Hot water and the decarbonisation challenge

Hospital and healthcare premises face unique challenges when it comes to decarbonising hot water to ensure a safe, reliable and efficient supply. Andy Green, technical director at Baxi, discusses the challenges and opportunities.

Remeha E-HP AW Air Source Heat Pumps

The NHS is committed to reaching net zero from its directly controlled emissions by 2040, with a target of an 80% reduction in emissions between 2028 and 2032. The high hot water demand frequently associated with healthcare premises makes it a clear target for emissions reduction. But it’s one that comes with certain challenges – from the huge variety of building types and hot water systems to the specific considerations relating to domestic hot water (DHW) provision in these hygiene-critical environments.

First and foremost is the need to prioritise patient safety and wellbeing. Delivering an adequate hot water supply quickly ensures that healthcare professionals have the water they need to keep patients safe and comfortable.

Related to this is the need to implement measures to prevent the build-up of waterborne pathogens, mostly associated with the control of legionella bacteria, in the hot water system. To reduce the risk of legionella, the Health and Safety Executive (HSE) advises that domestic hot water (DHW) should be stored at least at 60ºC and distributed so that it reaches 55ºC in healthcare premises.

As the risk of patient scalding or burning increases when water temperatures are too high, a further consideration is temperature control which needs to be provided at hot water outlets to protect patient safety.

With these factors in mind, let’s consider some of the options for decarbonising hot water across the NHS estate.

Energy efficiency

The first step, particularly in existing buildings, should always be to identify and act on any opportunities to reduce energy demand. This might include switching to low volume shower heads to lower water and energy usage or adding pipework lagging to reduce heat losses.

In buildings where a central boiler plant and calorifier provide both heating and hot water, separating out the hot water is advisable. Similarly, another early stage in the decarbonisation process might be to upgrade any non-condensing direct-fired water heaters to more energy-efficient condensing models to drive down energy consumption and emissions.

The role of heat pumps

Heat pumps are widely viewed as one of the favoured technologies to decarbonise heating and hot water across the NHS estate. But converting older hospitals that use gas-powered steam or high temperature water heating systems to a low-carbon solution will typically need to be carried out in several stages. For this reason, early engagement with hot water experts is advisable to help plot the most appropriate decarbonisation pathway for your project.

An all-electric approach using lower temperature air source heat pumps (ASHPs) with direct electric water heaters, for example, would achieve the required high efficiencies but involve much higher volumes of stored domestic hot water. Potential issues relating to available space and the weight of the larger cylinders should therefore be considered – particularly when dealing with roof top or non-basement plant rooms. Additional forms of legionella control within the larger volumes of stored water will also need to be managed.

High temperature heat pumps, a more recent introduction, are able to deliver the high flow temperature required to meet the design hot water temperature. This removes the need for an alternative technology to achieve the required 60ºC or higher storage temperature, bringing greater design flexibility. The caveat is that the coefficiency of performance (COP) of heat pumps falls off at higher temperatures.

Bivalent approach

On refurbishment projects where the natural gas supply might be maintained, using ASHPs to preheat direct gas-fired water heaters – especially those supplied with anti-legionella functions as standard – can provide several advantages. Legionella control is more straightforward, and storage is considerably reduced compared with other systems – as are recovery times and capital expenditure.


Point-of-use electric water heaters should also be considered as an efficient option for an immediate supply of sanitary hot water in hospitals and GP surgeries, as they only generate energy when required. Installing a point-of-use water heater that incorporates anti-legionella functionality will ensure that water is adequately stored, cycled and distributed.

In conclusion, the good news is that, when it comes to hot water, there are many achievable opportunities to improve energy efficiency within hospital and healthcare buildings. But due to the complexity of the challenge, it’s vital to engage with specialists from the outset. By working together, the industry can help hospitals and healthcare facilities plan and achieve their net-zero ambition while avoiding the risks associated with hot water provision.

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