Legionella flushing: how long should I run the taps?

Updated 12 September 2026

Flushing is the simplest and most widely used control for outlets that sit idle. The aim is to draw fresh water from the mains through the pipework, remove any biofilm that has started to form in the tap and shower head, and discharge the aerosols safely down the drain rather than into someone's breathing zone.

How long to run each tap

There is no single number that works for every property. The right duration depends on the length and diameter of the pipe run between the main and the outlet. The rule of thumb from HSG274 Part 2 is to run the outlet until the temperature stabilises (cold water below 20 degrees Celsius, hot water above 50 degrees Celsius) and then for a further minute at full flow. For most domestic and small commercial outlets this works out at between two and five minutes.

Which outlets to flush

Any outlet that is not used at least weekly should be on a scheduled flushing programme. In practice this catches:

• Disabled toilets and washrooms with low footfall
• Meeting room suites that are not in daily use
• Showers in bike-commuter facilities and gym changing rooms with limited occupancy
• Kitchen taps in breakout areas that sit idle out of hours
• Guest bathrooms during low hotel occupancy
• Outdoor taps during the winter
• Any system after a shutdown of more than a week

How to do it safely

The key safety concern with flushing is aerosol generation. Running a shower that has sat idle for weeks can produce a very high concentration of aerosolised water in the first minute. The safe approach is to:

• Remove the shower head or attachment before running the water
• Direct the first minute of flow down the plug hole rather than through the rose
• Allow the water to run fully before reattaching the head
• Where practical, open a window or ventilate the room first

Anyone carrying out this work should be aware of the risk and trained to follow the procedure. For higher-risk premises, flushing should be written into the control scheme and documented by the person doing the work.

After extended shutdowns

After any shutdown of a week or more (school summer holidays, building refurbishment, prolonged vacancy), every outlet in the system should be flushed before the building is reoccupied. For larger buildings this is a substantial task that should be planned in advance and allocated clearly in the control scheme.

Legal basis

HSG274 Part 2 paragraph 2.38 and table 2.2 on flushing of infrequently used outlets. ACOP L8 paragraph 63 on the written scheme including flushing.