What is central heating?
Central heating warms up the interior of an entire property, or a portion of the building, from one particular control panel, affecting usually several rooms.
Central heating is different to local heating because the generation of heat with central heating occurs in just one place. Usually, this is through a boiler. The high temperatures that this creates are then spread through the property, either through air ducts, by water through a pipe system, or by steam also going through a pipe system. For example, your boiler may provide your central heating system with hot water, which is then transported to your household’s radiators along a pipe system.
Central heating is commonplace in Europe, the USA and most MEDCs, and is usually based on gas heaters, district heating or oil-fired systems. In the USA in particular, central heating is usually powered by natural gas fired central forced air systems. Steam heating central heating systems, normally using coal, gas or oil, generally are used in larger installations, such as those heating factories or office blocks.
Electrical central heating systems are rarer - they are only economically viable when electricity is cheap, for example when it is provided by solar panels, or when geothermal heat pumps are used. The general efficiency is lower than the standard systems which burn fossil fuels, although electrical systems may be much better for the environment due to reduced emissions.
Water Central Heating
Central heating systems that are water-based, usually are comprised of the following components:
- propane tank or oil tank, and associated supply lines
- boiler, which heats up the water in a closed-water central heating system
- pump, which circulates the hot water once it has been heated by the boiler in a closed-water central heating system
- radiators, which are wall-mounted panels through which the hot-water passes, to heat up the property’s rooms
In Europe, it is common that the central heating system, if it’s water based, will also be combined with the household’s need for hot water. When this occurs, the pump will pass the hot water through a heat exchange before it gets to the radiators, which will then heat up the a tank, containing further water, which will then be fed directly into the household’s water supply to reach taps, showers, etc.
Sealed-water central heating systems provide a form of central heating in which water used for warming up the building is totally unrelated to the standard water supply for the taps etc. Expansion tanks are used, which contain a diaphragm separating compressed gas on one side from the sealed-system water on the other side. This compensates for variations of pressure in the system, since it means that if the water pressure drops (such as when the water is colder within the system), then the gas will push on the diaphragm to then increase the water pressure again. This means that your shower will not vary in pressure all the time and water will always flow at a good, predictable rate.
Electric or Gas Heating
Electric central heating is based on converting electrical energy directly into heat. Electric heating is often less economically viable than heating produced through combustion, like those heating systems that are based on gas, propane or oil, which are burned to produce high temperatures.
Electrical heating usually is based on having a fan coil, which heats up as electricity passes through it - it has a lot of resistance, which converts electrical energy into heat. This fan coil is part of a central air conditioning unit, which then distributes the hot air generated around the building.
Electric furnaces are sometimes used. They consist of several fan coils, all of which heat up. Air is passed over these coils, and therefore it too starts heating up. The heating elements activate only one at a time, because otherwise the electricity supply may become overloaded - it is standard for each coil to be rated at 5 kilowatts. A limit switch prevents overheating from occurring, which may happen for example if the blower fails and so the hot air stays within the furnace, getting hotter and hotter!
In bigger central heating systems, based on electricity, such as those used commercially, central heating with this method is provided by an air handler, which is a scaled-up version of the electric furnace mentioned above.
Hydronic and Steam Heating
Hydronic central heating circulate a medium for heating. For example, hydronic radiant floor heating systems firstly heat water, using either a boiler or district heating. The system then has a pump to circulate this hot water through plastic pipes installed in a concrete slab. These pipes, which are an integral part of the floor, are carrying hot water, which therefore warms up its surroundings, and therefore the concrete heats up gradually, and will warm the room above from below.
Steam heating systems work very similarly to hydronic central heating. The principle difference is that instead of circulating heated water, steam heating systems circulate steam as the heating medium. No circulating pumps are required with steam heating, but the rest of the set up is very similar - a boiler or district heating centre is required, along with a fan coil unit or a radiator located in the room to be heated.
Hydronic systems are closed loop. This means that these central heating systems do not need the heating medium to be constantly refreshed - the same water or steam is constantly reheated. They are often used outside, for example, underneath paths which are otherwise likely to get covered in snow or ice, or in car parks. The most common use, however, is for “spot heating” - heating up the floor in certain areas of the house that can otherwise prove rather chilly.
Heat Pumps and Central Heating
In mild climates, a heat pump can be installed to air condition properties during hot weather, whereas during times of cold weather, the same heat pump can be used to warm the building by extracting heat from the outdoor air during cold weather.
There are some issues with the economy of heat pumps. Air source heat pumps are considered inefficient for weather conditions below freezing. Geothermal heat pumps may be used in colder climates to extract heat from the ground, and for economic reasons, these systems typically are coupled with a more conventional central heating system.
The main advantage of heat pumps is that the level of energy purchased from a supplier for heating a building is reduced - it is essentially a long-term investment. Furthermore, geothermal systems often can provide hot water too, and overall heat pump systems can reduce carbon emissions, through the fact that they can produce from 70% to 85% of a household’s heating.
Central Heating and the Environment
There is a lot of criticism targeted at central heating because it means that often an entire building is heated, when only one room at a time is in use, hence needs to be warm. Certainly, it is possible to heat a single room via a central heating system, but a lot of households or businesses do not carefully consider their system settings.
By its nature, central heating requires heat to be distributed. Therefore, there are some inefficiencies occurring through distributive losses - pipes transferring hot water may heat up the rooms or areas they go through inadvertently. If a whole building needs to be heated, then this is no problem, but it is an issue for heating only one room. In this case, it may be better to consider spot heating - for example, buying individual electric heaters.
If a full building does need heating, however, then combustion central heating is generally considered the most efficient method than electric-air central heating, or any other direct electric heating devices (like an electric fan heater that you can just plug in where you want it). This is due to the fact that electricity is normally generated through the combustion of fossil fuels, and therefore, there is an extra conversion in terms of the energy required for electrical devices. Obviously, this would be a different story if solar panels were involved.
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