The new FHBS builds upon the interim 2022 uplift to Approved Document L (Conservation of Fuel and Power, Volume 2: Buildings other than dwellings), focusing on highly efficient, ‘zero-carbon ready’ non-domestic buildings. The core strategy for readiness involves a 'Fabric First' approach combined with a mandatory transition to low-carbon systems.
Hot water systems in commercial properties face the dual challenge of meeting stringent energy efficiency targets (Part L) while rigorously controlling health and safety risks, primarily Legionella (Health and Safety Executive guidance, notably HSG274).
Energy efficiency strategies under Part L are designed to ensure hot water plant contributes positively to the overall primary energy and CO² emission targets. Consultants are well versed in this and will now typically aim to utilise heat pumps for hot water. This sees the integration of ASHPs or potentially ground or water source alternatives into the hot water production chain, rather than relying on just an electric boiler or immersion for new build or gas water heaters when refurbishing plant in existing buildings. This is now deemed essential for meeting low-carbon targets.
Correctly designing hot water systems to precisely meet peak demand without oversizing is necessary for avoiding excessive storage volume that wastes energy and add considerably to a project’s capital and long-term operational costs. There has been in recent years considerable advance made in the prevention of oversizing; however, some applications are now suffering from under sizing. A 30kW energy source can heat 750 litres/hour by 34°C, so when the system draws hot water at a faster rate than it can be heated to 44°C, such as for hot showers, complaints of ‘cold’ water can be expected. This is a danger with systems deploying low-temperature ASHPs which must therefore work harder, whilst driving greater us of ‘primary’ top up heating. Electrical demand is managed by increasing the size of the hot water storage, which is then heated more slowly. Integrating a larger volume cylinder helps to overcome this under sizing, allowing for a two-hour reheat cycle that maintains enough water at 60°C to meet daily demand, whilst slowly heating reserves through the night when demand is minimal to meet the morning peak. This is a very different approach to the high energy input and low storage seen with traditional gas-fired systems.
All pipework and storage tanks must also be insulated to the highest possible standards to minimise waste through standing heat loss. Concerns over standing losses mean specification of instantaneous or point-of-use electric heaters for very low-use outlets or remote areas can be appropriate for avoiding running long, inefficient recirculation loops to little-used points, saving both energy and reducing Legionella risk. However, well-designed centralised systems with electric boilers and cylinder combinations will demonstrate similar, or less standing losses, as well as being able to take advantage of low carbon preheat, whether in the form of heat pumps or solar thermal. This contributes to the system’s ability to reduce dependency on energy, reducing carbon emissions and energy costs across its lifetime.
Health and safety and especially Legionella compliance remain absolutes. The key to hot water safety is temperature control and stagnation prevention. Owners and consultants must ensure the system is designed to facilitate ongoing management under the ACOP L8 and HSG274 guidance. Key to remember is that commercial hot water storage cylinders (and calorifiers) must store water at 60°C or higher, and that the primary heat source can achieve this consistently, especially when using heat pumps which can struggle to reach high temperatures efficiently. When distributing the hot water, it must reach a minimum of 50°C (55°C for healthcare) at all sentinel outlets (furthest/closest) within one minute. Designing effective hot water recirculation loops with temperature monitoring points and adequate pump sizing is therefore critical for maintaining flow and temperature throughout the system.
Water should not stand unused for sustained periods (a week or more) to prevent development of Legionella. Normally in hot water systems the risk is minimised by the temperature, constant flow and because the risk of Legionella in the incoming mains is relatively low. However, it can be present, and the risk to building occupants increases if a water system is fed from a cold water tank instead of the mains.
Overall system cleanliness is very important, from a health perspective but also to ensure system efficiency. Tanks and cylinders must be inspectable and drainable, so when specifying indirect cylinders (calorifiers) ensure drain valves or clean out access allows for the annual removal of accumulated scale and particulate matter which acts as a nutrient source for the bacteria.
Applications need to design out dead legs and dead ends in pipework where stagnation could occur and the system design must accommodate regular, weekly flushing at high temperature, preferably more than 70°C to curtail any possible development of Legionella, especially for all infrequently used outlets, such as vacant offices, school buildings over the holidays or seasonal facilities.
Future UK building regulations demand a highly integrated and performance-focused approach. For consultants, this means mastering low-carbon technology. For owners, it requires investing in quality installation and establishing rigorous monitoring and maintenance schedules. By proactively tackling the twin challenges of radical energy efficiency and stringent health and safety, commercial building projects can ensure not only compliance but also long-term value, lower operational costs, and a safe environment for occupants.
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