How do you make a water system safe after it has stood unused?

Flush every outlet to replace the standing water, bring stored hot water back to 60°C and cold below 20°C, then clean and disinfect showers and spray fittings. After a long closure, or in a higher-risk building, disinfect the whole system and sample it. Record all of it.

Why a stood-down system is high risk

Legionella grows in still, warm water. When a building is occupied, water moves: taps run, cisterns refill, hot water circulates, and temperatures stay at the extremes that hold the bacteria back. Close the building and all of that stops.

Standing water loses its temperature discipline first. Hot water in a stored cylinder cools towards room temperature, and cold water that is no longer being drawn warms up, especially where pipes run through heated voids, risers or ceiling spaces. Both drift into the 20°C to 45°C band where legionella multiplies fastest. At the same time, biofilm on the inside of pipes and fittings keeps growing, feeding on the sediment and nutrients that settle out of motionless water. The longer the standstill, the more of the system is affected, which is why reopening is treated as a whole-system problem rather than a single-tap one.

This applies to more situations than people expect: empty rental voids between tenants, holiday lets sitting idle between bookings, schools over the summer holiday, seasonal venues, show homes, and any building that was mothballed and is now being brought back into use.

Start with the risk assessment

Your legionella risk assessment should already have anticipated the shutdown and set out what to do on reopening. Review it before you touch anything. It tells you where the outlets are, which of them make a spray, where the stored water sits, and which parts of the system are most likely to have stagnated. If the assessment is silent on shutdowns, or the building has changed while it was closed, this is the moment to update it. The duty to assess and control the risk sits with the person responsible for the premises under the ACOP L8 framework and the guidance in HSG274 Part 2.

Flush every outlet

Flushing is the core reopening task. The aim is to replace all the standing water in the pipework with fresh water from the mains. Open each outlet in turn and let it run until the water is fresh and the temperature has stabilised: cold running genuinely cold, hot running genuinely hot. Work outward from the outlets closest to where the supply enters the building, so you are always pulling fresh water through pipes you have already cleared.

Flushing releases aerosols, which is exactly what you are trying to avoid breathing. Open taps gently to reduce splashing and spray, ventilate the room, and where possible remove the shower head or spray fitting so the water runs as a stream rather than a mist. Our guide on how long to run taps and the wider guidance on flushing outlets cover the practical detail.

Bring temperatures back into range

Turning the water over is not enough on its own if the system is still sitting in the growth range. Get the temperatures back to where the guidance expects them and confirm it with a thermometer rather than assuming.

For hot water, bring the stored temperature back to 60°C and check that water reaches 50°C (55°C in healthcare settings) at each tap and shower within a minute of running. For cold water, check it runs below 20°C at the outlet within two minutes. Where a calorifier or a cold water storage tank has sat idle, inspect it as part of the reopening. Our temperature chart sets out the target figures, and temperature control explains why they matter.

Clean and disinfect showers and spray outlets

Showers and other spray fittings are the highest concern because they turn water into a breathable aerosol. Standing water sitting in a shower hose and head over a closure is an ideal place for legionella and scale to build up. Before the shower is used again, remove the head and hose, clean and descale them, and disinfect them. The steps are in our guides on legionella in shower heads and cleaning shower heads.

When to disinfect the whole system

Flushing and temperature correction handle many short closures. A full system disinfection is a bigger step, and the risk assessment should decide when it is warranted: typically after a long shutdown, where the water has been standing for weeks or months, where the building serves people who are more vulnerable to infection, or where flushing alone has not restored the system to a good state. Disinfection usually means chlorination, dosing the system with a disinfectant and holding it for a set contact time before flushing through. That is a controlled procedure, and for anything beyond a straightforward domestic-scale system it is normally done by a competent contractor.

Sample where the risk justifies it

Sampling does not control legionella; it checks whether your controls have worked. After a long closure, or where the risk assessment calls for it, taking samples gives evidence of the water quality before people return. The recognised action levels are 100 and 1,000 CFU per litre for hot and cold systems; our guide on what to do if legionella is detected explains how the results are interpreted. Sampling is most useful when it confirms a system you have already flushed, heated and, where needed, disinfected, not as a substitute for those steps.

Write it all down

Every action above needs a record. Note the date of reopening, which outlets you flushed, the temperatures you measured, the showers you cleaned and disinfected, any full disinfection carried out, and any samples taken and their results. That record belongs in your log book alongside your written scheme of control. The whole reopening sits within the same duty as everyday control: ACOP L8 and HSG274 Part 2 expect the responsible person to identify the risk, act on it and keep records, and a reoccupation is simply that duty applied to a system that has been standing still.