In a heatwave, your cold water stops being cold

If the forecast shows a run of hot days, add cold-water temperature checks to systems you would normally leave until next month, and read each one against 20°C rather than against the comfortable margin you saw all winter. That is the short answer, and it is worth acting on before the hot spell rather than after. The longer answer is that your whole temperature-control regime rests on an assumption nobody writes on the logsheet: that cold water arrives cold. In a heatwave it doesn't. The control that holds all winter can fail while every procedure you follow stays exactly the same.

Start with the numbers the regime is built on. HSE is blunt about the targets: cold water systems should be maintained, where possible, at a temperature below 20°C, and hot water 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. The cold figure is the one that moves in summer. Cold mains normally reaches the building at around 8 to 12°C, which leaves a healthy gap below 20°C. Legionella multiplies between 20 and 45°C, so cold water at 12°C sits well clear of the band, and everyone relaxes.

The margin you rely on is a winter margin

Take that gap into a heatwave and it closes from both ends. The incoming mains is warmer, because the ground and the sources it draws from have warmed. At the same time the building itself gains heat, so cold water standing in a roof tank, a loft run or a riser picks up warmth from the fabric around it. Put the two together and stored cold water can rise above 20°C within hours, straight into the 20 to 45°C growth band. The system you signed off as compliant in February is out of parameter by early afternoon in July, and nothing about how you monitor it has changed to tell you.

That is the whole problem in a sentence: cold storage that is fine on a mild day is not fine on a hot one, and the difference is the weather, not the plant. A tank holding 14°C in April and 22°C in a July hot spell has not developed a fault. It is doing what an uninsulated or under-circulated store of water does when the air around it is hot.

What controlled stops meaning in July

The trap is treating "controlled" as a permanent property of the system rather than a reading you took on one day in one set of conditions. A cold-water log that reads 14, 15, 14, 13 month after month looks reassuring, and it tells you nothing about what that outlet does across four consecutive days of hot weather, because on those four days nobody went and looked. Year-round control means re-testing when the conditions the control assumed have gone away. Skip that, and a genuinely compliant building spends its highest-risk fortnight of the year completely unobserved.

It is an easy assumption to make, because the paperwork rewards it. A clean winter record builds confidence, and confidence is exactly what stops you booking an out-of-cycle check in August. The record that protects you is the one with a reading taken during the heatwave, not the unbroken run of comfortable numbers either side of it.

What to do when it's hot

Two things, in order.

First, run extra cold-water checks during the hot spell, off your normal monthly rhythm. Cover the sentinel cold outlets and stored cold water, and watch for anything drifting toward or past 20°C. A reading inside the band is a trigger for investigation in exactly the way a failed reading is at any other time of year, and what you record doing about it counts as much as the number itself.

Prioritise by where the heat gets in. Stored cold water is the first thing to look at, because a tank in a warm roof space or a plant room gains heat from every side and holds volume enough to keep it. After that, the outlets furthest from the incoming main, where water has spent longest in warm pipework before anyone draws it. The tap nearest the cold main is the last place a heatwave shows up; the roof tank is the first. If you can only add a handful of checks, start at the top of the building and work down.

Second, and this part has to be settled before the heat arrives: some buildings simply cannot hold cold water below 20°C in summer. A rooftop tank behind glass, a long cold main threaded through a warm plant room. If that describes a building you are responsible for, it should not be a discovery you make mid-heatwave. Your legionella risk assessment should already name it, and name the alternative control that takes over when temperature can't do the job, together with the sampling regime that verifies the alternative is working. That alternative might be a supplementary disinfection regime such as chlorine dioxide dosing. The doses, the design and the sampling schedule belong to your water treatment specialist and your written scheme, not to a blog post. The structural point for the duty holder is the one that matters: in a building like that, temperature is no longer your primary control through summer, and carrying on as if it were leaves you with no effective control for the hottest weeks of the year.

There are two opposite failure modes here, one at each end of the calendar. This is the summer one. Winter has its own version, for entirely different reasons, and it deserves its own piece rather than a footnote.

Monthly monitoring leaks in three predictable ways even in ordinary weather, and a heatwave widens every one of them, because the month you most need an extra visit is the month half the team is on leave. If it helps to see where these hot-spell checks slot into the baseline, we put every HSG274 task and its frequency in a single table.

Where BlueWave fits

When you ask whoever runs your monitoring for extra checks during a hot spell, the thing worth having back is evidence: the cold-water readings taken on the affected systems, dated to the days the weather actually turned, with anything above 20°C flagged rather than merely filed. The BlueWave engineer app captures those readings on site against the specific assets, and marks a reading that falls outside parameter so it stands out in the record instead of blending into a column of numbers. It records and schedules the checks. The water treatment decisions stay with your assessor and your specialist, where they belong.

The test at the end of summer is simple. Pull the cold-water records for the hottest week of the year and see whether anyone took a reading during it. If the log jumps from a comfortable June to a comfortable September with nothing in between, the system was not controlled through the heatwave. It was unobserved, which is not the same thing.