Legionella Risk in Domestic Hot Water Systems: Temperatures, Dead Legs and Control

Quick Answer: Legionella bacteria proliferate at 20–45°C and are killed above 60°C. Domestic hot water cylinders must store at ≥60°C and deliver ≥50°C at the tap within one minute. Cold water must be stored and delivered below 20°C. Dead legs (pipework with no flow) and stagnation in seldom-used outlets are the main risk vectors. ACoP L8 imposes a duty of care on landlords and employers — including HMO landlords — to assess and control Legionella risk.

Summary

Legionella pneumophila is the bacterium responsible for Legionnaires' disease, a severe form of pneumonia transmitted by inhalation of contaminated water aerosols. The bacterium occurs naturally in surface water at low levels but proliferates in warm, stagnant water in building services — particularly hot water cylinders, dead legs, and unused outlets. UK reports approximately 400–600 cases per year, with mortality around 10% in the general population and higher in vulnerable groups.

Legionella control is regulated by the Health and Safety Executive's ACoP L8 (Approved Code of Practice — "Legionnaires' disease: The control of legionella bacteria in water systems") and the supporting HSG274 technical guidance. The duty applies to landlords (Housing Act 2004, Health and Safety at Work etc Act 1974), employers, and any person in control of premises. Domestic homeowners are not legally bound by ACoP L8, but the underlying physics of bacterial growth applies regardless of who lives in the building — a poorly designed hot water system creates risk for everyone.

For tradespeople, Legionella control affects design and installation: storage temperatures, delivery temperatures, pipework sizing, dead-leg avoidance, and thermostatic mixing valve (TMV) selection. For landlords (particularly HMO operators), it affects ongoing management — risk assessments, temperature monitoring, and outlet flushing schedules. The TMV is the technical compromise that allows hot water to be stored at the bacterially-safe ≥60°C but delivered at the scald-safe ≤45°C. See legionella management for the management regime and hot water storage for cylinder design.

Key Facts

Quick Reference Table

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System Element Temperature Requirement Why
Hot cylinder storage ≥60°C Kills Legionella in storage
Hot delivery (within 1 min) ≥50°C Suppresses growth in pipework
Cold storage tank <20°C Below growth threshold
Cold delivery <20°C Avoid warming from hot pipes
Mixed (TMV outlet) 38–46°C Scald protection
Heat pump cylinder ≥60°C weekly anti-Legionella cycle Lower steady state = need periodic kill
Pasteurisation (event) 65°C × 1 hour Recovery from contamination
Shock chlorination 50 ppm × 1 hour Severe contamination response
Hot water cylinder bottom ≥55°C Avoid stagnation zone
Dead leg length ≤5 pipe diameters Avoid stagnant water

Detailed Guidance

Why temperature matters

Legionella has a sharp temperature response. Below 20°C it survives but does not multiply. Between 20°C and 45°C it multiplies, doubling every 1–4 hours in optimal conditions. Above 50°C growth slows; above 60°C the bacterium is killed within minutes.

Temperature response:
    < 20°C  →  Survives but does not multiply
   20–45°C  →  Multiplies rapidly (optimum ~37°C)
   45–50°C  →  Survives but reduced growth
   50–60°C  →  Slow death
      55°C  →  90% killed in 6 hours
      60°C  →  90% killed in 32 minutes
      65°C  →  90% killed in 2 minutes
      70°C  →  90% killed in seconds

The design principle is to keep cold water cold (<20°C) and hot water hot (≥60°C storage, ≥50°C delivery). The danger zone is 20–45°C — and that's where stagnant water in dead legs, tepid mixed water, or under-insulated cold pipes ends up.

Cylinder type and risk

Direct cylinder (electric immersion):

Indirect cylinder (boiler heated):

Heat pump cylinder:

Unvented cylinder:

Thermal store:

Dead legs

A dead leg is any pipe section that doesn't see regular flow. Typical examples:

HSE rule of thumb: any branch longer than 5 pipe diameters from a circulating main is a dead leg. Engineering control:

Thermostatic Mixing Valves (TMVs)

TMVs solve the conflict between Legionella control (need ≥50°C delivery) and scald prevention (vulnerable users harmed at >45°C). The TMV blends hot and cold close to the outlet, delivering 38–46°C tepid water for safe use while keeping the upstream pipework hot.

Requirements:

For HMOs, care homes and accessible bathrooms, TMVs are mandatory under HSG170/HSG274.

Risk assessment

ACoP L8 requires a written risk assessment of hot and cold water systems. The assessment covers:

For HMOs, the assessment is part of the licensing inspection. Re-do every 2 years or after any significant system change.

Monitoring

Routine monitoring confirms control measures are working:

Record results in a Legionella log book.

Response to contamination

If sampled water shows Legionella above action levels:

Domestic vs commercial duty

ACoP L8 imposes duties on:

Single domestic homeowners are not under ACoP L8 duty — but the system still needs to operate safely. The risks are the same. Cylinder set at 50°C "to save energy" is a hazard regardless of ownership.

HMO landlords ARE under ACoP L8 and HMO licensing inspections check Legionella risk assessments.

Frequently Asked Questions

My customer wants the cylinder set lower to save energy — what do I tell them?

50°C is genuinely a risk threshold. Below 50°C the growth zone is approached and storage at 60°C is recommended in all guidance. Lower temperature does save energy but at a real risk to health. The compromise: 60°C storage, TMV at outlets to deliver 40°C tepid water, accept the storage energy cost.

Heat pumps deliver lower temperatures — is that a problem?

Modern heat pump cylinders use a weekly anti-Legionella cycle: the immersion or a boost mode raises cylinder temperature to ≥60°C briefly, killing any Legionella. The steady state can be lower (45–55°C). Check the manufacturer's anti-Legionella programme is enabled and the cylinder reaches the kill temperature during the cycle.

How does Legionella enter the system?

Legionella is present in the environment at low levels in surface waters and soils. It enters domestic systems via the mains supply (usually at counts so low they pose no risk), or via stored water tanks contaminated by birds, rodents or vegetation. Open tanks without lids or screens are a major contamination route. Proper tank lidding and screening (BS EN 806) prevents most contamination.

Should I do annual disinfection?

Routine annual disinfection is not required where control measures (storage temp, delivery temp, flushing) are working. Disinfection is a response to suspected or confirmed contamination, or after major works (cylinder replacement, system installation, holiday let opening for the season).

Is a landlord legally liable if a tenant gets Legionnaires' disease?

Potentially yes. Where the cause can be traced to inadequate risk assessment or control (low cylinder temperature, dead legs, missing TMVs in HMOs), landlords face civil liability and potentially HSE prosecution. Insurance cover varies. Proper risk assessment and documented control is the legal protection.

What about Pseudomonas and other waterborne bacteria?

Pseudomonas aeruginosa and similar bacteria also colonise water systems and are particularly relevant for healthcare settings. Control measures (temperature, flushing, hygiene) are similar but with additional considerations (e.g. tap inserts and aerators harbouring Pseudomonas). For domestic and standard commercial, Legionella control measures generally cover these too.

Regulations & Standards