Pumped Systems are by far the most common systems in Britain. However, the sealed system is now becoming more and more popular. In this type of system the water is heated by the boiler and sent round the pipework by means of a pump. The water is then routed to the radiator circuit and/or the hot water cylinder by a motorised valve.
Normally found in older properties, gravity circulation is used to heat the hot water cylinder. This works on the principal that water expands when heated. The result is that it actually weighs less than cold water. This phenomenon is very handy as the cold heavier water sinks to the bottom of the system and causes the hot water to be pushed up.
The gravity system is pretty reliable as it doesn’t have a pump. It does however need larger 28mm pipes. For best results, the cylinder is installed upstairs above the boiler.
The same water recirculates round the system. There is a ‘feed and expansion’ cistern (usually in the loft) which keeps the system topped up when water is lost by evaporation or leakage. This same cistern is also used to cope with expansion of water from the system as it heats up.
Above the cistern you’ll see a pipe which hangs over it. This is called the vent pipe and acts as a safety measure so that excess pressure can be relieved.
These are gaining in popularity now for a number of reasons. As the name implies, the system is closed and has no feed and expansion cistern. It is most common for the boiler to be a combination type rather than connected individual components. These are very efficient and have the added advantage of saving space.
A pressure vessel copes with expansion and contraction of the water. In addition there is a safety valve to relieve excess pressure instead of a vent pipe.
On the upside, these systems run at a generally higher temperature than other types and therefore radiators can be a little smaller. These systems suffer less from corrosion. The other major advantage is that not only is less pipework needed, but also no space is required in the loft for a feed and expansion cistern.
On the downside, because the system runs under pressure, the installation requires higher quality fittings to ensure leaks do not occur. Also, since the system is not automatically topped up, in addition to the thermostat, the boiler must have a safety cut out to prevent over heating in the event of water loss.