What HSG274 is
HSG274 is the HSE's technical guidance on controlling Legionella bacteria in water systems. It's the practical companion to ACOP L8: where L8 sets out the legal framework and the 'what', HSG274 provides the detailed 'how' — the engineering measures, monitoring regimes and action levels.
It's published in three parts, each covering a different type of risk system. Together they cover the great majority of water systems a UK duty holder is likely to operate.
How HSG274 relates to ACOP L8
ACOP L8 is an Approved Code of Practice with special legal status — follow it, or have a documented equivalent, or risk being found in breach of the underlying Health and Safety at Work Act and COSHH. HSG274 is ordinary guidance that explains how to meet the L8 standard in practice.
In an audit or investigation, an HSE inspector will expect to see that your control measures align with HSG274. You don't have to follow it to the letter if you can justify an equivalent approach — but in practice, HSG274 is the path of least resistance to a defensible position.
Part 1 — Evaporative cooling systems
HSG274 Part 1 covers cooling towers and evaporative condensers — the highest-risk Legionella systems because they generate large volumes of aerosol that can travel significant distances.
It sets out the control regime: a documented treatment programme, weekly checks, monthly Legionella and microbiological sampling, and regular cleaning and disinfection. It also reminds duty holders that cooling towers must be notified to the local authority under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992.
Because of the aerosol risk, Part 1 systems carry the most rigorous monitoring expectations of the three parts.
Part 2 — Hot and cold water systems
Part 2 is the one most buildings deal with: domestic-style hot and cold water systems found in offices, schools, care homes, hotels and the like. It's the most widely-relevant part of HSG274.
The core controls are temperature and movement. Hot water should be stored at 60°C and distributed so it reaches at least 50°C (55°C in healthcare) at the outlet within a minute. Cold water should stay below 20°C. Outlets should reach temperature quickly, and infrequently-used outlets must be flushed regularly to prevent stagnation.
Part 2 specifies the monitoring regime around these controls: monthly temperature checks at sentinel outlets, periodic checks of stored water, calorifier inspections, showerhead cleaning and descaling, and tank inspections — all recorded in the Legionella logbook.
Part 3 — Other risk systems
Part 3 covers the systems that don't fit neatly into Parts 1 or 2: spa pools and hot tubs, humidifiers and air washers, emergency showers and eyewash stations, vehicle washes, fountains and water features, fire systems, and industrial process water among others.
These systems vary enormously, so Part 3 is organised system-by-system, giving the specific risks and control measures for each. If your site has anything beyond ordinary hot and cold water and cooling — a spa, a decorative fountain, a humidified archive — Part 3 is where you look.
Action levels and monitoring
For hot and cold water systems, Legionella sample results drive a clear response: up to 100 cfu/L is acceptable in a managed system; 100–1,000 cfu/L requires investigation and likely resampling; over 1,000 cfu/L requires immediate action including disinfection and a review of controls.
But sampling is a check on the system, not the primary control — temperature and flow are. A system kept at the right temperatures, with no dead-legs and a working flushing regime, should rarely produce a high result. When it does, HSG274 is the reference for what to do next.
Putting it together
For most duty holders, HSG274 compliance comes down to a competent risk assessment (BS8580-1), a site-specific written scheme of control built around Part 2's temperature and flow regime, disciplined monthly monitoring, and the right response when a result or an inspection flags a problem.
Cooling towers add Part 1's heavier regime; specialist systems add the relevant Part 3 controls. Get those foundations right and recorded, and you have a programme that satisfies both HSG274 and the ACOP L8 duty sitting above it.