What an HIU does and when you have one
A Heat Interface Unit (HIU) — also called a plate heat exchanger or flat station — is the in-flat equivalent of a boiler. It extracts heat from a communal pipe loop (district heating, communal boiler, communal heat pump) and transfers it to the flat's own radiator circuit and hot water taps via a plate heat exchanger (no fluid mixing). HIU is typical in: post-2015 new-build London flats with communal gas boiler or CHP plant; some pre-2015 new-build with district heating; refurbished local authority blocks where individual gas combis replaced with central plant. Common in Tower Hamlets, Newham, Greenwich, Lewisham regeneration schemes. Indicators: no individual boiler in flat; small white box (often hidden in cupboard or hallway) with two pairs of pipes (primary in/out, secondary to radiators) and a heat meter; flat receives separate heat bills (not via gas). Pricing: HIU supply £950–£2,400 (Danfoss EvoFlat, Alfa Laval Mini City, SAV FlatStation Plus); install £700–£1,500; total £1,800–£4,500 typically. Replacement common at 12–18 year age.
Sizing and selection
HIU sized by space heating demand + DHW (domestic hot water) peak. Typical 2-bed London flat: 8–14 kW space heat + 40 kW DHW peak (instantaneous combi-style HIU heating taps directly) = HIU rated at 35–50 kW DHW with 8–14 kW continuous. Storage-style HIU (charges in-flat cylinder) sized lower for DHW (10–14 kW continuous) but requires 120–180L cylinder in flat — cylinder takes space (typical 1700×600mm in cupboard). Modern flat designs almost universally instantaneous-DHW HIU — no in-flat cylinder, smaller plant cupboard. Primary side: communal loop typically supplies 75–85°C flow and 35–45°C return. Secondary side: HIU delivers 45–65°C to radiators (boilers' equivalent), 50–60°C to taps. Performance metric: VWART (Volume Weighted Average Return Temperature) — heat networks regulate HIU to deliver low return temperatures to maximise plant efficiency; HIUs with bypass valves or poor commissioning cause high return temperatures penalised by ESCo.
Heat metering and billing
Heat Network Regulations 2014 (amended 2020) mandate individual heat meters in every flat on communal heat networks. Heat meter measures flow rate × temperature difference = kWh of heat consumed; bill issued by ESCo (Energy Service Company — typically third-party operator like Switch2, INSITE, Pinnacle) monthly or quarterly. Heat meter cost £180–£480 supplied; install £85–£140; typically included in HIU package. Meter must be M-bus or wireless compatible for remote reading. Disputes: residents commonly query high bills (heat networks have higher per-kWh price than gas, often 12–18p/kWh) — Heat Trust offers consumer protection. From 2026: secondary regulations introducing price caps and competition; check with managing agent on regulatory status. Service charges: HIU maintenance typically covered by service charge (£50–£180/year share of communal plant maintenance + HIU servicing); replacement at end-of-life sometimes via sinking fund, sometimes additional levy.
HIU replacement and renovation considerations
Failed HIU symptoms: no heating or hot water, audible whirring/hissing, leaks, persistent low temperatures, error codes on controls. Lifespan 12–20 years. Replacement: not always possible to swap-in different brand — primary connections must match communal loop; agreement with ESCo required; usually like-for-like replacement easier than upgrade. Cost £2,200–£3,800 including drain-down of communal loop section, fitting new HIU, commissioning. Renovating a flat with HIU: consider relocating to allow better cupboard layouts — HIU footprint typically 600×400×900mm, can be moved 2–5m with extended pipework £450–£950. Adding radiators or zones in renovation: secondary side easily extended; smart controls (Tado, Hive) compatible with HIU output. Cannot add boiler — flat is locked to district heating. Cannot easily add heat pump (would require shutting off HIU primary supply and standalone plumbing route).
