Heating, Ventilating, and Air Conditioning (HVAC)

I

Introduction

Heating, Ventilating, and Air Conditioning (HVAC), related processes designed to regulate ambient conditions within buildings for comfort or for industrial purposes. Heating an area raises temperature in a given space to a more satisfactory level than that of the atmosphere. Ventilation, either separately or in combination with the heating or air-conditioning system, controls both the supply and exhaust of air within given areas in order to provide sufficient oxygen to the occupants and to eliminate odors. Air conditioning designates control of the indoor environment year-round to create and maintain desirable temperature, humidity, air circulation, and purity for the occupants of that space or for the industrial materials that are handled or stored there.

II

Heating

The heating process may be direct, as from a fireplace or stove in an individual room, or indirect, as in a central system in which steam, heated water, or heated air passing through pipes or other ducts transports thermal energy to all the rooms of a building. The earliest heating system was the open fire with which people warmed their dwellings. Stoves and braziers of various types that were developed by the ancient Romans are still employed in some parts of the world.

A

Fireplaces

The fireplace was developed as a method of heating rooms by means of an open fire. The first fireplaces were hearths, recessed into the walls of buildings, with short flues that communicated with the open air. Fireplaces with chimneys sufficiently high above the roof of the building to provide adequate draft for the fire were introduced during the 12th century.

Ordinary fireplaces consist of a hearth enclosed on three sides with brick and surmounted by a completely enclosed chimney or flue that carries away the smoke and other combustion products of the fire. On the hearth is either a metal grate, raised on legs, or a pair of metal supports called firedogs or andirons. Grates are used for such fuels as coal, coke, and charcoal, and andirons are used for wood. These devices promote combustion by permitting the circulation of air under the fuel.

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The useful heat given off by a fireplace consists of both direct radiation from the burning fuel and indirect radiation from the hot sidewalls and back wall. From 85 to 90 percent of the heat from the burning fuel is lost in the combustion gases that go up the chimney. Fireplaces are included in modern houses mainly for aesthetic reasons rather than thermal efficiency. To improve heating efficiency, however, some modern fireplaces are built with an arrangement of interior ducts in which cold air from the room is warmed and then recirculated through the room.

B

Stoves

The stove, an enclosure of metal or ceramic materials in which fuel is burned, is an improvement over the fireplace because its surfaces are in contact with the air of the room and by convection deliver heat to the air passing over them. An efficient stove delivers about 75 percent of the energy of the burning fuel. The fuels used include wood, coal, coke, peat, gas, and kerosene.

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