CPD PROGRAMME | DOMESTIC HOT WATER comply with Health and Safety Executive (HSE) requirements4 in order to minimise the growth of legionella bacteria. This requires that, if there is hot-water storage, it should be stored at least at 60C and distributed so that it reaches a temperature of 50C (55C in healthcare premises) within one minute at the outlets. In anything larger than small commercial systems, this would likely require DHW recirculation to maintain the minimum required temperatures at the outlets. UK hot-water systems will normally be arranged as one of three main types: Storage system this is often considered as a traditional system in the UK. In both CIBSE3 and Chartered Institute of Plumbing and Heating Engineering (CIPHE) guidance,5 the size of the store is established based on expected consumption over a particular time period that can be heated to the normal operational temperature from cold (10C) in, typically, two hours. Heat is generally provided from a gas- or oil-fired boiler, using a primary circuit to transfer low temperature hot water (circa 80C flow) through a heat exchanger within the hot-water store. Such systems normally operate with a daily or weekly cycle of heating up, DHW consumption, and then partial cool down before recommencing the heating as the next cycle begins. The store relies on maintaining a thermocline so that the hot water is drawn from the top while the cooler makeup water (or returning recirculating DHW) enters at the bottom of the store. The store can be topped up with heat at any time, and can provide good opportunities to aggregate output from various heat sources such as relatively low-output boilers, heat pumps, solar thermal panels and recovered heat. Storage systems can provide an excellent response to unexpected peak demands (so long as they occur when the bulk of water is heated) but come with the disadvantage of standing heat losses, specific legionella management challenges, and the physical space that they take up in the building. Part-storage these include an element of storage together with (typically) direct gas heating that allows the system to meet surge loads of limited duration while also being able to provide a continuous output that meets general, non-peak, requirements. An example of such an appliance is shown in Figure 1, which is able to deliver 440 litres of hot water (with a 50K temperature rise) in the initial 10 minutes, and then sustain a continuous flowrate of 29 litres per minute. The storage component also provides a limited buffer in case of interruption to the heat supply and, being relatively small, can be readily designed THE ORIGIN AND DEVELOPMENT OF UK LOADING UNITS In the 1960s, Harry Howick, past-president of the Institute of Plumbing (IoP, now CIPHE), developed demand compared to the base appliance (a wash basin used every 20 minutes set as one demand used to convert the cumulative load from connected appliances, in a distribution network, into a 7 4 (Developed from information in SOPHE TB 16-19) with low heat loss so that the stored hot water may be effectively maintained at the design operating temperature at all times when the building is occupied. An example schematic for such a system is shown in Figure 2. Instantaneous/continuous these are fed with a direct cold-water supply and are able to continuously supply hot water at anything from 0.2 litres per minute for an above-basin 3kW electric handwash unit, through to multi-point heaters (including combi boilers) serving several outlets at 50C to 60C, and up to dedicated modular gas hot-water heaters capable of producing, for example, 14 litres per minute of hot water (with a 50K temperature rise) for each wall-hung unit. Some units are designed to accept returning recirculating DHW as well as a cold-water feed, or potentially preheated but wholesome water from other sources. High-efficiency plate heat exchangers are also used for continuous-flow DHW heaters, with hot water from a boiler or other source as the primary fluid and the feed to the secondary wholesome DHW. There are hybrid systems deployed that, for example, employ renewable heat sources or recovered/waste heat to pre-heat or fully heat domestic hot water, together with heat exchangers and thermal stores. There is no lack of information sources that can be used to size DHW systems. The challenge is selecting the source of information, and estimation method, to provide a reasonable prediction of how a particular building and the expected occupants hot-water consumption are best represented. As discussed in the LUNA project6 stage 1 report published by the team at Heriot-Watt University, there are many different international standards and codes that are designed to provide information to facilitate hot-water system sizing. The methods of modelling vary (see Modelling methods boxout), and those commonly employed in the UK have evolved from the same root of a probabilistic modelling technique that was originally based around the concept of the loading unit (LU). The origin of the LU is more than 50 years old (see boxout), but the actual values and application of traditional LUs have been developed in an attempt to provide a better prediction of likely water use, principally to reduce oversizing particularly in larger (non-residential) installations. Many of the methods used around the world are empirically based and, with the increased maturity and availability of digital modelling techniques, there is a growing body of work employing stochastic procedures (such as that being developed at Heriot-Watt University1). In the UK, there are four predominant and potentially confusing choices for predicting hot-water needs: BS EN 806-3,7 the British adoption of the European standard, has LUs that have a different basis to traditional LUs and, although numerically similar to the CIPHE low category (see right), result in lower predicted hot-water demands when summed Figure 1: Example of a part-storage gas water heater. This example can provide 440 litres of hot water (with a 50K temperature rise) in the initial 10 minutes and then sustain a 76 May 2021 www.cibsejournal.com CIBSE May 21 pp75-78 CPD 179.indd 76 23/04/2021 18:08