Central heating and hot water systems explained
In a fully vented system (with gravity cold water feed to hot water cylinder and radiator circuit venting into a small cistern – often sited in the loft) water is typically heated by a boiler then circulated via (zone valves) two 2 port valve or a single 3 port valve (depending on whether you have a ‘Y’ or ‘S’ plan configuration) to either:
- The hot water cylinder’s boiler coil (heat exchanger) to generate hot water, and/or
- To the radiators for heating.
Control of what your boiler heats is by virtue of a programmer and the zone valves.
The term ‘fully vented’ comes from the fact that no part of this type of system is pressurised. There are usually two water tanks situated in the loft (one smaller than the other). The larger cold water cistern (or tank) supplies water to the hot water cylinder and does so purely by the force of gravity. The domestic hot water system (DHW) and cylinder vent back into this cistern. The smaller ‘feed and expansion’ tank (F&E) that vents the radiator circuit (and also keeps it topped up) also operates courtesy of gravity. Both cisterns will usually be fitted with a float valve so that a level may be maintained within the cistern and fed from a mains supply.
A gravity system is inherently safe. Because it should be impossible to have a dangerous pressure build-up in any part of the system there is no risk of something bursting under pressure – or at worst … an explosion! That’s the good news. There are however draw backs (trade-offs) as there are with each and any system.
The main draw back with a fully vented gravity system is the pressure you can expect from your domestic hot water. As mentioned, the cold water feed to your hot water cylinder is under the force of gravity. It is therefore going to be relatively low pressure.