Since their successful introduction to the UK market back in the 80s, pressurized water heating systems, alternatively known as unvented cylinders, have proven to be very popular. They are winning hearts both with the general public and the plumbing and heating industry. As an additional confirmation of their success, in 2007 unvented systems accounted for the first time for 50% of the hot water systems installed in the UK.

Unvented systems provide good pressure of hot water at home. They differ substantially from traditional systems in that they do not rely on a header tank. Traditional systems build the hot water pressure by having a feeder tank in the loft which builds the water head and creates the pressure for the system. However, there is typically not enough head room for the water to build up sufficient pressure resulting in, for example, low pressure in the shower (especially if it is on the top floor).

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.

Legally certified unvented cylinders must meet the UK standard for safety measures to ensure that pressure build up within the sealed system does not pose a problem. The systems therefore rely on a combination of technical solutions. An expansion vessel is normally attached (externally or sometimes internally within the cylinder) to absorb expansion in the volume of the water due the heating. These vessels can handle up to around ten bars of pressure, addressing major expansion in the system.

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.

The safety checks and valves ensure a safe and reliable hot water system. The Cold water enters the system at mains pressure, keeping the system (which is sealed) at a constant pressure throughout. The water heats up within the cylinder and is then delivered to the user (still at mains pressure). As the hot water is drawn to the user (normally from the top of the cylinder), fresh cold water enters the cylinder (normally from the bottom) to replenish the levels and sustain the pressure.

Unvented cylinders are typically grouped into one of the following two groups - 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.

Unvented systems have proven to be an improvement on the old hot water systems, offering higher pressure and better performance. With competition helping to improve availability and reduce prices, unvented systems are likely to grow in popularity.