Legionella Risk Assessments in Northwood

Legionella Risk Assessments in Northwood: what is different here

What the Northwood supply zone report says about cold water

Affinity Water's 2025 report for its Northwood supply zone, which serves about 62,000 people, records mains water temperatures between 7.6°C and 22.6°C across 156 samples, with a mean of 14.7°C. At the top of that range, the incoming cold water is already above 20°C, and HSE describes 20 to 45°C as the range suitable for legionella to grow.

The water arrives in good condition: no sample in the zone found coliforms or E. coli in 2025. Risk builds inside the house afterwards, where water warms in a loft tank under a hot roof or sits in a pipe nobody runs.

Loft tanks and cylinders in pre-war houses

The Arts and Crafts houses of Frithwood and the 1930s semis of Northwood Hills were built in the era of gravity hot water: a cold storage tank in the loft feeding a cylinder in an airing cupboard. Many have since gained a combi or an unvented cylinder, but plenty keep the tank, and some keep a redundant one, still connected and half full, after a refit.

We check that a working tank has a tight-fitting lid, insulation and a screened overflow, with no debris or sludge in the base, and we flag a redundant tank as a dead end to be drained and disconnected. HSE asks that stored hot water be kept at 60°C, so we check the cylinder setting and the temperature reaching the taps.

More bathrooms than people

Northwood has a lot of large homes. In the 2021 Census, 42.3% of households in the Ducks Hill area on Northwood's western side had four or more bedrooms, and 22.0% in Northwood Hills. Big houses tend to have a guest bathroom, an en suite in a spare room or a basement shower that goes unused for weeks.

HSE's guidance is that outlets should be used at least weekly to keep water moving. The assessment lists every rarely used outlet and sets out a simple flushing routine that a tenant can follow or that a managing agent can add to routine visits.

Very hard water, scale and showers

The zone's hardness of 348 mg/l leaves scale inside shower heads, on immersion elements and in the bottom of cylinders, and scale and sediment give bacteria somewhere to settle. A shower turns any that are present into a fine spray that can be breathed in. HSE asks tenants to clean and disinfect shower heads regularly and notes that instantaneous electric showers carry less risk.

In the written assessment we single out mixer showers fed from stored water, thermostatic valves at the end of long hot runs, and any shower in a room that is seldom used.

Empty houses and Hillingdon's three-month rule

Hillingdon's proposed HMO conditions require the licence holder to tell the council if the property becomes empty for more than three months. A long void matters for water too, because every part of the system stagnates at once. HSE says vacant properties may need a flushing regime, or draining down if they will be empty for a long time.

Before new occupants move in, every tap and shower should be run through, the cylinder brought up to temperature and the result recorded. We can make that re-occupation check and update the assessment in one visit.

When to review it, since there is no annual certificate

The law sets no annual or two-yearly review date. HSE asks landlords to review the assessment when something changes, and suggests checks can be fitted into routine visits such as the annual gas safety check. In Northwood the changes that should prompt a review are the ones big houses go through: a bathroom added or taken out, a loft tank removed, a gravity system swapped for a combi, or a family house turned into a shared let with twice as many people using the water.

Official sources

• HSE: legionella, landlords' responsibilities
• HSE: what is legionnaires' disease (conditions for growth)
• Affinity Water: 2025 water quality report, Northwood supply zone (AF048)
• Nomis: Census 2021 TS050 number of bedrooms, Northwood MSOAs
• House of Commons Library: MSOA names (Hillingdon 001 Ducks Hill, 002 Northwood, 004 Northwood Hills)
• Hillingdon Council: proposed HMO licensing conditions (Appendix 2 to the Cabinet report)
• Hillingdon Council: apply for an additional HMO licence (documents required)

Last reviewed October 2026 by EliteShires Property Services.