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.
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.
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.
An additional safety mechanism is the pressure and temperature relief valve. The PT valve is a mechanical instrument that is designed to open whenever the temperature or the pressure exceeds a preset level, thus creating a potential hazard. When the temperature or pressure becomes too high, the PT valve opens up, discharging excess water into the drain (normally through a tundish), and in doing so reduces the temperature and pressure.
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.
Unvented cylinders normally fall into one of two main categories - direct and indirect:
* Direct cylinders - typically using an electrical immersion heater to heat the water. In this arrangement the electrical unit is immersed in the water within the cylinder and is hence heating the water directly. Typical heating units come with a capacity of 2.5 to 3.6kW.
* 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. - 16069
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.
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.
An additional safety mechanism is the pressure and temperature relief valve. The PT valve is a mechanical instrument that is designed to open whenever the temperature or the pressure exceeds a preset level, thus creating a potential hazard. When the temperature or pressure becomes too high, the PT valve opens up, discharging excess water into the drain (normally through a tundish), and in doing so reduces the temperature and pressure.
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.
Unvented cylinders normally fall into one of two main categories - direct and indirect:
* Direct cylinders - typically using an electrical immersion heater to heat the water. In this arrangement the electrical unit is immersed in the water within the cylinder and is hence heating the water directly. Typical heating units come with a capacity of 2.5 to 3.6kW.
* 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. - 16069
About the Author:
Tal Potishman, editor of Heating Central, writes articles on central heating, plumbers, Hove boilers, underfloor heating and solar thermal. He specializes in helping save money by advising on efficient heating.
