Turning on energy savings for hot water: 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

Domestic 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 50°C to the outlet and stored at 60°C to pasteurise DHW supply. The risk of scalding increases from 43-44°C upwards, with the duration of safe exposure reducing rapidly as temperature increases. At 60°C, 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 energy-consumption 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 44°C 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.

The group hopes to bring a better balance to the risks of Legionnaires’ disease, scalding and excess energy consumption

An early observation was that a common approach to reduce the 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 low-risk 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 higher-power, 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 building’s 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 50°C, 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 50°C (with the 15-litre volume constraints remaining) and, therefore, building primary distribution circuits may be operated at around 55°C. This, in turn, leads to reduced distribution 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.

There is a balancing act to perform between these requirements

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 (50°C), 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 60°C 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.

  • 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: HSG 274 Part 2, HSE bit.ly/CJAug21DHW1