There are two main types of central heating system installed in UK homes: pumped or vented systems, and sealed or combi boiler systems.
Pumped or Vented Heating
Water can be pumped around the pipework system having been heated by the boiler. It passes through the radiator and/or the hot water cylinder, powered by a motorised valve.
With a gravity based system, gravity circulation is used to heat the hot water cylinder where the domestic hot water is stored. When the water is heated, it expands and thus weighs less than cold water. The cold water sinks and the hot water is pushed up.
The feed and expansion cistern is generally located in the loft and is open air, so helping the system work at natural atmospheric pressure. Its job is to handle any water losses or gains, which is measured by a low set ball valve in the tank. The pipe that hangs above the cistern is a vent pipe which serves to relieve excess pressure.
The storage cylinder holds a large amount of water and can provide it on demand (or as soon as its feed allows). The frustration however lies in the fact that once the store is used it must be reheated. In some properties that can take up to an hour. It is therefore not the most energy efficient heating system, hindered further by heat losses from the pipes and the cylinder itself.
The gravity system is fairly reliable as unlike the pump system it is not dependant on any such mechanism. It does however require larger pipes of 28mm, and ideally the cylinder should be installed above the boiler.
On vented systems the radiator temperature is controlled with individual thermostat valves. A thermostat is also used on pumped systems, triggering the motorised valve in accordance with the temperature of the surroundings. Although still widely used, the pumped hot water system is gradually being overtaken the prevalence of the sealed system.
Sealed Central Heating
The sealed system is closed off, not open air and vented like the older systems. There is no need for a feed or expansion cistern as most sealed systems involve a combination boiler that undertakes all the primary tasks. Combi boilers are designed to manage the system controls, expansion, pressure gauge and relief valve as well as operate as a normal boiler. The pressure gauge replaces the feed and expansion tank, while the relief valve does the same task as the vented system's vent pipe.
The system is pressurised at about 1 bar above atmospheric pressure, which is reached by a filling loop from the mains water to the system. The loop has a stopcock through which the heating system can be pressurised. It should retain a stable pressure unless the system leaks or the radiators are bled. The pressurised system will be at risk from leaks if high quality parts are not installed.
Without the tank, sealed systems do not have to undergo automatic top-up. This is a great advantage as hot water is on constant supply, but the boiler must have a safety mechanism installed so that it cuts out to prevent the water overheating.
Higher temperature sealed systems allow for smaller radiators, and also have less occurrences of corrosion. They generally run on mains pressure, so there is no need for the loft tank or the associated pipework giving you much more space and making it far easier and cheaper to install.
In the case of both vented and sealed central heating systems, the programme timer on the boiler can be set to switch on and off the central heating at desired times of the day.