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.
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 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 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 - direct systems heat the water through a direct contact between the heating unit and the water that is ultimately drawn by the user. The heating is done by an electrical element with a typical capacity of 2.5 to 3.6kW which is normally enough for a domestic installation.
* 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.
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