Use Hot Water Pressurized Tanks for Proper Hot Water Pressure
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.
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.
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.
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 – 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 – using a heat exchanger, the water in the cylinder acquires the heat from the heating medium. Typically a boiler is used to heat up water which is then pumped through the heat exchanger where it loses its heat to the water inside the cylinder. This system is widely used in conjunction with solar thermal systems.
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.
