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BSEE BOILERS & HOT WATER W


It has been 30 years since the UK water heating industry took a huge leap forward with the introduction of the unvented water heater. Stuart Doggrell, National Sales Manager at Fabdec, explores its evolution, bringing us right up to date with the latest technology.


hile the unvented water heating system was invented in 1861 by British engineer Thomas Hawksley


(and used extensively across Europe), heating water in a pressurised closed cylinder was prohibited in the UK because of the risk of explosions due to a build-up of pressure.


But as the technology evolved – and the industry was able to deliver devices that would ensure safety – laws were amended in 1986 to allow for heating water in unvented cylinders. This greatly improved the delivery and comfort of hot water to consumers, guaranteeing hot water at mains water pressure. People’s expectations increased and they wanted to recreate at home the luxury experience that they may only have had in hotels. Unvented water heaters quickly developed from leisure and commercial use to domestic households, where conventional vented systems with loft feed tanks were limited in the hot water pressure that they could deliver.


The change in UK legislation was accompanied by a set of rules and regulation to ensure the security and the safety of running this system. In accordance with BS 7206: 1990, to accommodate for the expansion of the water as it is heated inside the water tank, an external pressurised expansion vessel would need to be installed in the piping system.


Whilst there was also an allowance for limited internal expansion in the system, the expansion vessel was a necessary additional fitting that caused multiple installation issues. It needed extra space and was another functioning part of the system that would require regular maintenance – including having to be re-pressurised throughout its life cycle.


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30 YEARS OF UNVENTED WATER HEATERS A self‐sustaining system provides the solution


‘ As the


technology evolved – and the industry was able to deliver devices that would ensure safety – laws were


uFig 2: Unvented hot water cylinder with internal floating baffle within installation schematic.


The floating baffle enjoyed major success and is still used extensively today. However, it is important to recognise that it only minimises the issue and does not completely eradicate it. Utilising the floating baffle results in a short, medium or longer-term retention of the internal air expansion gap. The air gap or ‘bubble’ will need to be re-charged, which may or may not be frequent dependent on the conditions of use.


Because the baffle is designed to move up and down the dip tube outlet length (as the water level fluctuates) there is still the matter of wear-and-tear plus the outer ring can never fully seal against the inner wall of the vessel; so ultimately, the system would still require maintenance.


So, how could this be improved upon even further, to create, say, a totally self-sustaining internal air gap system? In the early 2000s, Fabdec started looking at ways to develop a self- sustaining system. How could air be ever-present in the vessel in a range of operating conditions and how could the absorption of this air into the water be eliminated over a period of time – an issue which has long been the downside of any unvented hot water system?


Fabdec proposed to install a device on the inlet valve which followed the Venturi effect, so named after Italian physicist Giovanni Battista Venturi (1746 – 1822) (Fig. 3).


This, in simple terms, is the reduction in pressure that results when a fluid flows through a constricted section of pipe with no discernible restriction to the volumetric flow rate.


uFig 1: Conventional unvented hot water cylinder installation schematic.


As pioneering as the original unvented heater was, research and development within the industry sought alternative solutions to cater for the expansion of hot water, leading to the next innovation – what is commonly known as the floating baffle (Fig. 2). This is an internal expansion device, which would provide for a volume of air to be accommodated above the water level inside the cylinder. Sitting on the water surface, the baffle would reduce the exposure of air to water to a minimum. With no external expansion vessel required, this would allow for a quicker and simpler installation, also reducing the amount of space required.


In the process, a fluid's velocity must increase as it passes through a constriction in accordance with the principle of continuity, while its static pressure must decrease. Any gain in kinetic energy a fluid may accrue (due to its increased velocity through a constriction) is balanced by a drop in pressure.


It is the mixing of liquid with air – or more specifically the induction of air bubbles into the liquid – that makes the patented ‘Self-Sustaining System’ 3S Technology unique, with the system now able to maintain the internal air gap as hot water is drawn off further upstream in typical usage.


As it replenishes the internal expansion device permanently, there is no need for recharging or regular servicing.


22 BUILDING SERVICES & ENVIRONMENTAL ENGINEER DECEMBER 2016


The resulting unvented water heater has no internal, moving parts in the cylinder and once the Venturi valve (air gap device) is installed, no additional parts are required. The absence of an external expansion vessel means the tank can fit into more compact installation spaces and is much simpler to fit.


This 3S Technology is now practically used within the Fabdec Excelsior i range of domestic water heaters, with scope to introduce larger bore Venturi valves for commercial applications where volumetric flow throughput requirements are that much higher.


www.fabdec.com


u3S Technology is now used within the Fabdec Excelsior i range of domestic water heaters, with scope to introduce larger bore Venturi valves for commercial applications where volumetric flow throughput requirements are that much higher.


VISIT OUR WEBSITE: www.bsee.co.uk


The organic development of the inlet valve led to prototype trials with the help of Birmingham University Chemical Engineering Division and the Advanced Engineering Cluster at Wolverhampton University, who carried out


Computational Fluid Dynamics to find the best geometry for the product. What is also important to consider is the dip tube length within the vessel itself. The relationship between ‘aerated water’ entering the vessel and the air gap maintained must ensure that firstly there is an adequate air gap for the volume of water to be heated and secondly that excess air may cause the water flow outlet to stall and starve demand for hot water upstream.


amended in 1986 to allow for heating water in unvented cylinders. This greatly improved the delivery and comfort of hot water to consumers, guaranteeing hot water at mains water pressure.





uFig 3: Installation schematic of ‘inlet group’ including Venturi valve.


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