District Heating & HIUs: Installation, Maintenance & Tenant Billing

Summary

Heat networks are an increasingly common form of heating in UK new-build developments, regeneration schemes, and urban apartment blocks. The Heat Network (Metering and Billing) Regulations 2014 (as amended 2020) require that individual consumers on heat networks are able to be billed based on their actual heat consumption — this has driven widespread HIU installation with individual heat meters.

For plumbers, heating engineers, and property managers, HIUs are a relatively unfamiliar technology compared to individual boilers, but are becoming more common as the Government's heat network zoning policy (requiring new heat networks in certain areas) takes effect from around 2025 onwards.

HIU maintenance and fault diagnosis is a growing area of work. Understanding how HIUs function, what can go wrong, and what tenants can and cannot do themselves is now a necessary skill for anyone working in residential building services.

Key Facts

• HIU (Heat Interface Unit) — a heat exchanger and control unit that connects a dwelling's internal heating and hot water system to the heat network primary circuit
• Primary circuit — the heat network pipework carrying hot water from the central plant at typically 60–80°C flow / 40–55°C return
• Secondary circuit — the dwelling's internal space heating circuit (fed via HIU); typically operating at 45–70°C flow/return depending on HIU type and radiator sizing
• Domestic hot water (DHW) from HIU — produced instantaneously via a plate heat exchanger within the HIU; no cylinder required in many HIU designs
• HIU types — indirecto (no water mixing; separate circuits via heat exchanger); some older systems are semi-direct (shared water with safety measures)
• Heat meter — measures energy transferred to the dwelling in kWh; mandatory under Metering and Billing Regulations for new connections; used to calculate tenant's heat bill
• HIU manifold pressure — primary circuit pressure typically 4–10 bar; secondary circuit 1–2 bar
• HIU service — annually; includes checking actuator valves, plate heat exchanger condition, flow rates, differential pressure, heat meter function
• Energy Loss Indicator (ELI) — some networks are assessed for efficiency; high standing losses indicate poor HIU function or network insulation
• Heat Network Regulation (from 2025) — Ofgem regulation of heat networks as an energy market; consumer protections including complaint rights
• Communal heating — smaller scale: a single boiler or heat pump in a building's plant room serving multiple apartments; functions similarly to district heating but within one building
• CHP (Combined Heat and Power) — gas or hydrogen-powered generator producing both electricity and heat; often the heat source for larger district heating networks

Quick Reference Table

Detailed Guidance

How an HIU Works

The HIU has two main functions:

1. Space heating: Primary hot water from the network enters the HIU. A modulating actuator valve opens to allow primary water to flow through the HIU's heat exchanger. Secondary circuit water (the dwelling's radiator circuit) is heated via the heat exchanger and circulates through the dwelling's radiators. The actuator valve modulates to maintain the secondary circuit at the desired flow temperature set by the room thermostat.

2. Domestic hot water: When a DHW tap is opened, a flow switch or pressure drop triggers the HIU's DHW mode. Primary hot water flows through the DHW plate heat exchanger, heating the secondary cold water from mains pressure to the set point (typically 55–60°C). A thermostatic mixing valve (TMV) at the HIU outlet blends this down to a safe delivery temperature (typically 41–43°C). This instantaneous process means no cylinder is needed — but it also means there is no storage reserve and very high flow rates must be maintained.

HIU Common Faults and Diagnosis

No heating — space heating only:

1. Check thermostat is calling for heat (room temperature below set point, timer in heat mode)
2. Check HIU display for fault codes — each manufacturer has their own (Hillross, Tahet, Dutypoint, Remeha, Caleffi)
3. Check the primary circuit is flowing: isolation valves open, pressure on primary gauge correct (typically 4–6 bar)
4. Check the actuator valve is operating — can be stuck open or closed; most HIUs allow manual override in service mode
5. Check the secondary circuit pump is running and circuit is properly balanced

No hot water:

1. Cold water supply isolated? Check isolation valve to HIU cold water inlet
2. Flow switch fault — dirty or stuck; usually cleanable
3. DHW plate heat exchanger blocked with scale — common in hard water areas; descale with citric acid or replace plate
4. TMV fault — delivery temperature incorrect; check with thermometer at a tap; adjust TMV or replace

Low flow temperature:

1. Network primary is returning too cold — network supply temperature issue (contact heat network operator)
2. HIU secondary pump inadequate for radiator system
3. HIU plate heat exchanger fouled — clean or replace

Balancing and Commissioning

HIU networks require balancing at both the network level (between buildings) and at the dwelling level. The HIU engineer's responsibilities:

Primary circuit balancing: Usually the heat network operator's responsibility; sets differential pressure at each HIU to ensure adequate primary flow.

Secondary circuit: The heating engineer balances the dwelling's radiator system using lockshield valves and flow meters as with any wet central heating system.

HIU commissioning settings:

• Primary differential pressure set point (depends on network; typically 0.3–0.8 bar)
• Secondary flow temperature (45–70°C depending on radiator sizing and weather compensation)
• DHW set point (55–60°C at HIU outlet before TMV blend)
• TMV delivery temperature (41–43°C)

Tenant Communication

Tenants moving into HIU properties often don't understand why there's "no boiler" and are confused when something goes wrong. Key points to communicate:

• "There is no boiler in your flat. Your heating and hot water come from a shared system outside the building."
• "If you have no heat or hot water, first check your own controls (thermostat, timer). If those are set correctly, it may be a HIU fault — call the managing agent."
• "Annual servicing of the HIU is the landlord/managing agent's responsibility — you don't need to arrange this."
• "Your heat bill is calculated from the heat meter on your HIU — read it the same way you would a gas meter."

Metering and Billing Regulations

The Heat Network (Metering and Billing) Regulations 2014 (amended 2020) require:

• Individual heat meters for new final customers from 1 August 2022
• Billing based on actual consumption
• Annual statement showing consumption and charges
• Access to meter readings
• Dispute resolution process

From 2025, Ofgem regulation will add further consumer protections. As a heating engineer, you may be asked to commission or replace heat meters as part of HIU servicing — ensure meters are MID-approved (European Measuring Instruments Directive) and installed per manufacturer instructions.

Frequently Asked Questions

Who is responsible for HIU maintenance?

The landlord or managing agent is responsible for HIU maintenance, not the tenant. This is analogous to the annual gas safety check for boilers. Annual HIU servicing is good practice and may become a regulatory requirement under Ofgem heat network regulation.

Can I fit an HIU in a property that doesn't have district heating?

No. HIUs are designed to connect to a heat network primary circuit. Without the network, there is no heat source. In a communal setting, a smaller version (communal heating HIU) connects to a boiler plant in the building — but this still requires the communal plant.

What qualifications do I need to work on HIUs?

HIUs have no gas connection and no combustion, so Gas Safe registration is not required for HIU work. A plumbing and heating qualification (City & Guilds Level 2/3 plumbing, NVQ plumbing) plus manufacturer-specific HIU training is the typical requirement. Some heat network operators require operatives to have completed training with the specific HIU model installed on their network.

Regulations & Standards

• Heat Network (Metering and Billing) Regulations 2014 (amended 2020) — individual metering requirements for heat networks

• Building Regulations Part G — hot water safety; TMV requirements within HIU

• Building Regulations Part L — energy efficiency requirements for heat networks in new buildings

• BS EN 834 — Heat cost allocators for the determination of consumption of room heating radiators

• CIBSE Heat Networks Code of Practice CP1 (2020) — design, installation, and commissioning guidance

• CIBSE CP1:2020 Heat Networks Code of Practice — Design and commissioning standard

• BEIS Heat Networks Guidance — GOV.UK consumer and landlord guidance

• ADE (Association for Decentralised Energy) — Industry body for heat networks

• Ofgem Heat Network Regulation — Regulatory framework (from 2025)

• system design — Sealed vs open vent systems; comparison with HIU-served systems

• thermostatic mixing valves — TMV requirements within HIU DHW delivery

• legionella management — Temperature management relevant to HIU DHW set points

• landlord certificates — HIU servicing within landlord compliance framework