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Pressurized Cylinders Offer Convenient Domestic Water Heating

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.

Compare this with pressurized systems which use the pressure of the mains system to provide hot water at convenient pressure levels. As the name suggests, pressurized systems sustain the incoming pressure of the cold water mains (normally around 2-3 bar in most residential areas in the UK). By keeping the system sealed, the hot water is pumped out to the user at the same flow as the cold water that comes out of the cold water tap.

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.

It is important to point out that the system is sealed and hence pressurized. As such it requires special training for correct design and installation. Since water expands upon heating it creates a few additional issues to account for. Unlike traditional systems where the header tank can take any expansion of the hot water, in sealed systems the technology has been designed to address this using a few checks and safety equipment along the hot water loop.

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.

Another safety measure is the temperature and pressure relief valve. The PT valve is a simple yet smart mechanical valve which senses whether the temperature or the pressure within the cylinder is too high (or is breaching a preset level). At that point the valve opens up to let the extra steam out into the drain, thus immediately reducing the temperature, the pressure and the volume of water in the system.

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.