How long should you run a tap to prevent legionella?

Run a tap to prevent legionella until the water reaches temperature: at least 50°C on the hot, which should arrive within one minute of full flow (55°C in healthcare), and below 20°C on the cold, which should happen within two minutes. When the temperature stabilises at the right value, the standing water in the branch has been replaced and the job is done, whether that took thirty seconds or three minutes.

The clock is the wrong instrument here because branches differ. A tap beside the rising main turns over almost immediately; a tap at the end of a long run holds metres of standing water that all has to be drawn off first. Temperature is the only measure that proves fresh water has actually arrived, which is why the HSG274 targets are expressed as temperatures within times rather than times alone.

Prevention is mostly use, plus temperature

It is worth separating two situations. A tap in normal daily use rarely needs deliberate running for legionella at all: the constant draw keeps fresh water moving through the branch, and as long as the system delivers hot at 50°C or more and cold below 20°C, the conditions legionella needs never establish. Prevention for the everyday tap is a matter of the system holding temperature, which is the subject of legionella temperature control.

The tap that needs deliberate running is the little-used one. Anything drawn off less than about once a week goes on the weekly flushing regime described in flushing little-used outlets, running hot and cold through to temperature and recording each flush.

What a slow hot tap is telling you

If hot water consistently takes well over a minute to arrive, the branch holds an unusually large volume of standing water, and that water spends its life sitting warm: cooled below 50°C but above 20°C, squarely in the growth range. Common causes are long pipe runs, uninsulated pipework, oversized pipes fitted for a fixture that no longer exists, and dead legs left when alterations were made. The right response is to investigate and, where possible, shorten, insulate or remove the branch, not simply to accept ever-longer flushing. The temperature chart shows the targets each reading should be judged against.

The cold tap deserves the same attention

Cold is easy to neglect because it feels safe, but cold water that has stood in a warm riser, a roof space or beside hot pipework can sit well above 20°C for days. Running the cold until it drops below 20°C, within about two minutes, confirms the branch has turned over and that the supply itself is arriving cold. A cold tap that will not run below 20°C in a reasonable time points at heat gain somewhere in the distribution, which is a system finding to record and investigate. Log readings in a log book or with the temperature checker, so that what was checked, when, and what it read is evidenced rather than remembered.