A forced-air

central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...

system is one which uses air as its

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

transfer medium. These systems rely on

ductwork Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, ''supply air'', ''return air'', and ''exhaust air''. Ducts commonly also deliver '' ...

, vents, and plenums as means of air distribution, separate from the actual heating and

air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...

systems. The return plenum carries the air from several large return grills (vents) to a central

air handler An air handler, or air handling unit (often abbreviated to AHU), is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning ( HVAC) system. An air handler is usually a large metal box containing a blow ...

for re-heating. The supply plenum directs air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems. Forced air heating is the type of central heating most commonly installed in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

. It is much less common in

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

, where hydronic heating predominates, especially in the form of hot-water radiators.

Types

Natural gas/propane/oil/coal/wood

*Heat is produced via combustion of fuel. *A heat exchanger keeps the combustion byproducts from entering the air stream. *A ribbon style (long with holes), inshot (torch-like), or oil type burner is located in the heat exchanger. *Ignition is provided by an electric spark, standing pilot, or hot surface igniter. *Safety devices ensure that combustion gases and/or unburned fuel do not accumulate in the event of an ignition failure or venting failure.

Electric

*A simple electric heating element warms the air. *When the thermostat calls for heat, blower and element come on at the same time. *When thermostat is "satisfied", blower and element shut off. *Requires very little maintenance. *Usually more expensive to operate than a natural gas furnace.

Heat pump

*Extracts heat from the environment, using either the ground or air as the source, via the refrigeration cycle. *Requires less energy than electric resistance heating and possibly more efficient than fossil fuel fired furnaces (gas/oil/coal). *Air source types may not be suitable for cold climates unless used with backup (secondary) source of heat. Newer models may still provide heat when coping with temperatures below 0 °C (32 °F). *A refrigerant coil is located in the air handler instead of a burner/heat exchanger. The system can also be used for cooling, just as any central air-conditioning system. *See

Heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...

Hydronic coil

*Combines hydronic (hot water) heating with a forced air delivery *Heat is produced via combustion of fuel (gas/propane/oil) in a boiler *A heat exchanger (hydronic coil) is placed in the air handler similar to the refrigerant coil in a Heat Pump system or a Central AC. Copper is often specified in supply and return manifolds and in tube coils. *Heated water is pumped through the heat exchanger then back to the boiler to be reheated

Sequence of operation

# Thermostat calls for heat # Source of ignition is provided at the boiler # Circulator initiates water flow to the hydronic coil (heat exchanger) # Once the heat exchanger warms up, the main blower is activated # When call for heat ceases, the boiler and circulator turn off # Blower shuts off after period of time (depending on the particular equipment involved this may be a fixed or programmable amount of time)

References

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