What Is a Heat Interface Unit (HIU)?

How a Heat Interface Unit Works

A Heat Interface Unit (HIU) is a compact mechanical device installed inside each individual flat or apartment within a building that is served by a communal or district heating network. Rather than each flat having its own gas boiler, the building has a central plant room that heats a large volume of water to a high temperature. That hot water is then circulated around the building through insulated pipework — the primary circuit.

The HIU in each flat connects to this primary circuit and uses a plate heat exchanger to transfer heat from the communal hot water into the flat's own secondary circuit. The primary and secondary water supplies never actually mix — heat passes through the thin metal plates of the exchanger, keeping the communal system entirely separate from the water in your taps and radiators.

This design means that one central plant room can serve dozens or hundreds of flats efficiently, and each resident gets independent control of their own heating and domestic hot water without requiring a gas supply to each individual property.

HIU vs Boiler — What's the Difference?

The key distinction between an HIU and a boiler is where the heat comes from:

• A boiler is a self-contained unit. It burns gas (or in some cases oil or LPG) to generate heat directly within your property. It has a flue, a gas supply connection, a heat exchanger, a pump and controls — all in one box.
• An HIU does not generate its own heat. It receives heat from the building's communal plant room and distributes it within your flat. It has no gas supply, no burner and no flue.

This brings some important practical differences for residents:

• No gas risk in the flat: Because there is no gas supply or combustion happening inside your home, there is no risk of gas leaks or carbon monoxide from the HIU itself.
• Lower individual maintenance costs: Annual HIU servicing is generally less complex than a full boiler service, and there is no Gas Safe certificate requirement for the individual flat.
• Reliance on the communal system: If the building's plant room goes down or the primary circuit has a fault, all flats on that system lose heating and hot water simultaneously. You cannot independently source heat the way a boiler owner can.
• Landlord or building management responsibility: In most leasehold buildings, the HIU and communal system are maintained by the building owner or managing agent, not the individual tenant — though practices vary between buildings.

The Main Components of an HIU

While HIUs vary in design between manufacturers, most units contain the same core components:

• Plate heat exchanger (PHX): The heart of the HIU. This is a stack of thin corrugated metal plates through which the primary and secondary water flow in alternating channels, transferring heat without mixing. Over time, the plates can scale up with limescale or block with debris, reducing efficiency and eventually causing no hot water.
• PICV (Pressure Independent Control Valve): This valve regulates the flow of primary water into the HIU, maintaining consistent heating regardless of pressure fluctuations in the communal system. A faulty PICV is one of the most common causes of poor performance.
• Actuator: An electromechanical device that opens and closes the PICV or domestic hot water valve in response to demand signals from the control board.
• Strainer / Y-filter: A mesh filter on the primary inlet that catches debris from the communal pipework before it can reach the heat exchanger. Strainers need to be cleaned at every service.
• Control board / PCB: The electronic brain of the HIU. It reads sensor inputs (flow temperature, return temperature, domestic hot water demand) and signals the actuator and pump accordingly. PCB faults can cause cryptic error codes on the display.
• Expansion vessel: A small pressurised vessel that absorbs pressure fluctuations in the secondary circuit, protecting components from damage.
• Secondary pump: Circulates the secondary water around your heating circuit (radiators and underfloor heating). If the pump fails, radiators will not heat even if the HIU is otherwise working.

Common HIU Brands in London

London's apartment stock — particularly the large residential developments built since 2000 — uses a range of HIU brands. The most common you will encounter are:

• Danfoss: One of the most widely installed brands in London. Danfoss HIUs are known for reliability and have a comprehensive range covering both indirect and direct systems.
• SAV Systems: A UK-based manufacturer whose units are common in social housing and private developments. SAV HIUs often feature integral metering.
• Switch2: Specialises in metered HIU systems, with a strong presence in local authority and housing association stock. Switch2 units include smart metering for district heat billing.
• Caleffi: An Italian manufacturer with a strong reputation in the European heat network sector. Caleffi HIUs are popular in higher-specification residential developments.
• Alfa Laval: A global heat exchanger specialist. Alfa Laval plate heat exchangers are used both as standalone components and within branded HIU assemblies.
• Heatweb: A UK manufacturer with a growing presence in new-build developments, particularly in the south-east.
• Thermal Integration: Known for bespoke and high-performance HIU assemblies, often specified on premium London developments.
• Redan: Manufactures a range of prefabricated HIU and heat meter stations used across the UK housing sector.

When Does an HIU Need Servicing or Repair?

Like any mechanical and electronic system, HIUs require regular maintenance and will occasionally develop faults. You should have your HIU serviced annually by a qualified engineer, regardless of whether it appears to be working correctly. Many faults develop gradually — a partially blocked heat exchanger or a sticking actuator will reduce performance and efficiency long before it causes a complete breakdown.

Signs that your HIU may need attention include:

• No hot water or lukewarm hot water at the tap
• Slow hot water flow rate
• Radiators not reaching the expected temperature
• Error codes or fault lights on the HIU display
• Unusual noises — banging, hissing or whistling from the unit
• Visible leaks around pipework connections
• Unusually high heat consumption on your meter

If you notice any of these symptoms, contact a specialist HIU engineer rather than a general plumber. HIUs require specific training and, in many cases, brand-specific knowledge to diagnose and repair correctly.