Short answer: hot water should leave the cylinder at 60 °C or above, reach 50 °C or above at the tap within one minute, and the cold should be below 20 °C within two minutes. Where hot water circulates, the return to the cylinder should not fall below 50 °C.
Those four numbers are the backbone of the monitoring regime in HSE guidance HSG274 Part 2, and they are what a monthly temperature check is actually checking.
The four thresholds
| What you are measuring | Target | Run the outlet for | Source |
|---|---|---|---|
| Hot water leaving the calorifier (flow) | 60 °C or above | — | Table 2.1 p76, para 2.82 p79 |
| Hot water returning to the calorifier | 50 °C or above | — | Table 2.1 p76, para 2.37 p66 |
| Hot water at a sentinel outlet | 50 °C or above | one minute | Table 2.1 p76, para 2.82 p79 |
| Cold water at a sentinel outlet | below 20 °C | two minutes | Table 2.1 p77, para 2.36 p65 |
The calorifier is the hot water cylinder. The return applies only where hot water circulates around a loop rather than sitting in a dead leg to each tap. Sentinel outlets are the taps chosen to represent the system; our guide on sentinel outlets explains how they are picked.
The wording matters at the boundary
This is where an app earns its keep, because the guidance does not use the same phrasing for each figure and the difference decides a pass or a fail:
- Calorifier flow is “as close to 60 °C as practicable without going below 60 °C”. 60.0 passes.
- Hot outlets and the calorifier return are “a minimum of 50 °C”. 50.0 passes.
- Cold outlets are “below 20 °C”. 20.0 fails.
A thermometer reading 20.0 °C on a cold tap is not a marginal pass. It is a fail, and the difference matters because it is the reading that shows a cold main is picking up heat somewhere. Legionella Logger evaluates the verdict on the exact value entered and never rounds a reading into a pass.
The run time is part of the measurement
“Within one minute” and “within two minutes” are not guidance on patience. They are the test.
A hot tap that eventually reaches 55 °C after four minutes has not met a target expressed as 50 °C within one minute; it has told you the draw-off leg is long, or the circulation is not reaching it. A cold tap that is 22 °C at two minutes and 17 °C at five has told you something is warming that pipe. Both are useful findings, and both are lost if the tap is simply run until the number looks right.
Run the tap, time it, take the reading at the time the guidance names, and record what you actually saw. Legionella Logger has the one and two minute timers built into the check screen for exactly this reason.
What if a reading fails?
A failed reading is not a disaster, it is the system telling you something. What the guidance expects is that you act on it and record what you did. Typical first checks:
- Calorifier flow below 60: the cylinder thermostat setting, or a failed thermostat or element.
- Hot outlet below 50 but the cylinder is fine: a long dead leg to that tap, a failed or bypassed circulation pump, or a mixed supply at the outlet.
- Cold above 20: cold pipework running near hot pipes or through a warm space, an uninsulated run, or a cold water tank sitting in a hot loft.
Record the reading, the action taken and the recheck. That trail is what an assessor or an inspector looks for, more than a page of straight passes.
Two important caveats
Scalding. Water at 50 °C and above at a tap can scald, and the risk is highest where children, older people or people with reduced mobility use the outlet. Balancing the scalding risk against the legionella control is exactly the sort of decision the property’s risk assessment exists to make, and the usual engineering answer is a thermostatic mixing valve fitted close to the outlet. Do not simply turn the cylinder down to make the tap safer; that trades one risk for another.
Frequency is set by your risk assessment, not by a table. HSG274 describes Table 2.1 as an indication of frequency, and says the risk assessment for the property determines what is actually done and how often (paras 2.79 p75 and 2.141 p90). Monthly is the common answer for these checks, but it is a starting point, not a rule handed down for every building.
Healthcare is different. Healthcare premises use 55 °C at outlets and on the return, with HTM 04-01 as the fuller reference. That is a separate regime and is not what this article, or Legionella Logger v1, covers.
Scope and sources
Figures here are from HSG274 Part 2, second edition 2024, published by the Health and Safety Executive, read together with the Approved Code of Practice L8. Page numbers are the printed ones in that edition. The documents are free to download: HSG274 on hse.gov.uk.
Thresholds checked against the archived HSE PDF of HSG274 Part 2 on 10 September 2026. Contains public sector information licensed under the Open Government Licence v3.0. This is general information, not a legionella risk assessment, and Plan CD is not affiliated with the Health and Safety Executive.