Introduction
As the UK government ramps up efforts to decarbonise homes in line with the Future Homes Standard 2025, the building services sector is experiencing a major shift.
Low-carbon heating solutions, particularly heat pumps and solar PV systems, are increasingly becoming the norm to swap out traditional combi boilers.
This move is also changing how we store and deliver domestic hot water. Since combi boilers deliver hot water on demand and don’t require storage, many homes currently lack a dedicated hot water storage solution. But with heat pumps, which operate most efficiently when paired with some form of thermal storage, hot water cylinders are gaining prominence again.
But this change also means old challenges coming back, as well as newer responsibilities, for specifiers in the UK.
First is, of course they’re bulky and difficult to accommodate into modern home designs and compact social housing flats. But there is another critical concern – how to make sure that reintroducing water storage into homes doesn’t bring back age-old risks such as Legionella that come with storing water in a cylinder?
How can specifiers, therefore, design systems that are not only energy-efficient but also inherently safe, easy to maintain and compliant with UK regulations?
In this article, we look at how Thermino heat batteries can help specifiers in delivering safer, low-carbon, and regulation-compliant hot water solutions that meet the industry standards, and enable low-carbon retrofitting in the built environment.
Let’s look at the Legionella risk and regulation landscape first.
Legionnaires’ disease is caused by inhaling droplets of water containing Legionella bacteria which thrive in traditional hot water systems, where water is stored at temperatures conducive to bacterial growth.
And an outbreak can have severe health and legal consequences for building owners and landlords – a recent case resulted in a fine of nearly £1 million for a Merseyside housing association on account of breaching the Health, Safety and Work Act 1974.
HSE ACoP L8 & HSG274
The UK’s Health and Safety Executive (HSE) has mandated rigorous control measures for water systems under the Approved Code of Practice (ACOP) L8 and its supporting technical guidance HSG274, making Legionella risk assessment and control a legal requirement.
ACoP L8 is a legal document that details the duty holders’ responsibilities such as Legionella risk assessment and record-keeping.
HSG274 is a series of technical guidance documents that support the ACoP L8.
Both regulations are the benchmarks for Legionella safety. They mandate that water services must be run at temperature hostile to bacteria, and for domestic hot water systems – this means hot water should be kept at ≥60C and distributed at ≥50C (55C in healthcare) – flushing little-used outlets weekly & monitoring outlet temperatures monthly – and cold water must be stored & distributed at <20C.
Building Regulations (Part G3):
Approved Document G (sanitation and water efficiency) outlines hot water safety and hygiene, including overheating prevention and safe discharge of hot water.
G3 Building Regulations outline the safe installation and operation of unvented hot water storage systems in the UK. Based on the guidance, hot water systems operating at mains pressure (typically above 3 bar) have to be installed by qualified professionals holding the G3 certifications, and need to be fitted with key safety devices such as temperature relief valves or combined temperature & pressure relief valve to prevent overheating or over-pressurisation. G3-certified installation ensures that a hot water system is robust enough to handle high pressures to protect residents.
PAS 2035/2030:2023: PAS 2035:2023 (with PAS 2030) is the UK standard/framework for domestic retrofit projects. It sets out best practices of how energy efficiency improvements should be assessed, designed, installed and evaluated.
Under PAS 2035, every energy efficiency measure, including new heating and hot water systems, must be appropriate to the dwelling in the sense that it should not introduce new risks or hazards.
This is a key aspect of ‘the whole house’ approach, ensuring that retrofit projects remain safe. For social housing refurbishments, this is crucial – any change to plumbing or heating may trigger a review of Legionella risk.
Future Homes Standard 2025: This regulation mandates low-carbon heating in all new homes, aiming to cut carbon emissions by 75-80% compared to current standards. As the government moves towards heat electrification, mainly through the adoption of heat pumps and heat networks, thermal storage is becoming a necessary component of domestic heating systems. It’s crucial, then, that these storage solutions must be Legionella-safe by design to ensure compliance and protect resident health as water storage makes a return to homes.
How Thermino helps specifiers achieve safety, compliance and decarbonisation compared to traditional hot water systems
The way Thermino heat batteries work:
Thermino stores heat, not water per se. And this distinction makes a big difference in Legionella and maintenance.
A Thermino heat battery contains a small internal pipe loop (heat exchanger), which is filled with water from the mains, and is linked to an electric heating element – powered by external energy sources. The heat exchanger is surrounded by our patented Plentigrade PCM. The only water stored is that which is in the internal loop, which is less than 15L in total. All the heating happens rapidly through the pipe wall into Plentigrade (the non-toxic PCM made of food-grade salt) surrounding it. No part of the heater is submerged in water, eliminating lime-scale build up and improving thermal performance. Limescale buildup can impact heat transfer efficiency – just 1mm of limescale on heating element can increase its energy consumption by 7-10%, leading to higher energy costs and carbon emissions.
Traditional hot water cylinders, on the other hand, require regular Legionella control checks, temperature monitoring, maintenance and record keeping, which add administrative and cost burden.
Because Thermino eliminates the need to store large water volumes and instead stores energy using phase change materials & heats water on demand, it inherently removes the key risk factor associated with stagnant stored water.
The ultra-high energy density of Plentigrade PCM also means Thermino units are up to 4x smaller than an equivalent hot water tank – ideal for retrofits and compact installations.
Thermino’s innovative design and its alignment with UK safety standards
HSE ACoP L8 and legionella risk
Thermino’s compact design ensures less than 15 litres of water is stored. With continuous flow and no stored bulk volume, Legionella bacteria doesn’t have a conducive environment to multiply. This design feature inherently meets the objectives of HSE L8 and HSG274, minimising risk without intervening.
Building regulations G3
As G3 covers hot water safety – including scald protection, overheating prevention and safety discharge – Thermino qualifies as a small unvented heater under G3, provided the mandatory external components, ie such as pressure & temperature relief valves are installed and applied correctly. Read more here.
PAS 2035 and Retrofit Programmes
In PAS 2035-compliant retrofits, hot water systems must avoid introducing new risks or complexity. Thermino supports this by minimising disruption to tenants (installs can be as as quick as a single day), eliminating legionella risk without new maintenance requirements – no tanks to clean or inspect, and by delivering hot water from renewable energy sources.
Overall, the compact size and ease of installation make it ideal for projects constrained by space, time or access.
While heat pumps themselves are not part of a fabric-first strategy, they are typically specified as the next step in a “fabric-first, technology-second” retrofit approach. Once the building fabric is improved to reduce heat loss, technologies like heat pumps and heat batteries can be introduced to meet the remaining demand more efficiently. This supports PAS 2035’s overarching goal of improving energy efficiency, safety and occupant comfort across the full retrofit journey, ie ‘whole-house retrofit’ approach.
BS EN 806 and BS 8558 Standards:
These standards provide guidance on the design, installation, testing and maintenance of hot water systems in buildings.
A key aspect is the proper flushing of new systems to remove debris, air, and potential contaminants that could encourage bacterial growth. It states that eliminating large stagnant water volumes reduces the potential for Legionella growth.
CIBSE Guidance (TM13 & TM14)
CIBSE’s TM13 guidance stresses the importance of maintaining flow, temperature control, and system cleanliness to reduce Legionella risk. It explicitly recognises that low-storage-volume systems, like those under 15 litres, pose minimal risk and often do not require thermal disinfection cycles.
Thermino’s compact design adheres to this approach, offering specifiers a low-risk solution.
In summary – Thermino is a future-proof solution for Specifiers
For specifiers designing low-carbon domestic hot water systems in a regulatory landscape that’s increasingly focused on health and compliance, Thermino offers a ready-made, future-proof solution.
The PCM heat battery is compact, scale-resistant and Legionella-safe, and simplifies compliance with HSE L8, G3, PAS 2035 and other key regulations for your upcoming social housing retrofit and net zero new build projects.
To support integration into your design process, Sunamp offers BIM models & SAP technical notes (available here) to help you assess compatibility and performance in your projects.
Need help evaluating project fit? Click here to get in touch with our technical team to discuss how Thermino can work for your next housing development or retrofit scheme.
