Legionella Risk Assessments in Mill Hill

Legionella Risk Assessments in Mill Hill: what is different here

Very hard water and scaled cylinders

Average hardness in Affinity Water's Stanmore/Mill Hill zone was 353 mg/l in 2025, measured as calcium carbonate. In a stored hot water system, that scale settles at the bottom of the cylinder and coats the immersion element and coil. Sediment gives bacteria somewhere to live, and a scaled cylinder struggles to hold the 60°C that HSE says stored hot water should reach.

When we assess a house with a cylinder, whether a cottage in Mill Hill Village or a semi near The Hale, we measure the stored and outlet temperatures rather than relying on the thermostat setting.

Loft tanks in the semis round the Broadway

VOA figures put 70.2% of homes in the Mill Hill Broadway area in the 1919 to 1939 bracket. Many of these semis kept their loft cistern and vented cylinder through later boiler changes. We check the tank's lid fits and its insulation is intact, that nothing has fallen in, and that water is not sitting warm in a hot loft over summer. HSE's benchmark for stored cold water is below 20°C, and an uninsulated tank under a south-facing roof can exceed it.

Millbrook Park flats with little stored water

Many Millbrook Park flats draw heat and hot water from the district network via a heat interface unit, or run a combination boiler, so there is often no cylinder or tank in the flat. The risk in these flats is usually low and the assessment short. The points that still matter are showers in second bathrooms that nobody uses, and the weeks between tenancies when the whole flat stands still. Communal tanks and plant are the managing agent's responsibility, not yours.

Converted buildings with shared supplies

Some of Mill Hill's larger buildings have been split up. The conservation area appraisal notes that the grand houses on Wills Grove, once Mill Hill School boarding and staff houses, are now partly private houses and apartments, and big Victorian and Edwardian houses elsewhere in NW7 have been converted too. Where flats share a tank or a pipe run, we trace which outlets it feeds and who controls it, so the assessment reflects the building rather than one flat in isolation.

Voids, shared houses and little-used outlets

Private lets make up about a quarter of Mill Hill ward's homes, so changeovers are frequent and a house can sit unused for weeks between tenants. Before new tenants arrive, run every tap and shower until the water is hot or cold enough. In Barnet shared houses, en-suite showers in vacant rooms are the classic low-use outlet. HSE does not set a fixed review date: review the assessment after a change, for example a new cylinder, a conversion or a long void.

Long pipe runs in the larger houses

The big detached houses round Mill Hill School, on Highwood Hill and along Nan Clarks Lane often have three or more bathrooms, a cloakroom, a utility sink and perhaps an outside tap in a distant corner of the garden. Over the years basins get removed and the pipe is capped behind the wall, leaving a deadleg where water sits still.

On the far bathrooms, HSE's benchmark is hot water reaching at least 50°C within a minute of running. A long run from the cylinder can fail that. We map the outlets, time the hot water at the furthest ones and flag deadlegs worth cutting back, which is usually a short plumbing job.

Official sources

• HSE: legionella and landlords' responsibilities
• HSE: legionella, things to consider (water temperatures)
• Affinity Water: 2025 water quality report, Stanmore/Mill Hill supply zone (AF052)
• Valuation Office Agency: dwellings by build period in the Mill Hill MSOAs, March 2025 (CTSOP4.1)
• Barnet Council: Mill Hill Conservation Area character appraisal (April 2008)
• Rapleys (lead consultant): Mill Hill East heat network, energy centre and CHP
• Nomis: Census 2021 TS054 tenure, Mill Hill ward (Barnet)

Last reviewed October 2026 by EliteShires Property Services.