Modern Building Services

FEATURE HEATING I n the majority of commercial buildings, the demand for space heating outweighs the demand for domestic hot water (DHW) but there are exceptions to this, especially in facilities such as hotels, leisure centres, and other hospitality venues. In such situations, it’s not only necessary to design the systems to address this balance of space heating and DHW, but it’s also essential that the chosen design takes full advantage of low and zero carbon technologies. Such an approach will enable the end customer to minimise carbon emissions and energy costs. In many cases, this necessitates a re-think of traditional strategies. It’s also worth pointing out that in all but the best insulated buildings there will still be a need for some form of backup heating, as low and zero carbon (LZC) technologies are only rarely able to meet 100% of heat demand. Of course, any such backups should also be as efficient as possible. Pre-heat or top-up? Because of these considerations, so-called mixed heating systems are becoming the rule rather than the exception. However, many mixed heating systems have been designed to use the LZC component as a supplementary top up source to traditional heating technologies. Inevitably, this imposes limitations on the use of the low carbon element and, in most cases, is not the optimum configuration. A better solution for these applications with high DHW demand is to use a low carbon heat source to pre-heat the DHW and then supplement it from other heat sources to bring it up to the desired temperature. Increasingly, heat pumps are being selected as the optimum low carbon heat source, ideally running on electricity from renewable energy sources. A key advantage of air source heat pumps, for instance, is that they are able to extract heat energy from the outside air at all but the most extreme winter temperatures. Solar thermal is also an option, though heat output is clearly dependent on variable levels of solar irradiation. Clearly, where heat pumps are to be used for pre-heating DHW, the heat pump needs to be inherently efficient and that can’t be said for many older heat pump designs. However, newer designs of heat pumps address such concerns by using, for instance, modulating inverter technology to enable continuous adjustment from40% to 100%of the load. In parallel, a hydrophilic coating on evaporator fins will support consistent efficiency and shorter defrosting times. Currently in the UK, the use of R32 refrigerant for heat pumps is becoming more widespread as its high volumetric refrigerating capacity is ideally suited to the temperate climate and due to its lower Global Warming Potential (GWP) relative to older refrigerants. The result is a high Seasonal Coefficient of Performance/Seasonal Energy Efficiency Ratio (SCOP/SEER). Thermal storage Whichever LZC technology is used, in this type of pre-heating configuration a mild steel vessel is used solely for storing the water heated by the LZC heat source. The DHW is then pre-heated via a plate heat exchanger. In buildings where the heat demand for domestic hot water exceeds that for space heating, a different design approach is required to ensure the optimum use of low carbon heating technologies. Ian Dagley , General Manager at Hoval Ltd, considers some of the options. Designing to meet high demands for hot water 18 MODERN BUILDING SERVICES FEBRUARY 2023