Unit II heating systems

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Heating Systems

Atmosphere–control systems in low–rise residential buildings use natural gas, fuel oil, or electric resistance coils as central heat sources; usually the heat generated is distributed to the occupied spaces by a fluid medium, either air or water. Electric resistance coils are also used to heat living spaces directly with radiant energy. Forced–air distribution moves the heat–bearing air through a treelike system of galvanized sheet–metal ducts of round or rectangular cross section; electric–powered fans provide a pressure differential to push the air from the heat source (or furnace) to the living spaces, where it is expelled from grills located in the walls or floors. The negative pressure side of the fan is connected to another treelike system of return air ducts that extract air from living spaces through grills and bring it back to the furnace for reheating. Fresh outside air can be introduced into the system airstream from an exterior intake, and odour–laden interior air can be expelled through a vent, providing ventilation, usually at the rate of about one complete air change per hour. To conserve energy, air–to–air heat exchangers can be used in the ex­haust–intake process. The heated air is usually supplied in constant volume, and the ambient temperature is varied in response to a thermostat located in one room. Central humidity control is rarely provided in this building type.

Another common heating system is the radiant hot–water type. The heat source is applied to a small boiler, in which water is heated and from which it is circulated by an electric pump in insulated copper pipes similar to a domestic hot–water system. The pipes can be connected to cast–iron or finned tube steel radiators within the living spaces. The radiators are placed near the areas of greatest heat loss (such as windows or outside walls) where their radiant energy heats the surrounding air and creates a convection cycle within the room, producing a roughly uniform temperature within it. The hot water can also be conducted through narrow pipes placed in a continu­ous looping pattern to create a large radiant surface; this pattern of pipes may be cast in a concrete floor slab or placed above a ceiling to heat the adjoining living space. Temperature control in hot–water systems uses a thermostat in the living space to adjust the pumped flow rate of the water to vary the heat supplied.

Radiant electric resistance heating systems use coils in baseboard units in the rooms, which create convection cycles similar to hot–water radiators, or resistance cables in continuous looped patterns embedded in plaster ceilings. Local temperature control can be much more precise with electric heating, because it is possible to install a thermo­statically controlled rheostat to vary the energy output of relatively small sections of baseboard units or cable.

A type of space heating that is increasing in use in residential buildings is passive solar radiation. On sunny win­ter days, south–facing windows let in substantial amounts of energy, often enough to heat the entire building. Wood–burning fireplaces with chimneys are still widely provided in residential buildings, but their use is mostly for aes­thetic effect.

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