DHW | INSTANTANEOUS HOT WATER Instantaneous hot-water systems can cut energy use and minimise the risk of legionella, says Huw Blackwell, co-author of a new CIBSE guide on reducing DHW temperatures safely in heat networks and buildings TURNING ON ENERGY SAVINGS FOR HOT WATER D omestic hot water (DHW) is the problem child of low carbon heat. Making hot water is energy intensive and accounts for a greater proportion of building energy use as insulation and renewables continue to reduce energy consumption in space heating. Some minor gains have been made in production efficiency (in Part L of the Building Regulations) and consumption reduction (through Part G), but the energy consumption of DHW cannot be reduced to minimal levels easily, as with PassivHaus-type interventions. A new guide published by CIBSE, however, will explain how DHW energy consumption can be reduced by using instantaneous hot-water systems to safely supply water at lower temperatures. The risks There are two potential health risks with DHW. The one writ large in the building services world is Legionnaires disease, which any sensible designer or facilities manager treats very seriously because of its potential implications for public health. This drives professional risk management, water treatment and monitoring for hot-water systems. The second risk less debated is scalding, even though this can cause significant injury or risk of death for the very young, the old, and the vulnerable in society. Typically, this is managed in modern design through the deployment of thermostatic mixing valves (TMVs) locally, on appropriate outlets, which themselves can be assessed within risk management and maintenance processes. Fundamentally, these two risks drive the operating temperatures of DHW systems. Prevention of Legionnaires disease requires that water is supplied at 50C to the outlet and stored at 60C to pasteurise DHW supply. The risk of scalding increases from 43-44C upwards, with the duration of safe exposure reducing rapidly as temperature increases. At 60C, water will scald an adult, leading to third-degree burns in a matter of seconds, and a child much faster. Balancing act As may be seen, there is a balancing act to perform between these two requirements in the design of systems. If we add in the energyconsumption factor, it becomes yet more complex. Water has a high specific heat capacity, so there are useful energy gains to be obtained from reduced water heating, particularly if it only requires a temperature of 44C at the outlet. However, this is not straightforward to deliver when also considering the Legionnaires risk. Last year, a group formed within CIBSE started looking at this problem, to understand the scope for clarifying guidance around all three issues, with the potential to unlock energy savings. CIBSE Journal covered this discussion (Taking the temperature, February 2020) and, since then, committee members have worked hard to prepare their first publication on the matter, which will be published imminently. The premise of the work is based upon what existing design solutions are: currently able to mitigate both risks; generally acceptable to relevant statutory bodies for example, building regulators and the HSE; and commercially available for wider deployment. An early observation was that a common approach to reduce the The group hopes to bring a better balance to the risks of Legionnaires disease, scalding and excess energy consumption www.cibsejournal.com August 2021 47 CIBSE Aug 21 pp47-48 Hot water.indd 47 23/07/2021 12:15