DHW | INSTANTANEOUS HOT WATER risk of Legionnaires and scalding, as well as energy consumption, is the use of point-of-use or low-storage ( 15 litres) water heaters. This was specifically discussed in HSE guidance1 as a lowrisk system. Typically, these are electric systems directly linked to mains potable water beneath a sink, serving a basin or similar with low volumes of hot water within a facility. The challenge with these systems is that, when demand for hot water is high, they usually struggle to provide a reliable supply. A similar system is the electric shower, a higherpower, instantaneous hot-water supply, where temperature is often regulated by varying the volume of cold water supplied directly. Although they tend to be volume-limited, compared with storage systems, and have a high instantaneous electrical load, these are particularly interesting because they heat water to the supply temperature required with no excess, and no mechanical TMV, with Legionnaires risk often controlled with a timed flush or equivalent approach. New guidance Guidance Note: Domestic hot water temperatures from instantaneous heat interface units seeks to extend this application to mechanical point-of-use systems, where a heat exchanger is used to produce hot water instantaneously from a buildings primary hot-water system. It is understood, that provided the water storage volume within the plate heat exchanger and between pipework and the hot water outlets is 15 litres, and the system can produce hot water at 50C, this may also be considered a low risk approach, equivalent to systems described above. None of the approaches in the applications manual are zero risk with respect to Legionnaires disease, and they do not release the designer or building operator from the need to undertake a risk assessment and appropriate management and maintenance under health and safety law. However, they may be considered acceptable and, at times, preferable design solutions to the production of hot water in certain situations. Together with other good design practices for example, the elimination or minimisation of cold water storage tanks, use of copper (biocidal) distribution pipework this may help minimise and control this design and operational risk. The mechanical approach, then, has the potential to solve the production and volume constraints of conventional point-of-use electrical systems, as well as offer a number of efficiency benefits to mechanical systems. For example, communal residential heat interface units may be operated to produce DHW at 50C (with the 15-litre volume constraints remaining) and, therefore, building primary distribution circuits may be operated at around 55C. This, in turn, leads to reduced distribution Bosch Commercial & Industrials Greenstar heat interface unit There is a balancing act to perform between these requirements losses (because of lower flow temperatures), increases the scope for the use of heat pumps in these systems, and improves operational efficiency for them, as production temperatures are also lowered. Even standalone heat pump systems at the domestic or commercial scale may benefit. Thermal storage is effectively transferred to the primary side (via system volume, a buffer vessel, or thermal store) and may be operated at lower temperatures with the associated energy benefits, including lower losses and higher efficiency production. Legionnaires risk may be reduced by removing the storage from the potable water supply. As water-supply temperatures are typically lower (50C), scalding risk is also reduced, though TMVs are likely to still be required in some circumstances. The paper covers this in greater detail, particularly considering approaches to compliance with Part G of the Building Regulations, where cut-off of the hot-water supply is required upon cold supply failure. System water quality One thing not to miss in the newer, low-flow system temperatures proposed in the draft Part L and the approaches discussed above is the increased importance of system water quality. Systems operated at below 60C primary flow temperatures have an increased risk of biological fouling of the system water, as there is no high-temperature pasteurisation effect. So, this risk also requires managing. The paper aligns with CIBSE CP1: Heat networks: Code of Practice for the UK (2020) with regards to a standard for quality of hot-water supply, with a minimum time to achieve a minimum hot-water supply temperature. This reduces water wastage from waiting for supplies to warm up. The working group believes it has been able to improve clarity for building service engineers and facilities managers who are applying the current regulations. In doing so, it hopes to bring a better balance to the assessment of the risks of Legionnaires disease, scalding, and excess energy consumption for hot-water systems. CJ Guidance Note: Domestic hot water temperatures from instantaneous heat interface units will be available on the CIBSE Knowledge Portal at cibse.org/knowledge HUW BLACKWELL is associate director at Anthesis References: 1 HSG 274 Part 2, HSE bit.ly/CJAug21DHW1 48 August 2021 www.cibsejournal.com CIBSE Aug 21 pp47-48 Hot water.indd 48 23/07/2021 12:15