How long should you run a shower to avoid legionella?

Run a shower until the water reaches temperature, not for a fixed number of minutes. Hot water should arrive at the outlet at least 50°C — 55°C in healthcare settings — and the guidance expectation is that it does so within about a minute. When the temperature stops climbing and holds steady, the standing water in the pipework and hose has been replaced with fresh water, and that is the point at which you can stop.

The reason there is no honest fixed time is that every shower sits at the end of a different length of pipe. An en-suite next to the cylinder may reach temperature in twenty seconds; a shower on the top floor of a converted building, at the end of a long branch off a distant riser, may take three minutes. A single "run it for two minutes" rule wastes water on the first and leaves stagnant water sitting in the second. Temperature is the only measure that tells you the turnover has actually happened, and the temperature checker lets you log exactly that at each outlet.

The hose is the part people forget

A mixer shower on a flexible hose holds more stagnant water than the branch behind the wall does. The hose hangs in a loop, often in a warm bathroom, full of water that has sat since the last use, and the spray head at the end of it traps scale and sediment that feed biofilm. When the shower is opened after days or weeks idle, that volume is the first water out — as a fine aerosol, at face height.

That is why the safe way to flush a shower after an absence is to take the head off first and run the hose as a plain stream, gently, until the temperature stabilises. You get the turnover without the spray. Clean and descale the head while it is off, then refit it. Our guide to legionella in shower heads covers why the head and hose are the highest-risk part of a domestic water system, and cleaning and disinfecting shower heads gives the method.

Daily use needs no routine; a weekly shower does

A shower in normal daily use is turning itself over every day, so there is nothing extra to do. The risk sits with the shower that is not being used: the guest en-suite, the shower room in a void flat, the changing-room cubicle nobody picks. Once an outlet drops below roughly one use a week, the water feeding it stands long enough to drift into the 20°C to 45°C range where legionella multiplies, and it goes onto the flushing regime: weekly, to temperature, recorded. The wider regime is covered in flushing little-used outlets.

What flushing a shower does not do

Flushing is a stagnation control, not a disinfection. It works by denying the bacteria the still, warm water they need to establish, so it prevents a problem rather than curing one. If a property has stood empty for weeks, or a shower is implicated in a positive sample, running it for a few minutes is not the answer: the response is a whole-system flush, cleaning and disinfection of heads and hoses, or full system disinfection, as set out in reopening a water system after a shutdown. Equally, if the hot water never reaches 50°C at that shower no matter how long it runs, flushing cannot fix it — that is a system fault to investigate, because water delivered permanently inside the growth range is the condition temperature control exists to prevent.

Keep the record

Whichever showers are on your regime, write down each flush: date, outlet, who ran it, and the temperature the water stabilised at. That log is the evidence the control happened, and it belongs alongside the rest of your monitoring in a log book, kept for at least five years. The flushing schedule itself should be named in the written scheme of control that follows from your legionella risk assessment, so the outlet, the frequency and the record all join up.