The role of HIUs in meeting the nation’s growing heat supply demands

By Building Engineer, 04 February, 2026

Building Engineer | February 2026

CABE Webinar Wednesday presenter Lance Colwyn advises consultants, engineers and energy professionals on how they can optimise heat interface unit (HIU) performance, as this will be critical in supporting the government’s ambitions to increase the supply of heat networks by 2050.

At present, the two main types of heat networks that operate in Britain – communal and district heating – supply around 2% of the nation’s growing demand for heating and hot water across commercial, industrial and residential buildings.

However, the UK government has ambitions to significantly increase this contribution to 20% by 2050, benefitting from a third “community hub” heating scheme that is becoming more prevalent in new builds and typically draws on a heat pump system to feed up to 80 properties. In London and Manchester, the ambition is even greater; heat networks could provide almost half of the total heat and hot water demand by this date.

In a recent CABE webinar that can be viewed below, Lance Colwyn, Heat Interface Unit (HIU) Business Unit Manager at leading manufacturer Vital Energi Ltd, explains these three systems in more detail, including how they operate, while also offering some case study examples, such as the Leeds Pipes project.

HIUs to power residential growth

As Lance explains, the residential sector will make up a significant proportion of the growth in all heat networks in the future, which is where wall-mounted heat interface units (HIUs), also known as heat stations and hydraulic boards, come in.
“HIUs can sometimes look like a combi-boiler, but they work very differently as there’s no combustion,” he explains.

“Inside is a plate heat exchanger that transfers heat from the primary heat network into the home’s heating and hot water system. The HIU can also reduce the pressure of the primary circuit before it enters the home, protecting domestic pipework and radiators.”

In the webinar, Lance advises on how to optimise HIU performance, with discussions around design, installation and how to commission it to the correct specification. He also discusses how to improve the HIU’s efficiency by covering issues like water quality, balancing systems and regular servicing. Finally, he offers readers tips on achieving energy efficiency savings.

After running through the individual components of a standard twin-plate HIU and the role each component plays, Lance explains how most HIUs are operated thermostatically or electronically, with hybrid models being phased out.

While thermostatic models are mechanically based, electronic versions, which are superior (and therefore growing in their usage), deploy a sensor to open and close the actuators and valves (and the required percentages) to supply heat and hot water.

“With the electronic versions, you get better control and efficiency, but more importantly, you have greater functionality,” he says.

“The pumps in thermostatic HIUs do not operate in summer, so come winter, many pumps seize because they haven’t operated for months. Whereas with an electronic HIU, you can trigger the system periodically to pulse the pump so that it runs on a regular basis. With an electronic HIU, you can also predict potential faults, which can be proactively managed as part of regular servicing to ensure a reliable supply of heating and hot water.”

Lance goes on to say that the performance level of HIUs in residential properties is incredibly important because the technology has a direct and measurable impact on the efficiency, reliability and running costs of the entire heat network. Ultimately, it affects customers’ bills.

To underline this importance, he explains how a single actuator valve that is stuck open can behave like a permanent bypass, dramatically increasing return temperatures, consequently dampening operational efficiency and results in higher bills for customers.

HIU standards and testing requirements

Although the capability and quality of different HIUs can vary considerably, Lance says the BESA 2023 HIU Test Standard has been pivotal in driving overall improvements as it provides consultants, engineers and energy professionals with the means to assess core performance indicators such as volumetric weighted average return temperature (VWART) and standing heat losses.

He explains how the standard’s 2023 version delivered an important upgrade on the 2018 iteration, which will be withdrawn in February 2026 when the new BESA 2025 HIU TEST standard replaces it.

To start with, the 2023 (and 2025) version assesses core areas against a modular testing system that covers HIU type, domestic hot water or heating tests. In addition, the new standard includes a pass/fail system that helps identify best practices.

Once the 2025 standard is introduced, HIUs that were previously tested to the 2018 standard will need to be retested, either to the 2023 or the 2025 version.

“There was a big difference between the 2018 and 2023 standards,” continues Lance.

“There were a lot more tests on the equipment, tests at 0.5 bar and 2 bar, and the temperatures were changed, so you now have to do a test at 55 degrees Celsius and 70 degrees Celsius.”

The BESA standard, however, is not the only legal requirement that consultants, engineers and energy professionals need to be aware of.

There is also the British Standard BS 8635-1: 2024, which specifies the design and manufacture of HIUs, as well as communal interface units. An important update is in the pipeline.

“Part 2 will come out later this year or early next year and that will give all the information that is required around the installation of HIUs, including maintenance and servicing,” he says.

The BS standards also introduce a naming convention that attributes each HIU with a generic model type. For example, a standard indirect twin plate HIU will be labelled HI/HWI (heating indirect and DHW indirect) while a direct heating with indirect DHW will be labelled HD/HWI. Similarly, each model will be attributed different references for kW rating, BOM and pipework material.

The introduction of a standard HIU naming convention brings clarity for clients, enabling a more transparent comparison between units, and will help ensure the correct unit is selected for a project.

Any other industry developments readers need to know about?

Currently, the Heat Network Code of Practice sets out many standards and requirements for heat interface units.

However, these have been further strengthened by the performance-based Heat Network Technical Assurance Scheme (HNTAS), which is currently in consultation, and will validate that minimum standards have been met during the design, construction and operation stages of HIUs.

“One of the HNTAS aims is to improve the efficiency and resilience of HIUs by setting minimal performance standards,” says Lance. HNTAS will do this by introducing 72 key performance indicators (KPIs) that all new heat network projects must comply with. It also references established standards such as CIBSE CP1, which will be replaced by a new mandatory assurance regime this year called HNTAS TS1.

This is on the regulatory backdrop of Ofgem becoming the statutory regulator for the heat network industry (27th January 2026), marking a significant step in the sector’s development to bring improved quality standards, greater customer protections and clearer routes of accountability across the sector.

All current heat networks will be “deemed authorised” and will be able to continue to operate. However, they (and any new heat networks) will need to register with Ofgem’s digital registration system by 26 January 2027, which is also the date when authorised status will no longer apply.

“After this date, no new networks or activities may operate unless a full operation is granted,” he says.

Under this new Ofgem-enforced regime, operators and suppliers will have to comply with several requirements, such as keeping records, being audited against minimum standards and providing notice to residents of any planned interruptions.

Significantly, they will be legally required to provide additional support to vulnerable residents in the event of a planned or unplanned interruption and supply restoration of services within a given timeframe. Having a heating infrastructure that can enable proactive maintenance to reduce outages will continue to be important.

“A failure to adhere to minimum standards or provide evidence of adherence will subject the operator to fines associated with breaching the Guaranteed Standards of Practice,” Lance says.

Final thoughts

While many existing HIUs do perform well at higher flow temperatures, the increasing introduction of fourth-generation and even fifth-generation (ambient loop) heat networks will require HIUs that can demonstrate efficient operation at lower flow and return temperatures to deliver the performance levels required.

Lance advises consultants, engineers and energy professionals that even the best performing HIUs will fail to deliver heat supply effectively if they are not operated and maintained properly. “Large heat networks may have thousands of connected dwellings, which means that small inefficiencies can quickly accumulate into substantial heat losses and increased operating costs,” he warns.

To safeguard against this, Lance suggests installing real-time monitoring, as this will identify issues with the HIU’s operation early on and will help maintain optimal system performance.

As a starting point for selecting an HIU, he recommends that readers purchase a BESA-tested and registered unit; not all HIUs are tested, and even those that have been tested won’t necessarily be as efficient as the user requires. This is because water quality and air in the system can dampen performance and also reduce its lifespan.

Lance also recommends that anyone looking to install a HIU should employ a suitably trained and competent person to commission, plan, service and repair the equipment onsite. It is not possible to detect leaks or exposed wiring from a remote service.

Looking to the future, he anticipates that the use of artificial intelligence algorithms could offer huge opportunities to enhance the operation of electronic HIUs and consequently the broader efficiency of the heat networks. Until then, he advises readers to focus on meeting the HNTAS KPI requirements.

Watch Lance’s CABE Webinar Wednesday presentation below.