Modern Central Heating

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2 Gas and Oil Layouts

The diagrams below show three central heating types. The vented boiler (regular boiler) and combination boiler systems probably account for over 95% of water-based central heating installations. System boilers are slowly becoming more common. You should read this page in conjunction with the page on boilers and the page on heating controls.  Most solid fuel central heating systems are not dissimilar to the first one shown below. They do however, incorporate one or two safety features because of the nature of solid fuel boilers. Solid fuel systems are shown on another page.
Vented systems. The majority of existing systems with a regular boiler and an indirect hot water cylinder are open vented. ‘Open vent’ refers to the separate vent pipe which is open to the atmosphere. The system also needs a feed-and- expansion cistern to allow for changes in water volume with temperature. This cistern has to be at the highest point of the system, usually in the loft space where it must be protected against freezing. Recommended controls on the system include:
  • programmable room thermostat with optional hot water timing capability
  • cylinder thermostat
  • TRVs on all radiators except in rooms with a room thermostat
  • automatic bypass valve (see Controls for more information)
  • boiler interlock (see Controls for more information)

The picture on the right shows just the heating system, the one on the left includes a vented hot water cylinder. It is possible to use a mains-fed pressurised hot water cylinder (with an expansion vessel) but there seems little point.

Sealed systems.  A sealed system normally operates at about 1 Bar above atmospheric pressure.  A stop cock in the link allows for the system to be pressurised - the pressure is only likely to drop if there is a leak or when the radiators are bled. As you can see from the drawings a sealed system does not require a feed & expansion tank nor does it require a vent. However, they need a number of safety devices in case faults develop (see right). The expansion vessel and safety features can be part of the boiler or separate from it. Note that that not all boilers are suitable for connecting to sealed systems: consult the manufacturers first. In these graphics the 'controls' have been omitted for simplicity.

With a sealed system the hot water for washing can be provided from either a vented (left)  or pressurised cylinder (right). More information on pressurised hot water systems can be found in the section on hot water.

Sealed system with instant hot water (combi system).  A combination boiler works in a similar way to a system boiler although the expansion vessel and safety features are all contained within the boiler. Unlike a system boiler it heats water for washing instantaneously. It can therefore provide a continuous flow of hot water, but at a lower rate than typical hot water storage systems. As such, they may be less suitable for dwellings where there may be simultaneous demands for hot water, ie multiple bathroom/shower room dwellings. The systems are easy to install because they do not require tanks in the roof or a cylinder. Like a system boiler a stop cock in the link allows for the system to be pressurised. A dial on the face of the boiler shows the pressure. Before choosing a combi system check with the maker to ensure the property has satisfactory water pressure and an adequately-sized water supply pipe.  As combi boilers use high gas inputs (when supplying heat for the rads and hot water), make sure the gas supply and pipework have adequate capacity, taking into account the demands of other gas appliances.
About half the boilers now sold are combination boilers. 20 years ago they were virtually unknown in the UK. They have been popular for a long time in France and Germany where more people live in flats.
©2006 University of the West of England, Bristol
except where acknowledged
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