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.
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.
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.
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.
Since their introduction, unvented cylinders have grown in their market share and their popularity. With competition in the market improving availability and reducing prices, it is fair to assume that unvented tanks have a bright future. - 15431
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.
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.
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.
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.
Since their introduction, unvented cylinders have grown in their market share and their popularity. With competition in the market improving availability and reducing prices, it is fair to assume that unvented tanks have a bright future. - 15431
About the Author:
Tal Potishman, editor of Heating Central, writes articles about central heating, plumbers, Watford boiler, underfloor heating and solar thermal. He focuses on helping cut costs by consulting on efficient heating.
