What seems like a basic hot water system in many countries around the world is finally catching up in the UK. Unvented hot water cylinders and central heating systems provide hot water at mains pressure level. Unvented systems eliminate cases where two users cannot draw hot water simultaneously, for example when one member of the household having a shower while another tries to do the washing up. Unvented systems use the pressure of the mains infrastructure to retain sufficient pressure of hot water around the entire house. This is in stark contrast to the traditional gravity fed systems with a feeder tank normally positioned in the loft, offering low pressure in the top floors, due to lack of head for the hot water to build up pressure.

Hectic lifestyles mean high demand from the basic systems in the home. These days we do not expect to wait for hot water, nor do we expect to get hit by a cold flush of water in the shower when someone else turns on the hot water tap in the house. Furthermore, many modern houses have two bathrooms, as well as a host of other appliances that put pressure on the hot water systems, such as a whirlpool bath or a jet shower. Only unvented systems can offer the reliable supply of hot water in the right quantity and pressure to allow for all this to work seamlessly.

Unvented cylinders are made of metals that can be broadly grouped into the following three groups: copper, stainless steel and sheet metal. All three are reliable and used around the world. They naturally differ and have unique advantages and disadvantages.

* Copper cylinders - the lightest material, and used to be relatively cheap until the recent sharp increase of the commodity prices of copper. Still they are very popular in the UK, despite having the lowest heat retention properties of the three.

* Stainless Steel shell - offers middle ground in terms of heat retention and weight compared to the other technologies. Their longer lifetime (over 20 years) is reflected in their substantially higher prices.

* Sheet metal cylinders - the most popular type in most European countries and the cheapest of the three. Inner enamel glazing ensures virtually no build up of scale. They provide the highest heat retention performance of the three, but also are the heaviest.

There are several important parameters to remember when choosing your unvented hot water cylinder:

* Volume and dimension of the cylinder - there is a wide variety of unvented cylinders sold in the UK. A typical household of 4-5 people would be well served by a 200 liter cylinder. It is also important to check that the cylinder would fit into the airing cupboard (replacing the existing cylinder).

* Heat retention properties - check the heat loss specifications of the cylinder. A good cylinder will lose very little heat (e.g. less than 3 kW per 24 hours). This will reduce energy wastage and cut your heating bills.

* Warranty and after-service - unvented hot water cylinders have good record of long life and little in the way of faults. Nevertheless it is important to check what warranty and service you get from the manufacturer.