Legionella Risk Assessment & Control: Temperature Management & Landlord Duties

Summary

Legionnaires' disease is a serious respiratory illness caused by Legionella pneumophila bacteria. It is contracted by inhaling fine water droplets (aerosols) containing the bacteria — showers, taps, and cooling towers are common sources. It is not spread person-to-person. Approximately 400–600 cases are reported in England and Wales each year, with a fatality rate of around 10%.

For plumbers, gas engineers, and heating engineers, Legionella management intersects with everyday work: hot water cylinder commissioning, thermostatic mixing valve (TMV) installation and maintenance, showerhead servicing, and cold water storage tank assessment. Understanding the temperature requirements and system design implications is now a core competency, particularly for those working in HMOs, care homes, and residential blocks.

For landlords and property managers, Legionella is a legal compliance issue. A risk assessment is mandatory. Simple domestic systems (where the water is frequently used and hot and cold supplies are direct from the mains) have low risk, but any system with storage — hot water cylinders, cold water tanks — requires formal assessment.

Key Facts

• Legionella growth range — 20°C to 45°C; optimal growth at 35–46°C; killed within 2 minutes at 60°C, instantly above 70°C
• Hot storage temperature — minimum 60°C throughout the cylinder (not just at top thermostat sensor)
• Delivery temperature — ≥50°C at outlets within 1 minute (except where TMV is fitted)
• Cold water storage temperature — must not exceed 20°C in storage tanks
• TMV compliance — TMVs blending hot and cold must be set to a safe delivery temperature (max 43°C in care/NHS settings; 41°C for domestic showers); must be maintained and tested annually
• Dead legs — long sections of pipe serving rarely-used outlets are high risk; eliminate wherever possible
• Risk categories — low (simple dwellings with direct mains supply, no storage, regular use), medium (storage cylinders, complex systems), high (healthcare, care homes, HMOs with complex systems)
• L8 Code of Practice — HSE Approved Code of Practice for controlling Legionella in water systems; landlords must comply
• ACOP L8 — Legionnaires' Disease: The Control of Legionella Bacteria in Water Systems (HSE)
• HSG274 — Technical guidance covering domestic hot and cold water systems (Part 2)
• Responsible person — landlord or their managing agent must appoint a responsible person for Legionella management
• Record keeping — risk assessments, maintenance records, temperature logs must be kept and produced on request
• Duty of care (residential) — not as onerous as commercial; simple domestic risk assessment is acceptable for straightforward single-household dwellings

Quick Reference Table

Detailed Guidance

Domestic Risk Assessment — What Is Required

For landlords of private rental properties, the Health and Safety Executive and the Residential Landlords Association guidance is clear: a written Legionella risk assessment is required. For simple properties (mains-connected hot and cold, regular occupancy, no storage tanks), a one-page risk assessment identifying low risk is acceptable.

High-risk features requiring more detailed assessment:

• Cold water storage tanks (cisterns) in the loft
• Hot water cylinders (any type)
• Underused outlets or dead legs (guest bathrooms, second kitchens)
• Calorifiers or large commercial-style cylinders
• Multiple tenants or HMO status
• Any cooling tower or evaporative cooling equipment (extremely high risk — separate regime)
• Hot tubs, spa pools, ornamental fountains

For a simple modern property with a combi boiler (no storage), mains cold supply to all outlets, and regular occupancy, the risk is inherently low. A basic written assessment noting this is sufficient.

Hot Water Cylinder Temperature Control

Where a vented or unvented hot water cylinder is present:

Storage temperature: The thermostat must maintain storage at 60°C or above throughout the cylinder. Many domestic cylinders use a thermostat in the side of the cylinder at mid-height — if this reads 60°C, the top of the cylinder may not be up to temperature due to stratification. Set thermostat to achieve 60°C at the flow (top) connection.

Weekly pasteurisation (where applicable): In some commercial and HMO settings, a weekly heat-up cycle to 60°C throughout the system followed by a flush of all outlets is specified. In simple domestic settings, maintaining continuous hot storage at 60°C is sufficient.

Cylinder lagging: Adequate insulation maintains cylinder temperature without excessive energy use, but does not affect Legionella management directly.

Thermostatic Mixing Valves (TMVs) and Legionella

TMVs create a paradox: they blend hot water down to a safe delivery temperature, reducing scalding risk. But if the TMV malfunctions and cold water fails, the full hot supply at 60°C would reach the outlet — that's scalding risk. And if the TMV is set too low or leaks cold water past the hot seat, delivery temperature drops into the danger zone.

Annual TMV servicing must include:

1. Check delivery temperature at each outlet with a calibrated thermometer
2. Verify the thermal shut-off (fail-safe) operates — simulate cold water failure by closing the cold inlet; TMV should shut off flow
3. Dismantle and descale the cartridge if in a hard water area (limescale restricts the thermostatic element)
4. Replace cartridge if delivery temperature cannot be maintained
5. Record the test date, delivery temperature measured, and any corrective action taken

This testing is required under BS EN 1287 (TMV2) and BS EN 1111 (TMV3) and must be documented.

Cold Water Storage Tanks

Cold water storage tanks (typically 227L or 450L polyethylene tanks in the loft) are a Legionella risk if:

• Water sits stagnant (outlet demand low relative to tank volume)
• Tank insulation allows loft heat to raise water above 20°C in summer
• Tank is uncovered or the cover is ill-fitting, allowing contamination
• Inlet float valve is malfunctioning, causing stagnant water

Management measures:

• Keep the tank covered and sealed against dust and light
• Insulate the tank (but not the underside — cold from the ceiling is beneficial in winter)
• Ensure turnover: if demand is low, fit an outlet valve and flush through regularly
• Inspect annually: check cover, ball valve operation, any sediment, visible contamination

Where cold water tanks are not required (e.g., combi boiler provides all hot water, cold water supplies are mains-direct), consider removing the tank and capping the connection — this eliminates the risk entirely.

Dead Legs and Infrequently Used Outlets

A dead leg is a section of pipe that goes to an outlet which is rarely used. The water in that pipe cools (or heats) to ambient temperature and stagnates. This is ideal for Legionella growth.

In rental properties: flush all outlets weekly if the property is vacant. If a bathroom is consistently unused (e.g., a second bathroom not used by the tenant), either decommission the outlet or implement a weekly flushing protocol.

When extending or altering plumbing, avoid creating dead legs. If a tee is required for a future connection, cap it at the tee and fit it as short as possible.

Frequently Asked Questions

Does a combi boiler property need a Legionella risk assessment?

Yes — a written assessment is still required for rental properties. However, for a property with a combi boiler (no storage), mains-cold-supply throughout, single household, and regular use, the assessment will almost certainly conclude "low risk, no further action." This can be a simple document completed by the landlord themselves or their managing agent. It should be reviewed when the system changes or on a periodic basis (every 2 years is common guidance).

Can I just run the hot tap hot to kill Legionella?

Partially effective as an occasional measure, but not as a substitute for system management. Running hot water at the tap raises the temperature at the fixture but doesn't address the cylinder temperature, cold storage tanks, or long pipe runs. If the system is properly maintained at 60°C storage and 50°C delivery, Legionella risk in domestic systems is very low.

Who can carry out a Legionella risk assessment?

For simple domestic properties, the landlord can complete their own assessment using HSE guidance templates. For complex systems (HMOs, care homes, residential blocks, systems with cooling towers), a competent person with Legionella awareness training (e.g., Legionella Control Association member) should carry out the assessment.

At what temperature should I set a cylinder thermostat?

Set to achieve 60°C at the flow connection (top of cylinder) — this typically means setting the thermostat to 60–65°C. Do not set above 65°C without ensuring the TMV is properly calibrated — higher cylinder temperatures increase scalding risk at outlets without TMVs.

Regulations & Standards

• Health and Safety at Work Act 1974 — overarching duty of care including Legionella management

• Control of Substances Hazardous to Health Regulations 2002 (COSHH) — Legionella is a biological hazard under COSHH

• HSE Approved Code of Practice L8 — Legionnaires' Disease: Control of Legionella Bacteria in Water Systems

• HSG274 Part 2 — Technical guidance for hot and cold water systems

• BS 8558:2015 — Design, installation, testing, and maintenance of services supplying water for domestic use

• BS EN 1287 — Thermostatic mixing valves for sanitary purposes (TMV2)

• BS EN 1111 — Thermostatic mixing valves for healthcare applications (TMV3)

• HSE ACOP L8 — Free download

• HSG274 Part 2 — Domestic water systems

• Legionella Control Association — UK industry body, member directory

• WRAS Legionella Guidance — Water regulations context

• thermostatic mixing valves — TMV selection, installation, and testing

• hot water systems — Hot water system types and cylinder selection

• cold water storage — Cold water storage tank sizing and management

• unvented cylinders — G3 cylinders and safety devices