Open Vented Pumped Central Heating Systems
Most properties are centrally heated by hot water from a boiler being pumped through radiators. The water in the central heating system is separate from the water from the hot taps.

A programmer is used to switch on the boiler automatically at convenient times of the day so that there will be hot water in the morning and to warm the house when temperatures drop in the evening. A thermostatic control, known as a roomstat is placed in a room where the temperature usually remains fairly stable, such as a living room, and monitors any rise or fall in the temperature. The room thermostat then maintains the temperature at a set level by switching on or off a pump or motorised valve which allows water heated by the boiler to flow through pipes to the radiators.

The same water flows around the system Open Vented Boiler Systemtime and time again until the desired temperature is reached and the roomstat shuts down the pump or valve.
In an open system small amounts of the water in the radiators can evaporate or leak, this water is topped up from a feed and expansion tank which also allows for the expansion that occurs when the water is heated.
If the boiler overheats, usually due to a mechanical failure, the open safety-vent pipe allows boiling water and steam to escape safely without expanding in the pipes or the boiler and causing them to crack often resulting in extensive damage.

Gravity Fed Central Heating Systems
In some older central heating systems water is circulated through the radiators by gravity rather than by a pump. When the water is heated it expands becoming less dense and lighter than cold water. The cold water sinks down the return pipe forcing the lighter hot water up the flow pipe and around the radiators.

Sealed Central Heating Systems
A sealed system can be used in small houses and flats where space is in short supply as there is no need for a water tank.

Water can be manually added to the central heating system as required via a filling loop which is connected to the mains water supply. The water is added until the required pressure is reached on the boiler pressure gague. In place of a feed and expansion tank, a pressure vessel copes with the expansion of the water as the temperature rises. Should the system become over pressurised by leaving the filling loop on, a safety valve allows the water to escape before damage is caused.

The boiler has an over-heat thermostat to prevent boiling should the standard thermostat fail, this is an especially important safety feature on a sealed system as it would result in there being nowhere for the boiling water to expand into. The most common type of boiler used on a sealed system is a combination boiler because there is no need for a feed and expansion tank or a hot water cylinder as hot water is provided by the boiler for the taps aswell as the radiators thereby saving additional space.

As with the exhaust on a car the gases which are a byproduct of the combustion which occurs within the boiler can be harmful and must be removed safely. In the case of a boiler the gases are removed via a flue which is fitted to the boiler. The location and correct use of these flues is vitally important as allowing any of the gases back into the property could be fatal.

Room-Sealed Flues
With a room sealed flue the room is, as the name suggests, sealed from contact with the gases expelled via the flue. There are two types of room sealed flue.

A natural draught flue 
must be positioned directly on an outside wall so the gases do not need to travel any great distance thereby doing away with the need for and the noise from a fan.

A fan-assisted flue allows the boiler to be located up to 4m (13ft) away from an outside wall. It can also be run through a roof space allowing for more flexibility in the location of a gas appliance. A fan-assisted flue may be noisier than a natural draught flue but is more efficient and can result in greater fuel efficiency.

Open Flues
An open-flue takes air from the room in which it is located. The flue can be run up the outside of a building or through an existing chimney stack enclosed in a flexible flue liner. Levels of ventilation via airbricks or vents must be kept to a specific level or fumes can return into the room rather than to the outside of the property. The importance of available fresh air cannot be overstated on an open flue appliance. Many deaths are caused every year simply by blocking up airbricks, if you are in any doubt please contact an expert.

Source material and graphics were obtained from Housingnet
Comments & Suggestions?