Gravity-fed heating refers to a heating system that is based on the natural effects of cool air sinking and warm air rising. This phenomenon results in passive cycle of drawing cool air into a heating system (usually located at the lowest point of a structure) and allowing the warm air to rise and heat other parts of the building. Gravity-fed heating is typically employed in central heating systems.
A central heating system provides warmth to the whole interior of a building (or portion of a building) from one point to multiple rooms. When combined with other systems in order to control the building climate, the whole system may comprise an HVAC (heating, ventilation and air conditioning). Central heating differs from local heating in that the heat generation occurs in one place, such as a furnace room in a house or a mechanical room in a large building (though not necessarily at the "central" geometric point). The most common method of heat generation involves the combustion of fossil fuel in a furnace or boiler. The resultant heat then gets distributed, typically by forced air through ductwork, by water circulating through pipes, or by steam fed through pipes. Increasingly, buildings utilize solar-powered heat sources, in which case the distribution system normally uses water circulation.
In much of northern Europe and in urban portions of Russia, where people seldom require air conditioning in homes due to the temperate climate, most new housing comes with central heating installed. Such areas normally use gas heaters, district heating, or oil-fired systems. In the western and southern United States, natural-gas-fired central forced-air systems occur most commonly; these systems and central boiler systems both occur in the far northern regions of the US. Steam-heating systems, fired by coal, oil, or gas, are common in the US, Russia, and Europe and are used primarily in larger buildings. Electrical heating systems occur less commonly and are practical only when low-cost electricity is available or when geothermal heat pumps are used. Considering the combined system of central generating plant and electric resistance heating, the overall efficiency will be less than for direct use of fossil fuel for space heating.
Water Heating
Common components of a central heating system using water circulation include the following:
- Gas supply lines (sometimes including a propane tank), oil tank, and supply lines or district heating supply lines
- Boiler (or a heat exchanger for district heating)—heats water in a closed-water system
- Pump—circulates the water in the closed system
- Radiators—wall-mounted panels through which the heated water passes in order to release heat into rooms
Engineers in the United Kingdom and in other parts of Europe commonly combine the needs of room heating with hot-water heating and storage. These systems occur less commonly in the US. In this case, the heated water in a sealed system flows through a heat exchanger in a hot-water tank or hot-water cylinder, where it heats water from the normal water supply before that water gets fed to hot-water outlets in the house. These outlets may service hot-water taps or appliances such as washing machines or dishwashers.
Sealed Water-circulating System
A sealed system provides a form of central heating in which the water used for heating usually circulates independently of the building’s normal water supply. A pressure vessel contains compressed gas, separated from the sealed-system water by a diaphragm. The diaphragm 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 can open to replenish water from the normal water supply if the pressure drops too low. Sealed systems offer an alternative to open-vent systems, in which steam can escape from the system, and gets replaced from the building’s water supply via a feed and central storage system.
Electric and Gas-fired Heaters
Electric heating or resistance heating converts electricity directly to heat. Electric heat is often more expensive than heat produced by combustion appliances using natural gas, propane, and oil. Electric resistance heat can be provided by baseboard heaters, space heaters, radiant heaters, furnaces, wall heaters, or thermal storage systems.
Electric heaters are usually part of a fan coil that is part of a central air conditioner. They circulate heat by blowing air across the heating element and then supplying it to the furnace through return air ducts. Blowers in electric furnaces move air over one to five resistance coils or elements, which are usually rated at 5 kW. The heating elements activate one at a time to avoid overloading the electrical system. Overheating is prevented by a safety switch called a limit controller or limit switch. This limit controller may shut the furnace off if the blower fails or if something is blocking the air flow. The heated air is then sent back through the home through supply ducts.
In larger commercial applications, central heating is provided through an air handler, which incorporates similar components as a furnace but on a larger scale.
Hydronic and Steam Systems
Hydronic heating systems are systems that circulate a medium for heating. 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 the heated water, which conducts warmth to the surface of the floor, where it broadcasts energy to the room.
Hydronic systems circulate hot water for heating. Steam heating systems are similar to heating water systems, except steam is used as the heating medium instead of water.
Hydronic heating systems generally consist of a boiler or district heating heat exchanger, hot water circulating pumps, distribution piping, and a fan coil unit or a radiator located in the room or space. Steam heating systems are similar except no circulating pumps are required.
Hydronic systems are closed loop: The same fluid is heated and then reheated. Hydronic heating systems also sometimes use antifreeze solutions to make ice- and snow-melting systems for walkways, parking lots, and streets. They are more commonly used in commercial and whole-house radiant floor heat projects, while electric radiant heat systems are more commonly used in smaller "spot warming" applications.
Heat Pumps
In mild climates a heat pump can be used to air-condition a building during hot weather, and to warm the building using heat extracted from outdoor air during cold weather. Air-source heat pumps are generally not economical 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 energy furnished for heating is supplied from the environment, with only 15 to 30% being purchased
Environmental Aspects
From an energy efficiency standpoint, considerable heat gets lost or goes to waste if only a single room needs heating, since central heating has distribution losses and (in the case of forced-air systems particularly) may heat some unoccupied rooms without need. In buildings that require isolated heating, therefore, one may wish to consider noncentral systems such as individual room heaters, fireplaces, or other devices. Alternatively, architects can design new buildings to use low-energy building techniques, which can virtually eliminate the need for heating.
If a building does need full heating, combustion central heating offers a more environmentally friendly solution than electric-air central heating or other direct electric heating devices. This stems from the fact that most electricity originates remotely using fossil fuels, with up to two thirds of the energy in the fuel lost (unless utilized for district heating) at the power station and in transmission. In Sweden proposals exist to phase out direct electric heating for this reason. Nuclear and hydroelectric sources reduce this type of energy loss.
In contrast, hot-water central heating systems can use water heated in or close to the building using high-efficiency condensing boilers, biofuels, or district heating. Wet underfloor heating has proven ideal. It offers the option of relatively easy conversion in the future to use developing technologies such as heat pumps and solar combisystems.
Typical efficiencies for central heating are 85–97% for gas-fired heating; 80–89% for oil-fired heating; and 45–60% for coal-fired heating.
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Central heating."
