BOILERS & HOT WATER
cannot fall clear, becoming trapped in the rods and damaging the element. Where limescale forms and remains on the heat transfer surface, because it is non- conductive, the surface becomes insulated leading to overheating of the element or heat exchanger. Over time this will cause it to rupture if the heat cannot be dissipated. For electric immersion hot water systems scale formation can happen in hard water areas in as little as six months so should be a major concern. It is common for protection from limescale
formation to be provided by a vigorous cleaning regime, but this option carries both a cost and system downtime that is not acceptable for many commercial buildings. For this reason, an approach which minimises formation of scale, reducing the need for cleaning, is more advantageous. There are three main options available, use of a water softener, a scale inhibitor or consider using alternative heat exchange.
A water softener is the best way of dealing with limescale and provides a benefit not only for the water heater but for the whole system. However, water softeners require regular maintenance, filling them with salt, which if neglected cancels all benefits. As a low-maintenance product, scale inhibitors are often selected to protect hot water systems as a ‘fit and forget’ technology. This is a common mistake. Whilst inhibitors do reduce scale formation and the required amount of water heater maintenance, they do not replace the maintenance regime, nor provide enough protection to ignore possible scale formation.
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For many sites neither a water softener or a scale inhibitor provides a satisfactory response, whether because of space, maintenance, downtime, or cost. A better option for these sites would be to replace the immersion heaters with a low limescale forming hot water system.
The case for using an electric boiler
An electric boiler, such as the Adveco ARDENT, heats water using immersion heaters located in a small tank within the boiler housing rather than directly installed into a hot water tank. This creates a sealed ‘primary’ loop to an indirect coil in the cylinder, eliminating the common problems of direct electric heating.
The electric boiler heats the same water continuously so there is only a small, finite amount of scale in the system which will not damage the elements. The heat exchanger in the cylinder is a large coil operating at a relatively low (80°C) temperature. Extensive experience with indirect coil use in the UK has shown that scale is not usually a significant problem in these systems. The electric boiler operates at the same efficiency as an electric immersion heater (100%) and so the only overall difference
in system efficiency is the minimal pump electrical consumption and a negligible amount of heat loss in the pipework An electric boiler hot water system will take up a little more space than an all- in-one electric cylinder, but it has more versatility and requires less clearance for the cylinder. Similarly priced to an immersion heater, an electric boiler system can cost slightly more due to the small amount of additional installation work. But with the cylinder forming significantly less scale, vastly improving reliability and drastically reducing maintenance demands, operational and maintenance savings will quickly offset any additional capital costs. The electric boiler additionally offers a level of redundancy that is not achieved with a single immersion heater. Scale formation is significantly reduced to a level that scale control can be adopted or not, depending on other building fittings and equipment that may benefit from it.
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BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2022 35
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