A heat interface unit (HIU) heats domestic hot water on demand through a plate heat exchanger, so no flat on the network stores hot water and there is no cylinder or calorifier to colonise. That removes the single biggest stored-water hazard. It does not remove the duty: the communal cold feed, the branch pipework to each HIU and the short runs to taps and showers still sit in Legionella’s growth band, and a void flat still stagnates.

If you specified a heat network partly because it deletes the cylinder, you have solved one problem and quietly inherited three smaller ones.

What the HIU removes, and what it leaves behind

A heat network has a central energy centre generating low-temperature hot water, the “primary” circuit, pumped continuously through insulated flow and return risers to every dwelling. Inside each flat the HIU contains a brazed-plate heat exchanger. Open a hot tap and primary water flushes one face of the plates while incoming cold mains passes the other, picking up heat in seconds. The hot water you draw was cold mains a moment earlier. Nothing is stored.

Like a Combi boilers and instantaneous water heaters: do they remove Legionella risk?, the HIU makes the stored-cylinder hazard disappear at the point of use. The stored-water controls that dominate the hot and cold guidance, calorifier inspection, storage at around 60 °C, destratification, simply have nothing inside the flat to act on [1].

So far so reassuring. The catch is that “instantaneous” describes only the heat exchanger. Everything upstream and downstream of it is ordinary plumbing carrying water that warms, cools and sits.

Where the risk actually sits

The communal cold feed. This is the one schemes miss. Cold branches and risers serving the HIUs frequently share a service shaft with the primary heating pipes, which run warm around the clock. Even well-lagged primary pipework sheds heat, and a riser shaft can sit several degrees above the rest of the building. Cold water that should arrive below 20 °C can creep into the low-to-mid twenties, where Legionella starts to multiply [1]. On a heat network the cold side is often the warmest “cold” you will measure anywhere in the building.

Branch dead-legs to each HIU. Every HIU is fed by a tee off the cold riser and, on the secondary side, feeds short DHW runs to kitchen and bathroom. When a flat is occupied and used daily, those legs turn over. When it is empty, a void, an unsold unit, a show flat, a buy-to-let between tenancies, they do not. A developer handing a block over in phases can have a quarter of the HIUs sitting dead for months while the shared cold feed dead leg behind each one warms to room temperature.

The DHW the HIU just made. Instantaneous heating fills the pipe between the exchanger and the showerhead with hot water. The moment the tap closes, that slug starts cooling, and in a heated flat it drifts straight down through the growth band. The volume is small. It is also the exact water that reaches the spray you breathe in.

The plate exchanger and keep-warm. Many HIUs run a keep-warm function that holds the exchanger or a small bypass warm so hot water arrives quickly. That convenience keeps a little water continuously in a warm band, and the plates themselves scale and accumulate biofilm over time. A keep-warm loop behaves like a miniature Hot water recirculation systems: ensuring consistent temperatures: good for performance, but a feature the risk assessment has to look at rather than assume away.

Tracing the water: an HIU from energy centre to shower

Picture it as two separate water systems that meet at the plates and never mix. Anyone who can sketch this can see exactly where the cylinder framing stops describing the building.

  • Start at the energy centre. It produces primary low-temperature hot water and pumps it out as a continuously circulating flow-and-return pair. This is a sealed heating loop carrying treated water nobody drinks or breathes as aerosol, so it is not itself an exposure route. Label it “hot, always moving”.
  • Run flow and return up a riser shaft. In the same shaft, draw the cold-water riser feeding the flats, fed either direct off the incoming main or, in a taller block, from a cold tank and booster set. Label the shaft “warm”. That adjacency is the first risk note on the page.
  • At each floor, tee off the cold riser into the flat and into the HIU’s cold inlet. That tee-to-HIU branch is your dead-leg candidate. Tag it “stagnates when the flat is empty”.
  • Inside the HIU, draw the plate heat exchanger: primary on one face, cold mains on the other, producing communal heating DHW on demand. Add the keep-warm bypass as a small loop on the primary side and tag it “small warm volume, continuous”.
  • From the HIU’s DHW outlet, draw the short secondary runs to kitchen tap, basin and shower. Tag the shower “aerosol, the exposure point”.
  • Finally, mark the two sentinel readings a heat network frames differently from a cylinder system: the cold temperature at the outlet furthest along the warmed riser, and the time a DHW outlet takes to run genuinely hot. Those two numbers tell you most of what the design has done to your risk.

The cylinder hazard is gone. The cold riser, the branch and the shower run are all still on the drawing.

What this changes for monitoring and control

The control regime shifts rather than shrinks. With no storage to inspect, attention moves to four things:

  • Cold-water sentinel temperatures, prioritising outlets on or near the warmed riser, because that is where “cold” fails first. The general expectation that cold reaches outlets below roughly 20 °C within a couple of minutes still applies [2].
  • DHW outlet performance, confirming a hot tap runs genuinely hot within a reasonable time. That is your evidence the HIU and its short run are turning over rather than sitting lukewarm [2].
  • Voids and low-use flats, handled with a defined flushing regime exactly as little-used outlets are anywhere, recorded per unit until the flat is occupied. Isolating and draining a long-term void can be cleaner than flushing it.
  • Plate-exchanger maintenance to the manufacturer’s schedule, since scale and biofilm degrade both performance and hygiene.

Who owns those tasks is its own question on a shared system. On a block of flats the Who is the duty holder for Legionella in a block of flats? Freeholders, RMCs and leaseholders is rarely the resident; it is usually the freeholder, RMC or managing agent controlling the communal plant and risers. On a Legionella in mixed-use residential and commercial complexes, the heat network can cross the boundary between residential and commercial duty holders. Settle that before the first void appears, not after.

A word on scope

This is general guidance, not a design opinion or a legal ruling on your scheme. Heat networks vary enormously in plant temperature, riser layout, void rates and how the cold side is configured, and only your building tells you which of the risks above bites hardest. A competent person should carry out a site-specific Legionella risk assessment under ACoP L8 and BS 8580-1, sketch your actual pipework, and set the sentinels, temperatures and flushing frequencies for the system you have [3][4]. Treat the HIU manufacturer’s data as input to that assessment, not a substitute for it.

Common questions on HIU heat networks

Do heat interface units still need Legionella temperature monitoring?

Yes. There is no cylinder to monitor, but you still record cold-water sentinel temperatures, especially near warmed risers, and confirm that DHW outlets run hot. The checks change shape rather than disappear [2].

Is the communal heating circuit itself a Legionella risk?

The primary loop is a sealed heating circuit carrying treated water that nobody drinks or breathes as spray, so it is not a direct exposure route. Its relevance is indirect: it runs hot through the shafts and warms the adjacent cold-water pipework, which is the part that does matter [1].

What about an empty or unsold flat on the network?

Its cold branch and the DHW run behind the HIU stagnate and warm up. Treat each void like any little-used outlet: flush it on a defined schedule, or isolate and drain it, and keep a per-unit record of what you did until the flat is occupied.

Does the HIU keep-warm function create a Legionella risk?

It holds a small volume of water continuously warm to speed hot delivery, so by design it sits in the growth band. It is rarely the dominant risk on its own, but the assessment should note the setting and the manufacturer’s cleaning interval rather than ignore it.

Sources

[1] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [2] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [4] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1