Legionella Temperature Monitoring: Thresholds and Frequency

Temperature is the single most important control measure for legionella in a normal hot and cold water system. Get the temperatures right and recorded, and you have done most of what ACoP L8 asks of a duty holder. The thresholds are fixed: store hot water at 60°C, deliver it at 50°C, and keep cold water below 20°C. What trips people up is not the numbers — it is knowing how often to check each part of the system, and what to actually write down.

This guide covers the thresholds, the monitoring frequency for each component, and how to keep a record that an HSE inspector would accept.

The temperature thresholds, in one place

Legionella bacteria multiply in water between 20°C and 45°C. HSE guidance is direct on this: legionella is "dormant below 20°C and do not survive above 60°C". The whole point of temperature control is to keep your water out of that 20–45°C window.

Part of the system	Threshold	Why
Hot water storage (calorifier/cylinder)	Stored at 60°C or above	Above 60°C legionella does not survive
Hot water at outlets	Reaches 50°C within 1 minute (55°C in healthcare premises)	Hot enough at the tap to keep the distribution leg safe
Cold water at outlets	Below 20°C within 2 minutes of running	Below 20°C legionella stays dormant

The exact HSE wording is that "hot water should be stored at least at 60°C and distributed so that it reaches a temperature of 50°C (55°C in healthcare premises) within 1 minute at the outlets," and that "cold water systems should be maintained, where possible, at a temperature below 20°C." The healthcare figure (55°C) is higher because the people in those buildings are more vulnerable to infection.

You can check any individual reading against these thresholds with our free Temperature Compliance Checker — enter a reading and it tells you instantly whether it passes.

How often to check each part

This is where most logs go wrong. There is not one single frequency — different components are checked on different schedules. The frequencies below come from HSG274 Part 2, the HSE's technical guidance for hot and cold water systems.

Check	Frequency
Hot water at sentinel outlets	Monthly
Hot water storage (calorifier flow temperature)	Monthly
Cold water at sentinel outlets	Monthly
Cold water storage tank temperature	At least every 6 months (and tank inspection annually)
Cold water storage tank inspection	Annually

"Sentinel" outlets are the taps nearest to and furthest from the hot water source (and from the cold water tank). You do not check every tap in the building every month — you check the sentinels, which represent the temperature extremes of the system. On a circulating hot water system, HSG274 Part 2 asks for representative outlets across the rest of the system to be checked on a rotating basis so that, over time, the whole system gets covered.

A practical monthly routine for a typical small building: check the calorifier storage temperature, then check the hot and cold sentinel taps. That is three to four readings a month for a simple property, more for a larger building with several loops.

What "within one minute" and "within two minutes" mean

The timing matters as much as the temperature. Hot water should reach 50°C within one minute of running the tap. Cold water should be below 20°C within two minutes of running. A tap that eventually reaches the right temperature after five minutes is not compliant — slow temperature recovery is itself a warning sign (a dead leg, a failing TMV, or heat gain on a cold line).

When you log a temperature, note the time it took to stabilise, not just the final reading. A reading of "52°C" tells an inspector nothing on its own. "52°C, reached within 40 seconds" tells them the outlet is working as it should.

What to record

A temperature monitoring record needs enough detail that someone else could repeat the check and that an inspector can see the system was under control on the day. For each reading, log:

• Date of the check
• Outlet or component (e.g. "bathroom basin — far sentinel", "calorifier flow")
• Temperature reading
• Time to reach temperature (hot: target within 1 min; cold: within 2 min)
• Who took the reading
• Action taken if the reading was out of range

That last column is the one people skip and the one that matters most. A failed reading is not a compliance problem in itself — failing to act on it is. If a sentinel tap reads 44°C, the record should show what you did: re-checked, adjusted the thermostat, flushed the line, or escalated to a plumber.

What happens when a reading fails

An out-of-range reading is information, not a disaster. The response depends on which way it failed:

• Hot water too cool (below 50°C at the outlet): check the calorifier is storing at 60°C, look for a TMV blending in too much cold, or a long dead leg cooling the water before it reaches the tap.
• Cold water too warm (above 20°C): look for the cold line running alongside a hot pipe, sunlagged pipework in a warm void, or low usage letting the water sit and warm up.
• Storage below 60°C: check the thermostat setting and the heating element or coil — this is the highest-priority failure because storage temperature protects the whole system.

Record the failure, the cause you identified, and the corrective action. Then re-check to confirm the fix worked, and log that too.

Temperature monitoring is not the same as legionella testing

A common confusion: monthly temperature monitoring is part of routine control, but it is not "legionella testing" in the laboratory sense. HSE is clear that microbiological sampling — sending water away to a lab to be tested for legionella bacteria — "is not usually required for domestic hot and cold water systems, and is only needed in very specific circumstances". Temperature monitoring is the routine job. Lab sampling is the exception. For the full picture on when sampling is and is not needed, see our guide to UK legionella testing requirements.

Practical takeaway

1. Thresholds: store hot at 60°C, deliver hot at 50°C within 1 minute, keep cold below 20°C within 2 minutes.
2. Frequency: sentinel outlets and calorifier monthly; cold tank temperature 6-monthly; cold tank inspection annually.
3. Record: date, outlet, reading, time to stabilise, who checked, and action taken on any failure.
4. Act on failures: log the cause and the fix, then re-check and record the re-check.

Consistent monthly temperature records are the backbone of a defensible compliance file. They are cheap to produce and they are the first thing an inspector asks for.

This guidance covers legionella temperature control under ACoP L8 and HSG274 for England, Wales, and Scotland. This is general compliance guidance, not legal or professional advice — for site-specific assessments, consult a competent person as defined by ACoP L8.

Sources

• HSE — Hot and cold water systems
• HSE — Hot and cold water systems: things to consider
• HSE — HSG274: Legionnaires' disease technical guidance (Part 2)
• HSE — Legionella and landlords' responsibilities