Central Heating Systems Central heating systems are all the same aren't they? Well, er.., no actually. There are vented and unvented systems, ones which do heating alone and ones which also do hot water. The heat source no longer has to be a boiler - you can now have a ground-source heat pump, while much of the hot water can be provided by solar panels. Then there are the types and number of motorised valves you have and their corresponding thermostats which determine the amount of control you have over the system. And so on.
Designing a System Central heating system design is conceptually very simple. You need to know the total heat loss from the building - from the walls, windows, roof, etc over a given period. This is the amount of heat that has to be put back in in order to maintain a given temperature. Thereafter, it is a matter of determining how much heat is required in each room, and how it should be provided - often radiators, seldom these days by warm air, but increasingly by under floor heating. Finally, the hot water requirement needs to be determined, along with the best way to provide it - tank-fed, mains pressure, or combi boiler. Boilers in a Nutshell 
There are two main types of domestic boiler - 'Combis' (or combination), and 'Conventional'. There is also a bit of a hybrid known as a 'System Boiler'. Most domestic boilers are now 'room-sealed' ie air for combustion and cooling are drawn from outside the building, and exhaust gases are (fairly obviously) flued to outside. If you want to know more about fluing, click here. Also, boilers now have to be 'condensing' in order to meet government efficiency criteria. As such, flue temperatures have fallen from the 120-180oC of old, to between 50o & 60oC with modern condensing boilers. | Conventional Boilers Conventional boilers consist purely of a heat source (ie a burner), and a gas to water heat exchanger to provide a source of hot water (in the case of a gas boiler). The hot water produced is pumped round the 'primary circuit' - through the radiators and through a coil inside the Domestic Hot Water (DHW) cylinder (the big thing in the airing cupboard). This primary water does NOT come out of the hot taps! The hot water taps are fed from the DHW cylinder which is, in turn, fed by the large tank in the roof. The DHW cylinder will normally incorporate an immersion heater that can be used to maintain a hot water supply should the boiler fail. Most conventional boilers will be installed in open-vented systems, however all modern ones will be capable of operating on sealed systems. Combi Boilers Combi boilers provide central heating in the same way as conventional boilers - ie radiators and/or under floor heating - but provide hot water by heating mains water as it passes through the boiler, ie when you open a hot tap, the boiler heats the water so there is no stored hot water in the form of a DHW cylinder This makes for an efficient system (water is only heated as and when it is required), however such a system requires sufficient mains water pressure and flow rate to operate effectively. There is also the slight downside in that there is no back-up to provide hot water should the boiler fail. Combi boilers invariably operate on unvented (or pressurised, or sealed) systems. | Combi boilers have a heat source and heat exchanger in much the same way as conventional boilers, but also include other elements of the heating system. Eg they include a circulating pump to pump the hot (primary) water round the rdiators. They also incorporate an expansion vessel (normally 10-12 litres capacity), and a diverter valve which 'diverts' the flow from the pump (ie the heated primary water) to either the heating circuit, or to a secondary heat exchanger that heats the domestic hot water. System Boilers System boilers are essentially conventional boilers that also incorporate an expansion vessel and a circulating pump. [They generally also include a by-pass and the odd other bits of this and that, but that's not important just now]. This can make for simplified installation and pipe runs, however in all but comparatively small systems, an additional expansion vessel will be needed to allow for the amount of expansion required. |