How long should you run a shower for legionella?
On this page
Key points
- Run the shower to temperature, not to the clock: hot reaching at least 50°C (55°C in healthcare), cold dropping below 20°C.
- Hot water should arrive at the outlet within one minute of full flow; if it takes longer, the branch holds too much standing warm water.
- A shower used less than about once a week needs running weekly, on both hot and cold, until each reaches temperature.
- Through a thermostatic mixing valve you will never see 50°C at the head, so check the hot supply before the valve, not after it.
- Every run should be written down with the date and the temperature reached, because an unrecorded flush cannot be evidenced.
Run a shower for legionella control until the water at the head reaches temperature: at least 50°C on the hot (55°C in a healthcare setting) and below 20°C on the cold. On a healthy system the hot should arrive within about a minute of full flow, so in practice you are running the shower for a minute or two, but the real measure is the thermometer, not the clock.
The reason a guessed time fails is that every shower sits at the end of a different length of pipe. A shower next to the cylinder turns over in seconds; one at the end of a long upstairs branch may take several minutes to draw fresh hot water all the way through. Temperature is what tells you the standing water in the branch has actually been replaced. Our temperature checker records exactly this, and the temperature chart sets out the target figures.
Why the shower is the outlet that matters most
Of all the outlets in a building, the shower is the one most directly linked to Legionnaires' disease. It combines the three things legionella needs in one device: water that sits warm between uses, a spray plate that harbours biofilm and scale, and a fine aerosol that carries bacteria deep into the lungs when inhaled. A tap you run for a few seconds is a lesser risk; a shower you stand under for ten minutes is the exposure route that counts.
That is why showers get their own controls on top of flushing. The head and hose should be dismantled, cleaned and descaled regularly, as set out in our guide to cleaning shower heads, and any shower used less than about once a week goes on the weekly flushing regime described in flushing little-used outlets.
The mixing valve catch: you may never see 50°C at the head
Many showers, especially in care settings, schools and newer bathrooms, run through a thermostatic mixing valve (TMV) that blends hot and cold down to a safe outlet temperature of roughly 38 to 44°C to prevent scalding. That is correct for scald protection, but it means the water coming out of the head will never read 50°C, however long you run it.
When you check such a shower, measure the hot supply upstream of the valve, not the blended water after it. The hot system feeding the TMV still needs to be at 50°C or more, the valve itself needs regular servicing because its blended side sits in the legionella growth range, and both points are covered in TMV servicing and balancing scalding against legionella.
Running a shower that is not in regular use
A shower that is used daily largely looks after itself, because the constant draw keeps fresh water moving. One used less than about once a week needs running weekly. Open the mixer gently, or remove the head and run the water as a stream, and keep it running until the hot stabilises at 50°C or more and the cold drops below 20°C. Do both sides: the cold branch stagnates just as surely as the hot, and a cold feed that has warmed up standing in a hot riser or roof space is a classic problem.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
What running the shower does, and what it does not do
Running the shower is turnover: it moves fresh water through the branch before the standing water can stagnate and grow biofilm. It does not kill bacteria, clean the head, or fix a system fault. If a shower will not reach 50°C on the hot within a minute, that is telling you something is wrong: a long uninsulated branch, a blending problem, or a hot water system that is not holding temperature. Investigate the cause rather than simply running it for longer, and log the finding. The record of each run, kept in a log book, is the evidence that the control happened; a flush you did not write down cannot be evidenced later.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
How long should you run a shower for legionella?
Run the shower until the water at the head reaches its target temperature, not for a fixed number of minutes. On hot, that is at least 50°C (55°C in healthcare), which HSG274 expects to arrive at the outlet within one minute of full flow. On cold, run until the water drops below 20°C. When the temperature stabilises at the right value, the standing water in the branch has been replaced and you can stop.
How often should a shower that is not in use be run?
At least once a week. Any shower used less than about once a week is classed as a little-used outlet, and weekly flushing turns the water over before it can stagnate and warm into the legionella growth range. Run both the hot and the cold through to temperature, and record each run with the date and the temperature reached, because an unrecorded flush cannot be evidenced.
Does running the shower get rid of legionella?
Running a shower replaces the standing water in the branch with fresh water from the main, which removes the stagnant conditions legionella needs. It does not sterilise the pipe or the shower head, and it is not a substitute for temperature control or for cleaning and descaling the head. Think of it as turnover that supports the real controls, which are hot water kept at 50°C or more and cold kept below 20°C.
Should you remove the shower head when flushing?
Where the head or hose can be removed, yes: running the water as a plain stream produces far less aerosol than running it through the spray plate, and flushing a colonised shower is itself a release of spray. If the head cannot be removed, open the mixer gently so the flow does not atomise, and ventilate the room. Clean and refit the head afterwards.
Related water hygiene products and services from trusted UK providers will appear here.