HSG274 is the Health and Safety Executive's technical guidance on controlling legionella bacteria in water systems. It sits alongside the Approved Code of Practice L8 (fourth edition, 2013) and comes in three parts. Part 2 covers hot and cold water systems, which is where most buildings' temperature monitoring comes from. The current HSG274 is the second edition, published in 2024; HSE notes that only Part 1 was revised in it.
This guide sets out what Part 2 asks you to measure, where, and how often, using HSE's own text. It then covers where continuous sensors make those checks easier, and what they cannot do for you.
The temperatures that control legionella
Temperature control is the traditional way to keep legionella in check. Paragraph 2.6 sets the basic rule: keep cold water below 20 °C where possible, store hot water at 60 °C or above, and distribute it so it reaches at least 50 °C at the outlets within one minute (55 °C in healthcare premises).
| Point in the system | HSG274 Part 2 target |
|---|---|
| Calorifier flow | At least 60 °C (thermostat as close to 60 °C as practicable, not below it) |
| Calorifier return | Not below 50 °C |
| Hot water at outlets | At least 50 °C within one minute of running (55 °C in healthcare premises) |
| Cold water at outlets | Below 20 °C within two minutes of running |
| Point-of-use heaters up to 15 litres | 50 °C to 60 °C (55 °C in healthcare premises) |
Sentinel outlets: where the readings come from
You do not measure every tap every month. HSG274 asks for readings at sentinel points chosen to represent the condition of the whole system (paragraphs 2.59 and 2.62). In a simple system these are the outlet nearest to the tank or calorifier (the near sentinel) and the furthest (the far sentinel). Branched systems add the outlets at the ends of significant spurs. HSE stresses that the layout of the pipework decides which outlets are sentinels, not where the outlets happen to be in the building.
In circulating hot water systems the far sentinels are the return legs towards the end of each loop, checked monthly (paragraph 2.63). Smaller subordinate loops should be tested quarterly, at a return leg or a representative outlet (paragraph 2.64). Everything not covered by a sentinel is picked up by temperature profiling: a rotating selection of other outlets, so the whole system is checked over a defined period (paragraphs 2.65 and 2.66).
What to check and how often
Table 2.1 of HSG274 Part 2 gives a checklist with an indication of frequency. These are the temperature checks in it:
| Check | What HSG274 asks for | Indicated frequency |
|---|---|---|
| Calorifier flow and return | Flow at least 60 °C; return not below 50 °C | Monthly |
| Hot sentinels, non-circulating systems | Nearest, furthest and long-branch outlets at 50 °C or more within one minute (55 °C in healthcare) | Monthly |
| Return legs of principal hot loops | At least 50 °C (55 °C in healthcare); can be measured on the surface of metal pipe | Monthly |
| Return legs of subordinate hot loops | As above; if the pipe cannot be measured, the last outlet on the loop should exceed 50 °C within one minute | Quarterly, ideally on a rolling monthly rota |
| Cold sentinels | Below 20 °C within two minutes of running | Monthly |
| Other hot and cold outlets | The same targets, rotated to build a temperature profile of the whole system | Rotating selection |
| Cold water storage tank | Stored water temperature away from the ball valve, and the incoming mains temperature | Annually in summer, or as profiling indicates |
| Infrequently used outlets | Flush until the temperature stabilises; covers anything unused for seven days or more | Weekly, or as the risk assessment says |
The same table lists tasks that are not temperature readings: inspecting and cleaning calorifiers and cold water tanks annually, descaling showerheads and hoses quarterly, maintaining thermostatic mixing valves (TMVs), and checking insulation. Where people are at higher risk, such as in healthcare and care homes, HSE notes that more frequent flushing may be needed.
What your records need to show
- Every inspection and measurement should record who took it, verified by a signature or other means such as electronic verification, the date, and enough detail of the location to repeat it (paragraph 2.54).
- Records can be written or electronic (paragraph 26 of the introduction).
- Monitoring and inspection records should be kept for at least five years (paragraph 25).
- Once a flushing regime for little-used outlets starts, it has to be kept up and logged, because lapses can cause a sharp rise in legionella at the outlet (paragraph 2.78).
Where continuous sensors help
HSG274 already allows for automated monitoring. Paragraph 2.53 notes that "building management systems are increasingly used to provide an automated monitoring programme", which allows early detection when the control regime fails. Sensors on calorifiers, return legs and sentinel outlets add four things to manual rounds:
- Readings between rounds. A monthly check records one moment. A sensor on a calorifier or return leg records all month, so a flow temperature that dips below 60 °C overnight shows up.
- Evidence that outlets were used. Temperature or flow changes at a little-used outlet show whether its weekly flush happened, which supports the log HSG274 asks you to keep.
- Alerts. A reading outside its target band can alert the responsible person the same day, rather than at the next round.
- Records that are already in order. Each reading is stored with its time and location. Check that your platform's data retention, or regular exports, cover the five years HSE asks for.
What sensors do not replace
- The risk assessment, the written control scheme and the competent responsible person who manages them.
- Flushing itself. A sensor can show that an outlet has not been used; the water still has to be run.
- The inspection, cleaning and maintenance tasks in Table 2.1, from calorifier inspection to TMV servicing.
- Judgement on method. Whether sensor readings can stand in for any manual check, where sensors go and how they are calibrated are decisions for your risk assessor.
How SIMBIOSYS fits
SIMBIOSYS brings temperature and flow readings from sensors on outlets, return legs and calorifiers into one dashboard, alerts when a reading leaves its target band, and keeps the history for audits. See how it works on the Legionella monitoring page. If you are planning a sensor rollout, the Sensor Integration Readiness Checklist in our Resource Library covers protocols, power and gateways.
Source: Health and Safety Executive, HSG274 Legionnaires' disease: Technical guidance, Part 2, second edition (2024): introduction paragraphs 24 to 26, paragraphs 2.6, 2.53 to 2.66, 2.78 and 2.79, and Table 2.1.