Pressurized Cylinders Hold a Bright Future for the UK Hot Water

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 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.

However, pressurized systems, as the name suggests, are effectively passing the incoming pressure of the mains cold water system into the hot water system. A typical residential area in the UK would have a mains cold water pressure of around 2-3 bar. By keeping the system sealed, unvented systems can sustain this pressure within the hot water loop, providing sufficient pressure in the shower, kitchen or wherever else it is need in the house.

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.

The above safety checks and measures ensure safe operations and full protection for the users. By replenishing hot water drawn by the user with fresh cold water, the system sustains a steady pressure (which is equal to the mains pressure). Hot water is normally drawn from the top of the cylinder (which is where the rising hot water accumulates due to gravity) and the cold water is normally entered via the inlet at the bottom of the cylinder rising up as it warms up.

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 - this set up is based on a heat exchanger. Since the water inside the tank is heated by another medium using the heat exchanger, it is considered to be done indirectly. Typically a boiler is used to heat up water which is then pumped through the heat exchanger where the heat is transferred to the water inside the cylinder.

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.