Pressurized Cylinders Hold a Bright Future for the UK Hot Water

Written by Tal Potishman

Tuesday, 25 November 2008

Pressurized Cylinders Have a Great Future for the UK Hot Water

Pressurized hot water storage tanks, also known as unvented cylinders, have seen a remarkable increase in demand since their introduction to the country in the 1980s. They have proven to be highly popular both with the wider population and the plumbing and heating industry in equal measures. Latest results show 2007 was the first year where unvented systems have accounted for 50% of the hot water systems installations in the UK.

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.

Unvented systems are sealed and thus have an internal pressure which is likely to increase as the water within the tank heats up. In traditional systems, expansion of water as it heats up is handled by the feeder tank, which easily accepts any such expansion. For unvented systems, the solutions come in the form of specially designed checks and safety equipment to ensure safe and reliable operations even as water heats up and expands.

The sealed nature of the system led to some technological developments that were needed to address issues such as water expansion. As water heats up it expands and increase the volume it occupies. Traditionally this was easily addressed by the feeder tank absorbing any such expansion of the water. For unvented systems, however, a set of safety solutions has been designed to ensure safety is maintained at the highest level while still allowing for reliable and efficient operation.

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.

There are two main types of unvented cylinders - 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.

Article Author:

Tal Potishman, editor of Heating Central, writes articles about central heating, plumbers, Watford boiler, underfloor heating and solar thermal. He specializes in helping save money by consulting on efficient heating.

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