In the case of heated water or steam, piping is used to transport the heat to the rooms. Most modern hot water boiler heating systems have a circulator, which is a pump, to move hot water through the distribution system (as opposed to older gravity-fed systems). The heat can be transferred to the surrounding air using radiators, hot water coils (hydro-air), or other heat exchangers. The radiators may be mounted on walls or installed within the floor to give the floor heat. The use of water as the heat transfer medium is known as hydronics. The heated water can also supply an auxiliary heat exchanger to supply hot water for bathing and washing.
- Forced hot air
Warm air systems distribute heated air through duct work systems of supply and return air through metal or fiberglass ducts. Many systems use the same ducts to distribute air cooled by an evaporator coil for air conditioning. The air supply is typically filtered through air cleaners to remove dust and pollen particles.
A gas furnace, or forced-air heating system, reacts when the room air temperature drops below your programmed setting on the thermostat. The silicon nitride igniter lights a burner inside the combustion chamber. The heat created is then pushed into the heat exchanger, where the air is then heated. This newly heated air moves through the ductwork and into the rooms of the house. The combustion gases used to create the heat are vented through a flue in the roof or wall. The air supply is typically filtered through air cleaners to remove dust and pollen particles.
From a storage tank, oil is pumped into the furnace where it is turned into a mist, sprayed through the blast tube and ignited. The fine droplets mixed with the air allow it to burn very efficiently. The flame heats the air within a heat exchanger, and the combusted gasses exit through a flu or chimney. The air supply is typically filtered through air cleaners to remove dust and pollen particles.
- Water heating
Active indirect water heater:Circulating hot water can be used for central heating. Sometimes these systems are called hydronic heating systems:
- Common components of a central heating system using water-circulation include:
- A supply of fuel, electric power, gas or oil
- A Boiler which heats water in the system
- Pump to circulate the water
- Radiators which are wall-mounted panels through which the heated water passes in order to release heat into rooms or radiant floor tubing
The circulating water systems use a closed loop; the same water is heated and then reheated. A sealed system provides a form of central heating in which the water used for heating circulates independently of the building’s normal water supply. An expansion tank contains compressed gas, separated from the sealed-system water by a diaphragm. This allows for normal variations of pressure in the system. A safety valve allows water to escape from the system when pressure becomes too high, and a valve that can open to replenish water from the normal water supply if the pressure drops too low.
Hydronic radiant floor heating systems use a boiler or district heating to heat water and a pump to circulate the hot water in plastic pipes installed in a concrete slab. The pipes, embedded in the floor, carry heated water that conducts warmth to the surface of the floor, where it broadcasts heat energy to the room above. Hydronic heating systems are also used with antifreeze solutions in ice and snow melt systems for walkways, parking lots and streets. They are more commonly used in commercial and whole house radiant floor heat projects, whereas electric radiant heat systems are more commonly used in smaller “spot warming” applications.
- Heat pumps
In mild climates an air source heat pump can be used to air condition the building during hot weather, and to warm the building using heat extracted from outdoor air in cold weather. Air-source heat pumps are generally uneconomic for outdoor temperatures much below freezing. In colder climates, geothermal heat pumps can be used to extract heat from the ground. For economy, these systems are designed for average low winter temperatures and use supplemental heating for extreme low temperature conditions. The advantage of the heat pump is that it reduces the purchased energy required for building heating; often geothermal source systems also supply domestic hot water. Even in places where fossil fuels provide most electricity, a geothermal system may offset greenhouse gas production since most of the heat is supplied from the surrounding environment, with only 15–30% as electrical consumption