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There are three different types of systems: forced air, and hot water or hydronic.

Forced air heating
This system uses ducts and an air blower to circulate warm air. The blower pushes the air through the ducts to registers into the rooms. The cooled air is then pulled back to the furnace through return ducts. The furnace warms it once again and the cycle is repeated.

Forced air systems may be located in the basement, a utility closet, a crawl space or an attached garage. Supply registers are usually installed in the ceiling, floor or wall to prevent cold spots and maintain an even air distribution. An advantage of a forced air system is that a central air conditioning system can be attached.

Hot water/hydronic heating
In a hot water system, a boiler heats water and circulates it using either a one- or two-pipe system, or a “series loop.” Either system uses the pipe to supply radiators with hot water and return the cool water to the boiler. A circulator is used to pump the water through the pipes.

There are three types of radiators in this system. The vertical tube, cast iron type, is installed standing on the floor. Baseboard radiators can be cast iron or copper tube, the latter having thin metal “fins.” They are located at floor level against the outside wall in an enclosure. Convector radiators also have finned tubing enclosed in a rectangular box, usually metal. Air circulates up from the floor through the tubing and then flows out the top into the room. Some convector systems use a fan to force the heat out of the convector.

Central heating equipment is available as: furnaces (gas, oil, electric); boilers (gas, oil); and heat pumps (electric only). There are also combination heating/cooling systems that combine both in a single package.

How different units work
A gas unit has a burner through which gas is delivered and burned, a heat exchanger where the heat produced from the burning gas is transferred to the distribution system (air or water), and a vent pipe through which combustion waste is vented outside of the home.

A typical warm-air furnace will use gas, air for burning the gas, and circulating air (or water if a boiler is used) that goes through the furnace, is heated and then is circulated throughout the house. The hot exhaust gases travel through the vent and out the chimney.

An electric furnace has resistant wire strips that are heated by electricity. A fan blows through the strips and heats the air, which is then distributed through ducts to the rooms via a fan/blower. Since there are no fumes, the system does not require a flue or chimney.

An electric heat pump extracts heat from the outside and delivers it to the inside (in winter), and does the opposite in summer. In winter, the pump uses an outdoor coil containing a liquid refrigerant. When a fan
blows across this coil, the refrigerant absorbs heat from the air and turns into hot vapor, which is pumped through an indoor coil. Because the vapor is hotter than room temperature, it condenses and releases the heat which is blown through the ducts to heat the house. The process is
reversed for air conditioning.

An oil furnace is similar to gas in that it has a fan and blower system to distribute the heat throughout the house. With oil, a power burner is used to mix oil with air. The mixture then is sprayed or “fogged” out a nozzle where the fuel is ignited by electrodes. Air is blown into the
heat chamber, heated, and then exits outdoors through a vent pipe.

Like a gas furnace, an oil unit has safety devices to shut down the oil supply if there is a problem at the burner. A fuel pump is part of the burner used to pump the oil from a storage tank; this supply line is filtered so the oil stays clean.