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Turning on Energy Savings for Hot Water: Instantaneous Hot Water

September 9, 2021 | Insights,

Huw Blackwell

Huw Blackwell, Associate Director at Anthesis
Huw is co-author of a new CIBSE guide on reducing domestic hot water temperatures safely in heat networks and buildings



This is an adaptation of an article that first appeared in the CIBSE journal in July 2021.

Making hot water is energy intensive. As insulation and renewables continue to reduce and decarbonise energy consumption, heating hot water now accounts for a greater proportion of domestic energy use.

Some minor gains have been made with the introduction of energy efficiency guidance, such as Part L and Part G of the Building Regulations in the UK, but energy consumption cannot be reduced to minimal levels easily. However, a new guide published by the Chartered Institution of Building Services Engineers (CIBSE) explains how instantaneous hot water systems could provide a solution to reducing energy consumption from domestic hot water by safely supplying water at lower temperatures.


The risks of domestic hot water

There are two potential health risks with domestic hot water.

The one commonly cited in the building services world is Legionnaires’ disease, which comes with potentially severe implications for public health. Legionnaires’ disease is a notifiable disease caused by inhaling Legionella bacteria in water aerosol (i.e., water spray). The disease may be contracted through water systems and needs to be treated seriously by any sensible designer or facilities manager.

The second, less debated, risk is scalding, which can cause significant injury, particularly for vulnerable people. Typically, the risk of scalding is managed in modern design using thermostatic mixing valves (TMVs), devices that control the temperature of water before it reaches a tap, which can be assessed within risk management and maintenance processes.

Fundamentally, these two risks drive the operating temperatures of domestic hot water systems.

Prevention of Legionnaires’ disease requires that water is supplied at 50°C to the outlet and stored at 60°C.
The risk of scalding increases from 43-44°C upwards, with the duration of safe exposure reducing rapidly as temperature increases.



The premise of the work is based upon the existing design solutions that are:

  • Currently able to mitigate both risks
  • Generally acceptable to relevant statutory bodies – for example, building regulators and the Health and Safety Executive
  • Commercially available for wider deployment


Balancing act

There is a balancing act to perform between these two requirements in the design of systems. If we factor in how much energy is used to heat water, it becomes yet more complex. Water has a high specific heat capacity, meaning that it requires a lot of energy to increase the temperature compared to other substances. This highlights that there are useful energy efficiency gains to be obtained from reduced water heating, however, it 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 hope of unlocking energy savings while balancing the associated risks. The group members have just released their first publication.

Early observations

An early observation made by the CIBSE committee was that a common approach to reducing 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. The ‘lower temperatures’ used in these systems are relative to current practice, with water temperatures still warm enough for baths, showers and washing up, but by generating water instantaneously, there is the potential to avoid the temperatures required to prevent keep stored hot water safe. HSE guidance specifically states that these are low-risk systems. Typically, these are electric systems directly linked to mains drinking water beneath a sink, serving a basin or similar with low volumes of hot water. 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, which supplies instantaneous, higher-power hot water, where temperature is often regulated by varying the volume of cold water supplied directly. Although these systems tend to be limited by the volume they can produce, compared with storage systems like water tanks, and have a high instantaneous electrical requirement, they are particularly interesting because they heat water to the supply temperature required with no excess. They also remove the issue of potential failures of mechanical TMVs. Legionnaires’ risk is often controlled with a timed flush of water from the system or equivalent approach.

As well as reducing the risks described above, the mechanical approach has the potential to solve the production and volume constraints of conventional point-of-use electrical systems, as well as offer efficiency benefits to mechanical systems.

New guidance

The new CIBSE guidance on domestic hot water temperatures seeks to extend this current practice for electrical point-of-use systems to mechanical point-of-use systems. Mechanical, or conventional heating systems, can typically provide greater volumes of hot water centrally at a lower cost than electrical point-of-use systems.

The mechanical analogue and electrical point-of-use system may be considered a plate heat exchanger, where plates are used to transfer heat to the water, producing hot water instantaneously from a building’s primary hot water system. Provided the water storage volume within the plate heat exchanger and between pipework and the hot water outlets for mechanical systems is ≤15 litres, and the system can produce hot water at 50°C, it is understood that this may also be considered a ‘low risk’ approach, equivalent to the electrical point-of-use systems described above.

None of the approaches provided in the guidance 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, for certain situations, preferable design solutions to the production of hot water. 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.

As well as reducing the risks described above, the mechanical approach has the potential to solve the production and volume constraints of conventional point-of-use electrical systems, as well as offer efficiency benefits to mechanical systems.

Examples of energy efficiency benefits

Using the example of communal residential properties, such as a block of flats or apartments, we can see the potential energy efficiency benefits of mechanical point-of-use systems. Communal residential properties can be served by heat interface units, which provide heating and hot water from a central heat source (e.g., heat pump) rather than a heat source in each property. Using mechanical point-of-use systems, system flow temperatures may be lowered to produce domestic hot water at 50°C (with the 15-litre volume constraints remaining) and, therefore, building primary distribution circuits may be operated at around 60°C. This leads to reduced thermal losses through the distribution system (pipework and storage) compared to conventional systems operating at 80°C. It also increases the scope for the use of heat pumps in these systems as the operational efficiency of heat pumps is improved as the production temperature is lowered.

Even standalone heat pump systems at the domestic or commercial scale may benefit. As thermal storage is effectively transferred to water in the heating system (primary side), not the hot water being directly consumed (which requires storing at a minimum 60°C), it may be operated at lower temperatures, providing energy benefits including lower thermal losses and higher efficiency production. In this solution, Legionnaires’ risk may be reduced by removing the storage from the drinking water supply. As water-supply temperatures are typically lower (50°C), the scalding risk is also reduced, though TMVs are likely to still be required in some circumstances. The guidance covers this in greater detail, particularly considering approaches to compliance with Part G of the UK 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 approaches discussed above (and the draft Part L of the UK Building Regulations) 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 when considering overall system design.

The paper aligns with CIBSE CP1: Heat networks: Code of Practice for the UK (2020) with regards to requiring 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 guidance improves clarity for building service engineers and facilities managers who are applying the current regulations. In doing so, the working group hopes to bring a better balance to the assessment of the risks of Legionnaires’ disease, scalding, and excess energy consumption for hot water systems.

See the article in the CIBSE Journal

See the full guidance

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