Central Heating Systems

Dry Heating Systems

Dry heating systems are normally electrical - though there are electric storage systems, which do heat hot water and circulate this around pipework. The most common form of 'dry' central heating is the storage unit. This runs off cheap rate electricity - usually at night when demand is at its lowest - and heats up refractory blocks with electric elements. These in effect are electric radiators: the heat stored in them at night is emitted throughout the day, but they can also be switched over to 'full-price' electricity if extra heat is required.

Some of these storage units are fan assisted, but they do not provide instantaneous heat. A ducted system - which can be operated by any kind of heating source - is where the heat exchange occurs by air circulation and fan assistance through electrically heated elements, out through grills with circulating hot water. Ducts or 'registers' of different sizes are placed in rooms to provide the heat. Ducted systems are normally installed as the house is constructed because of the ducting needed to circulate the warm air throughout the house.

Wet Heating Systems

The wet system derives heat from heated water circulated along pipes from a boiler to various heat exchangers - panel radiators, skirting or fan convectors or ducts. The water finally returns to the boiler along return pipework, to be re-heated and re-cycled. In modern small-bore and micro-bore systems - these terms refer to the size of pipes used - a pump is used to circulate and accelerate the water through the pipes and radiators.

Modern heating systems also provide domestic hot water - called the primary circuit. In some systems, the hot water is pumped for the primary, while in others gravity, or natural circulation, is set up by the expansion of hot water. Many early 'piped' heating systems relied on gravity circulation through large-bore pipes, which was neither economic nor efficient.

Open small and micro-bore systems make use of a feed-and-expansion cistern, usually situated in the loft, providing for the thermal expansion of hot water, which then discharges into this cistern to compensate for loss of water through evaporation. Because small- and micro-bore systems utilize separate flow and return circuits, an even temperature of flow water is thus maintained.


At the heart of any wet central heating system is the boiler, which can be gas, oil or solid fuel. Solid fuel boilers can provide very high levels of heat output. Where solid-fuel room heaters are used, however, an independent water heating system, such as an immersion heater, is necessary for the summer, when the room heater is not in use.

For both solid fuel - whether wood or coal - and for oil-fired boilers, the fuel must be stored in a large-capacity oil tank, bunker or shed. Some solid fuel boilers are 'hopper fed' a constant supply of fuel, which otherwise would have to be stoked by hand.

Gas can be mains supplied or, as in many rural areas, supplied in the form of bottled gas. Gas and oil-fired boilers can be either balanced flue or conventional flues. Conventional flues are free-standing and designed to be connected to an existing chimney - or a new prefabricated one.

Balanced flue boilers don't require a chimney. Instead their gases are passed through a horizontal duct in an outside wall. In the duct are two passages: one for incoming combustion air, the other for outgoing flue gases. Solid fuel boilers must be connected to a chimney.


Radiators come in a range of sizes and styles. The larger the radiator, the greater its heat output and most radiators have fluted faces to increase their surface area even more.

An ordinary radiator is simply a double skin of metal through which hot water flows. At the top is a bleed valve to let out any air. There are also double and even triple panel radiators - panels mounted one behind the other and those with 'finned' rear faces to increase the amount of convected heat.

For each room in your home, there are 'ideal' temperatures. Central heating designers will calculate the heating requirements for each room and recommend radiators of appropriate outputs. All the heat outputs are then totalled, to give the boiler capacity.


There are various control devices available for use with wet central heating systems that can save you a lot of money if used efficiently. Room thermostats should be placed in rooms where the temperature is fairly constant (i.e. not in a hall or kitchen). They can switch the boiler on or off to maintain the temperature they have been set for.

It is possible to divide your house into 'zones' so that you aren't heating the whole house during the day when only downstairs rooms are in use, or at night when just the bedrooms need to be heated. Zone valves (or heating-circuit controllers) linked to individual thermostats can provide a separate temperature for each zone, and can be linked to an automatic timer or programmer.

Simple timers have two 'on' and two 'off' settings, which are repeated each day, so you can set them to come on for a couple of hours in the morning before you leave for work, and during the evenings when you are at home. There is a manual override that can be used at weekends or other times when your routine changes.

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