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 provide good pressure of hot water at home. They differ substantially from traditional systems in that they do not rely on a header tank. Traditional systems build the hot water pressure by having a feeder tank in the loft which builds the water head and creates the pressure for the system. However, there is typically not enough head room for the water to build up sufficient pressure resulting in, for example, low pressure in the shower (especially if it is on the top floor).
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.
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.
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 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 are typically grouped into one of the following two groups - 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 - this set up is based on a heat exchanger. Since the water inside the tank is heated by another medium using the heat exchanger, it is considered to be done indirectly. Typically a boiler is used to heat up water which is then pumped through the heat exchanger where the heat is transferred to the water inside the cylinder.
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. - 15478
Unvented systems provide good pressure of hot water at home. They differ substantially from traditional systems in that they do not rely on a header tank. Traditional systems build the hot water pressure by having a feeder tank in the loft which builds the water head and creates the pressure for the system. However, there is typically not enough head room for the water to build up sufficient pressure resulting in, for example, low pressure in the shower (especially if it is on the top floor).
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.
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.
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 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 are typically grouped into one of the following two groups - 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 - this set up is based on a heat exchanger. Since the water inside the tank is heated by another medium using the heat exchanger, it is considered to be done indirectly. Typically a boiler is used to heat up water which is then pumped through the heat exchanger where the heat is transferred to the water inside the cylinder.
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. - 15478
About the Author:
Tal Potishman, editor of Heating Central, writes articles about heating systems, plumbers, Watford boiler, heat pumps and solar thermal. He specializes in helping save money by consulting on efficient heating.
