33 HPM 0813

HPM August 2013

Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com TRAINING/TECHNICAL Technical issues solved easily Brian Murphy, engineering services manager at Worcester, Bosch Group, answers some of the questions most commonly asked of the manufacturer’s team of technical advisors... I’m fitting a combi boiler in a home where a water meter is fitted. Do I need a mini expansion vessel? As we know, water expands when heated. This is true of domestic hot water too, so when it is heated by a combi boiler, it usually expands back down the cold main. When there is a backflow prevention device in place, however, there is no space for natural expansion to take place, and in extreme circumstances, this could lead to taps starting to drip or shower valves starting to pass. The solution to this issue is to fit a small expansion vessel on the cold main, as close to the boiler as practically possible. These are also known as shock arrestors, which are only small and must be suitable for potable water. Must I powerflush every time I fit a new boiler? Without doubt, it is a competitive market out there and the extra expense of a power flush on a quote can be the difference between securing a job and losing one to a competitor. With this in mind, it is important to point out to your customers that not every system needs a powerflush; it is entirely your decision as the installer to determine whether one is required, which will depend on the condition and layout of the system. There are some systems out there that are difficult, if not impossible, to power flush effectively. In some instances, where the system has been well cared for, a chemical cleanse and flush will be all that is necessary. The terms of a manufacturer’s guarantee will often state that the system should be ‘adequately cleansed and flushed’, which leaves it at the discretion of the installer on site to make a decision on what is required. I am replacing a regular boiler on an older open-vented system. Can I put the pump on the return as it’s much easier? Generally this is not advised. Putting the pump on the return means that most of the system itself is under negative pressure. This means there’s a much higher chance of pulling air in through joints or valves, which will accelerate corrosion within the system. If the vent pipe tees into the flow (as illustrated above) then there’s a higher chance of ‘pumping over’ as well. Air, of course, accelerates corrosion, so a system should be deemed as being in an acceptable condition before the boiler is replaced. Consider changing the system layout for the good of the new boiler, and to prolong the life of the system. Connecting the pump to the flow rather than the return will mean that most of the system will be under positive pressure. There is, therefore, less chance of air ingress, and this also means that space saving pipework layouts can be used, such as combined feed and vent, or close-coupled feed and vent. How can I make sure that the expansion vessel I fit to a system is the right size, and charged to the right pressure? As a rough rule of thumb, the vessel should be ten to 15% of the total system volume. The Domestic Heating Design Guide provides worksheets for calculating system size. The charge pressure should be as defined by the manufacturer, but generally depends on the static head of the system. The formula for establishing the right pressure is the pressure of the static head plus 0.2 bar when the system is cold. For example, if there is five metres of static head, that gives you 0.5 bar, plus 0.2 - so the initial charge pressure of the expansion vessel should be 0.7 bar. It is good practice to check the expansion vessel’s charge pressure when servicing the boiler. The recommended method for checking the charge pressure is as follows: • Isolate the appliance from the system • Drain the appliance through the appliance drain point (leave it open) • Remove the dust cap from the Schraeder valve and, using an appropriate gauge, check the pre-charge pressure • If the pre-charge pressure is too low, replenish using a bicycle pump or similar • If the diaphragm has failed, you will hear air being expelled from the vessel through the boiler as it is repressurised. If it holds pressure, check the Schraeder valve with LDF and replace the cap • Close the drain point, open the isolation valves and repressurise the system to its normal operating pressure. As an additional point, do not simply ‘dab’ the Schraeder valve to see if it lets water out - this isn’t a reliable method of testing the condition of the vessel. One of my customers needs some help with a new build in Scotland. I know that legislation can vary through the UK, and I need to be sure that the system and controls will comply. Can you help? There are variations - for example when part L of the English building regulations was revised in 2010, and new build properties below 150m2 required zoning of both upstairs and downstairs circuits, Scotland didn’t follow suit. Manufacturers can be a good source of information. It is also worth checking with organisations like TACMA (www.beama.org.uk), and the Building Regulations web pages (www.planningportal.gov.uk/buildingregulations). Regional government web pages are useful but if you are unsure, the best option is to check with the local building controls office. * To contact Worcester’s team of technical experts for guidance with installation, servicing, or maintenance, call 0844 892 3366. WWW.HPMMAG.COM HEATING & PLUMBING MONTHLY AUGUST 2013 33 enquiry number 125


HPM August 2013
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