam.

Vent Valve (Water):
    A device permitting air to be pushed out of a pipe or heating unit and which closes when water fill vent.

Warm Air Heating System:
    A warm air heating plant consists of a heating unit (fuel-burning furnace) enclosed in a casing, from which the heated air is distributed to the various rooms of the building through ducts. If the motive head producing flow depends on the difference in weight between the heated air leaving the casing and the cooler air entering the bottom of the casing, it is termed a gravity system. A booster fan may, however, be used in conjunction with a gravity-designed system. If a fan is used to produce circulation and the system is designed especially for fan circulation, it is termed a fan furnace system or a central fan furnace system. A fan furnace system may include air washer, filters, etc.

Wet Bulb Temperature:
    The lowest temperature which a water-wetted body will reach when exposed to an air current.

Wet Return (Steam):
    That part of a return main of a steam heating system which is completely filled with water from condensation.