Water hygiene logbook: why the run time matters more than the temperature
HSE HSG274 Part 2 makes the Legionella temperature checks time-bound: cold below 20C after running for two minutes, hot at least 50C within one minute. A reading with no run time evidences nothing. Plus flushing without release of aerosols, and why the record is per outlet.
By Hovermarks team
Quick answer. HSE guidance HSG274 Part 2 sets two monthly sentinel-outlet temperature checks, and both are time-bound: cold water should be below 20C after running the outlet for two minutes, hot water at least 50C within one minute of running. The run time is what makes the reading meaningful, because any tap eventually reaches its supply temperature. Infrequently used outlets are flushed weekly, and HSE is explicit that flushing should happen without release of aerosols, since Legionella is contracted by inhalation. The record is kept per outlet, which is what makes it a logbook rather than a survey.
Most water hygiene content says "record the temperature". That instruction, on its own, produces records that evidence nothing at all, and the reason is worth understanding rather than just working around.
Why a temperature with no run time is not evidence
Run any cold tap long enough and it will eventually reach the temperature of the water feeding it. Run any hot tap long enough and it will reach the cylinder temperature. So a bare reading of "18C" or "52C" tells you what the system can eventually deliver, not whether it is delivering it in the way that controls risk.
HSG274 Part 2 therefore makes both checks time-bound:
- Cold: below 20C after two minutes of running. If the cold water is still above 20C at two minutes, the cold system is being warmed somewhere, by proximity to heating, poor insulation, or a long dead leg.
- Hot: at least 50C within one minute of running. If it takes three minutes to get there, the hot water is sitting cool in the pipework between the cylinder and the outlet.
The number that matters in both cases is the one at the stated time, not the one you eventually get to.
Why 20C, and why 50C
Legionella multiplies between roughly 20C and 45C. That is the whole reason for the thresholds. 20C is not a comfort figure or an arbitrary standard, it is the bottom of the growth range, and cold water held below it is water in which the organism does not proliferate. Hot water at 50C or above at the outlet reflects storage and distribution hot enough to control growth through the system.
Understanding that changes how a failed reading reads. A cold outlet at 23C is not "slightly warm", it is an outlet inside the growth range, and the useful question is what is warming it.
Flushing without release of aerosols
The weekly duty on infrequently used outlets is to flush them, and HSE is specific that this should be done without release of aerosols.
This is the part almost nobody writes about, and it inverts the intuition. If the system may contain Legionella, the flush is the moment of highest exposure risk, because Legionellosis is contracted by inhaling contaminated aerosol, not by drinking water. A shower head run at full pressure into an unventilated bathroom to "clear it through" is an exposure event dressed as a control measure.
Practically: remove or bag the shower head where appropriate, run at low pressure, keep the space ventilated, and avoid spray. A flushing record that does not distinguish between a careful flush and a careless one is recording the wrong thing.
Why the record is per outlet
The asset here is the individual outlet, not the building and not the system. A logbook that records "flushed all outlets in Block B" cannot answer the question that gets asked after an incident, which is always about a specific tap or shower: when was this one last flushed, what did this one read last month, and by whom.
That is also the difference between the logbook and the risk assessment. The Legionella risk assessment style survey looks at the system and its hazards periodically. The logbook is the operational record that the controls the assessment specified are actually happening, outlet by outlet, week by week.
Per-outlet records mean each sentinel outlet carries its own monthly temperature history, each infrequently used outlet carries its own weekly flushing history, and a new outlet added to the estate arrives with an empty history rather than disappearing into a building-level tick.
What a defensible water hygiene record holds
- The outlet, identified individually and locatable
- Whether it is a sentinel outlet, an infrequently used outlet, or both
- For temperature checks: the reading, the run time it was taken at, and which check it was
- For flushing: the date, the duration, and confirmation it was done without aerosol release
- The person who did it
- What happened where a reading was out of range, and what was done about it
Where Hovermarks fits
Hovermarks runs the water hygiene logbook as QR-tagged assets at outlet level: each sentinel outlet on its monthly temperature cycle and each infrequently used outlet on its weekly flushing cycle, with the run time captured as a field alongside the reading rather than left implied. Scanning the tag at the outlet opens that outlet's own history.
What it does not do: it produces the record, it does not discharge the duty. Whether the control scheme is right, whether an outlet is correctly classified as sentinel, and what to do about a reading out of range are judgements for the responsible person and whoever holds the water safety role. The facilities management solution shows where the logbook sits alongside the rest of a statutory schedule.