Legionella Temperature Monitoring Log: UK Compliance Guide (2026)
How to keep a legionella temperature monitoring log — sentinel outlet checks, the ≥50°C hot / ≤20°C cold targets, monitoring frequency, and what HSE expects to see.
Quick answer: A legionella temperature monitoring log records water temperatures at sentinel (representative) outlets to confirm the system stays in the safe range: hot water ≥50°C at the tap (≥55°C healthcare), hot storage ≥60°C, and cold water ≤20°C. Monitoring is typically monthly at sentinel outlets, set by the risk assessment. The log records date, outlet, temperature, who checked, and any corrective action. It's the ongoing evidence that backs up your legionella risk assessment.
Why temperature is the key control
Legionella thrives between 20-45°C. The single most effective control is keeping water out of that range — hot water hot, cold water cold. The monitoring log is the evidence that you're achieving this consistently.
The target temperatures
| Point | Target | Within |
|---|---|---|
| Hot water at tap (sentinel) | ≥50°C (≥55°C healthcare) | 1 minute |
| Hot water storage (calorifier) | ≥60°C | — |
| Cold water at tap (sentinel) | ≤20°C | 2 minutes |
| Cold water storage tank | ≤20°C | — |
Sentinel outlets
You don't check every tap. You check sentinel outlets — the nearest and furthest outlets from each water heater or tank. These are representative: if the nearest and furthest are within range, the system between them generally is too.
What to record in the log
- Date and time of check
- Outlet location/identifier
- Hot and/or cold temperature reading
- Whether within target range
- Name/initials of person taking the reading
- Any corrective action where out of range
Monitoring frequency
- Sentinel outlets (hot and cold) — monthly typically
- Stored hot water — monthly
- Incoming cold water — at least every 6 months (more in summer)
- Little-used outlets — flush weekly; temperature check periodically
- Thermostatic mixing valves (TMVs) — checked and serviced periodically
Exact frequency is set by your legionella risk assessment.
What to do when temperatures are out of range
- Hot too low — check the heater thermostat; raise to ≥60°C storage; investigate why distribution dropped below 50°C
- Cold too high — check for warm pipework runs, proximity to heat sources, tank insulation
- Record the issue and the corrective action in the log
- Re-check after remediation
Equipment
- A calibrated digital thermometer (surface probe or immersion)
- Calibration checked periodically for accuracy
- Consistent method (run the tap, time it, record at the specified interval)
Retention
Keep monitoring logs for at least 5 years for higher-risk systems. For domestic rentals, retain for the duration of the tenancy plus a reasonable period.
FAQs
Do I need to monitor every tap?
No — sentinel outlets (nearest and furthest from each heater/tank) are representative. Little-used outlets are flushed and checked periodically.
What thermometer should I use?
A calibrated digital thermometer. Surface-probe types are common for pipework; running-water readings for taps.
Is monthly monitoring always required?
Monthly is typical for sentinel outlets but the risk assessment sets the exact frequency. Lower-risk simple domestic systems may need less.
What if I'm a landlord with a combi boiler?
Combi systems (no storage) are lower-risk. The risk assessment may require only basic checks and flushing after void periods rather than ongoing monthly logs.
Related guides
- Legionella risk assessment: landlord & business guide
- Legionella temperature monitoring log
- Water hygiene records: L8 ACoP compliance
Last reviewed 2026-06-08 by Jamie Dawson, Editor.
