Legionella Control: The Complete UK Guide

Legionella control in the UK is built on three things: keep cold water below 20°C, store hot water at 60°C and distribute it so it reaches at least 50°C at the outlet within a minute, and do a written risk assessment proportionate to the system. There is no legal "Legionella certificate" — that's a myth. What's required is sensible, documented control under ACOP L8 and HSG274.

This is the pillar guide for the whole Legionella section. Below we walk through the law, who's responsible, the actual bacterium and how it hurts people, the practical controls, and the everyday tasks — tank cleans, temperature checks, flushing — that keep a system safe. Every section links to a deeper sibling guide.

Why Legionella matters

Legionella is a naturally occurring waterborne bacterium. In low numbers it is everywhere. The problem starts when a building's plumbing — a poorly heated cylinder, a stagnant cold tank, a long dead leg, a rarely used shower — lets it multiply, and the contaminated water is then breathed in as a fine spray. The illness it causes, Legionnaires' disease, is a severe pneumonia with a fatality rate of roughly 10% in the general population and higher in vulnerable groups. Read the background in what is Legionella.

What the law actually says

There is no single "Legionella Act". The duty to control Legionella comes from general health and safety law, with industry-specific guidance:

• Health and Safety at Work etc. Act 1974 — sections 2 and 3: employers and the self-employed must, so far as reasonably practicable, protect employees and others (including tenants and the public).
• COSHH 2002 — Legionella is a biological agent; you must assess and control exposure to it.
• Management of Health & Safety at Work Regulations 1999 — the duty to make a suitable and sufficient assessment of risk.
• ACOP L8 — the Approved Code of Practice on Legionnaires' disease. Special legal status: a court will use it to judge what was reasonable.
• HSG274 Parts 1-3 — practical guidance for evaporative cooling, hot & cold water systems, and other risk systems (spa pools etc.). Plain English explainer: ACOP L8 and HSG274 explained.

Landlords are covered by the same general law. There is no separate "landlord Legionella law" and no statutory annual certificate. The detail is in landlord responsibilities.

The risk assessment

The risk assessment is the legal cornerstone. It identifies the system, who could be harmed, what could go wrong, and what controls are in place — and it must be kept under review. For most single-let domestic homes with mains-fed combination heating and no stored water, the assessment is short and the controls are routine. For larger premises with stored cold water, calorifiers, showers, TMVs, spa pools or cooling towers, the assessment and the control scheme are much more involved.

The full how-to is in Legionella risk assessment. The blunt myth-buster: nobody is required to buy a "Legionella certificate" for an ordinary rental — what you need is a competent, documented assessment.

Temperature control — the 20/50/60 rule

Temperature is the primary control. The standard is short enough to memorise and is covered in depth in the 20/50/60 rule and our pillar guide on safe hot water temperatures.

"Sentinel" outlets are the nearest and furthest taps on each loop. Monthly hot/cold sentinel checks are the routine backbone for any system with stored water or significant pipework — and any TMV-served bath/basin needs to be checked too, because a failed TMV is a failed safety device. See TMVs explained.

Other controls beyond temperature

• Keep the system clean. Sediment, scale and biofilm shelter Legionella from heat and biocides. Cold water storage tanks need inspection and, when conditions require, a clean and disinfection — see cleaning a cold water storage tank and our blog how often should water tanks be cleaned.
• Avoid stagnation. Remove dead legs, flush rarely-used outlets weekly, and have a written regime for empty / void properties.
• Avoid aerosol exposure where you can. Where you can't (showers, spray taps), keep them clean, descaled and on the temperature regime above.
• Keep records. Temperatures, inspections, cleans, remedial work — kept for at least five years for non-domestic premises.

When to test (and when not to)

Routine microbiological sampling is not required for most low-risk systems — HSG274 is clear that temperature control and a clean system are the primary measures, not chasing test results. Sampling is appropriate when control is in doubt (out-of-range temperatures, an outbreak, after major works, or for higher-risk systems like healthcare or where the water is being treated by another method). The detail, including how a compliant sample is taken and how to read CFU/L results, is in Legionella testing and sampling.

What a working water-hygiene visit looks like

1. 1

   Walk-the-system survey

   Map every cylinder, tank, calorifier, TMV, outlet, dead leg and sentinel point. Photograph and schematic the system if there isn't one.

2. 2

   Temperature audit

   Hot and cold at sentinels and at representative outlets, plus calorifier flow and return — recorded against the HSG274 targets.

3. 3

   Tank and cylinder inspection

   Inside the cold tank: lid sealed, insect screens, no debris or biofilm, water temperature below 20°C. Cylinder thermostat verified at 60°C.

4. 4

   Risk assessment & scheme of control

   A site-specific written risk assessment, plus a practical scheme of control telling whoever runs the building what to do, when, and how to record it.

5. 5

   Remedials

   Fix the things that need fixing — chop out dead legs, fit lagging, replace failed TMV cartridges, clean and disinfect tanks where conditions justify it.

Who we are and what we cover

Pipe Assassin is an Enfield-based, G3-certified and WRAS-approved water hygiene and electric-boiler specialist. We do Legionella risk assessments, schemes of control, temperature audits, TMV installs and servicing, cold water storage tank cleans, dead-leg removal and remedial pipework across London, south Hertfordshire and west Essex. We are not Gas Safe — we don't do gas work — but everything around water hygiene, unvented cylinders (G3) and electric heating, we do every day.

Where to go next

Read the siblings in this cluster: what is Legionella, risk assessments, landlord duties, testing & sampling, ACOP L8 / HSG274, the 20/50/60 rule, tank cleaning, and empty / void properties.

Frequently Asked Questions

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