Hot Water System Safety

Hot water systems in buildings operate at temperatures from 55°C to over 80°C, causing scald burns on contact.
Legionella bacteria grow in water systems between 20°C and 45°C — systems must be stored above 60°C.
Unvented hot water systems operate under pressure and require specific safety devices to prevent explosion.
The Building Regulations Part G and BS 6700 set the standards for hot water system installation in the UK.
Incorrect installation of expansion vessels, temperature relief valves, or discharge pipes creates explosion risk.
Working on live hot water systems exposes installers to scald burns from pressurised hot water release.
G3 qualification is required for any person installing or working on unvented hot water systems.
Legionella risk assessments under the HSE L8 guidance (ACoP) apply to all hot water installations.
Commissioning must verify temperature, flow rates, and safety valve operation before handover.
Thermostatic mixing valves (TMVs) at outlets protect building users from scald injury.

Prevent scalding	Hot water above 55°C causes severe scald burns within seconds — correct TMV installation protects users.
Prevent explosion	Unvented systems under pressure can fail catastrophically if safety devices are incorrectly installed or missing.
Legionella prevention	Incorrect temperature settings allow deadly Legionella bacteria to colonise the system and infect building occupants.
Legal compliance	Building Regulations Part G and the G3 qualification scheme require competent installation of hot water systems.

Do

Don't

Ensure only G3-qualified installers work on unvented hot water systems and cylinders
Install temperature and pressure relief valves in accordance with manufacturer instructions
Set hot water storage temperature to 60°C minimum to prevent Legionella growth
Install TMVs at all outlets serving vulnerable users to limit delivery temperature
Drain down and isolate hot water systems before carrying out maintenance or modifications
Test all safety valves and discharge pipework during commissioning before handover
Carry out a Legionella risk assessment for the hot water system under HSE L8 guidance
Label all hot water pipework clearly to prevent confusion with cold water services
Commission the system fully and record temperatures and flow rates for the O&M manual
Wear gloves and use caution when working near hot pipework and pressurised components

DON'T install or modify unvented hot water systems without a valid G3 qualification
DON'T omit temperature or pressure relief valves from unvented hot water installations
DON'T set storage temperature below 60°C as this allows Legionella bacteria to multiply
DON'T work on pressurised hot water systems without draining down and isolating first
DON'T discharge safety valve outlets into locations where scalding water could injure people
DON'T bypass or remove TMVs from outlets serving care homes, schools, or hospitals
DON'T commission the system without testing every safety device and recording the results
DON'T leave dead legs in pipework where stagnant water allows bacteria to proliferate
DON'T ignore dripping safety valves — they indicate a system fault requiring investigation
DON'T hand over hot water systems without a complete Legionella risk assessment in place

See also: Legionella Awareness | Pipework Installation Safety

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