These are an ingenious space-saving idea, and an increasingly popular choice in UK homes. They now account for well over half of all new domestic boilers installed in Britain every year. They provide central heating and hot water directly from the boiler, heating water on demand, and they don’t require a hot water cylinder or cold storage tank in the loft. These boilers can be more efficient as there’s minimal hot water wastage, although they are only as good as the cold water mains pressure coming into the property.
When people refer to “open vent boilers” they generally mean a conventional boiler. This type of system comprises of a boiler and separate hot water cylinder, with a separate system pump, separate diverter valve and a back-up immersion heater. There will be two tanks in the loft, one large, one small. With this type of system you will normally find a pump on the shower. These boilers are often chosen when replacing old like-for-like boiler systems in order to minimise disruption.
A system boiler works in a similar way to an open vent boiler with a separate boiler and cylinder, but without the small tank in the loft as the boiler houses an internal sealed expansion vessel. It also incorporates the system heating pump making installation relatively quick and easy. With System boilers there is also the added benefit that the major components will be covered by the boiler guarantee.
If a household requires a separate cylinder then this type of boiler would generally be used, as once flushed and filled with protective chemicals, the system will stay clean and will not suffer from air locks and sludge problems.