GuidanceWater hygiene, September 2026
School water hygiene: when cold water will not stay below 20 degrees
Six weeks of warm, still water is the one thing legionella needs. Reopening well takes an afternoon and a written record.
Legionella multiplies between roughly 20 and 45 degrees in water that is sitting still. A school over the summer offers both.
The number that counts
Cold water should be below 20 degrees at the outlet within two minutes, and below 20 in storage. Hot water leaves the calorifier at 60 and should reach 50 at the tap within a minute.
When cold water will not stay cold
Cold water is now arriving in buildings above 20 degrees, through roof tanks and pipework in warm ceiling voids. HSG274 accepts the incoming main can reach around 23 degrees in a very hot summer, so the useful measurement is the gap between the main and the water we draw. Main at 22 and tank at 23 is the supply. Main at 15 and tank at 23 is heat picked up inside the building, and that we can fix.
Either way, 20 degrees stays the action point. Anything above it is recorded, investigated, and the action written alongside it. What changes is the remedy: insulation, tank lids and loft ventilation, keeping the water moving, removing dead legs, and separating cold pipework from heat sources.
The outlets that get missed
Science prep taps, the accessible toilet used twice a term, the caretaker's shower, outside taps, bottle filling stations, and anything out of service but never disconnected.
The reopening flush
Every outlet run for long enough to draw fresh water through, working outwards from the incoming main. Showers and spray taps need the head removed or submerged first, because the risk is the aerosol.
Reopening checks
- System flushed before occupation, outwards from the incoming main.
- Shower heads and spray taps descaled and disinfected.
- Tank, sentinel and incoming mains temperatures taken and compared.
- Monitoring log brought up to date and signed.
Above 20 degrees: what we do
- Above 20
- Recorded with the mains reading beside it, flushing stepped up, and everything flushed again before reoccupation
- More than 2 above the mains
- Heat gain in the building. We find the cause, programme the work, monitor fortnightly
- Above 25, sustained
- Control failure. Weekly monitoring, assessment reviewed now, tank turnover examined, sampling considered
How we help
We check the monitoring log rather than the certificate, and where cold water cannot be held below 20 degrees we write the compensating controls into the water hygiene risk assessment.
General guidance only. It does not replace the fire risk assessment, water hygiene risk assessment or specific advice for a particular building.
