Pressurised cylinders were introduced to the UK market as long ago as the 1980s. They are often referred to as unvented cylinders and are proving to be highly popular. Their popularity is evident both in the domestic market and the professional space with plumbing and heating engineers. In 2007 unvented systems accounted for around 50% of the installed systems for the first time.

Unvented systems make a far better solution for water pressure compared with traditional hot water systems. In traditional domestic systems, the water feeder tank, normally positioned in the loft is responsible for filling the hot water tank and building up the pressure in the system. However, the feeder tank is rarely high enough to build enough of a head to create substantial pressure. This leads, for example, to an annoying drizzle from one's shower rather than a powerful shower.

Pressurized systems use the pressure that is provided by the mains water system, producing a steady and strong pressure throughout the hot water loop. Most residential areas in the UK have mains pressure of 2-3 bar which is the pressure of the cold water from your tap. Unvented systems ensure the pressure is maintained and transferred into the hot water loop by keeping the system sealed and sustaining a fixed pressure along the entire loop.

The UK certification of unvented systems has strict requirements from manufacturers to ensure that proper safety measures are in place to handle the excess expansion in the system when the water heats up. Expansion vessels are attached to the loop (either externally or within the cylinder) to absorb volume expansion of up to ten bars. This level is normally enough to account for most cases of volume increase due to overheating of the water.

UK regulations for unvented cylinders set out a detailed spec of safety measures to ensure all unvented systems sold in the UK meet the minimum level of safety. One of the main measures to deal with water expansion within sealed systems is the introduction of an expansion vessel. Whether external or internal (within the cylinder) an expansion vessel can absorb the expanding volume of water due the increase in the water's temperature. Expansion vessels vary but most can handle up to ten bars of pressure.

In addition, the legal safety requirements request a pressure and temperature relieve valve (PT valve) to be pre-installed in the cylinder. The valve is designed to open automatically whenever the pressure or the temperature reaches a point which is deemed to exceed the recommended levels of operation. In such case, excess water is discharged and is drained away from the cylinder, thus reducing the pressure and the temperature inside.

These safety mechanisms ensure unvented cylinders are highly reliable and safe. The hot water is delivered to the user at a pleasant pressure based on the cold water pressure. Effectively, the cold water passes through the cylinder and the system at a constant pressure throughout. The cold water within the cylinder heats up and is directly delivered to the user at the mains pressure. Fresh cold water is then drawn into the cylinder replenishing the system and at the same time ensuring the pressure remains constant.

Unvented cylinders normally fall into one of two main categories - direct and indirect:

* Direct cylinders - normally use an electrical heating unit to heat the water. The electrical unit (often referred to as an immersion heater) is immersed within the water inside the cylinder and has a typical heating capacity of 2.5 to 3.6 kW to heat up the water electrically.

* In-direct cylinders - these cylinders use a heat exchanger format to transfer the heat from another heat source. Typically a gas boiler heats up water which is then run through the heat exchanger inside the cylinder. Heat from the boiler-heated water is then used to heat the water inside the cylinder in-directly through the heat exchanger.

Over the last two decades unvented systems have carved a strong position in the plumbing and heating arena. Since strong competition is expected to drive prices down and availability up, unvented systems have a bright future in the UK.