How do you reopen a building's water system after closure?
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Key points
- A closed building is a building-sized little-used outlet: every branch has stagnated, so every outlet needs attention, not just the rarely used ones.
- Recommission in order: inspect, flush from nearest outlet to farthest, restore hot storage to 60°C, verify outlet temperatures, clean showers, then reoccupy.
- Short closures in healthy systems are usually managed by flushing alone; long closures and higher-risk premises may need disinfection and sampling, decided by the risk assessment.
- A planned closure can be kept safe by a documented weekly flush throughout, which avoids a large recommissioning exercise at the end.
- Every step should be recorded: an unrecorded recommissioning cannot be evidenced to an insurer, an inspector, or a court.
Reopening a building after closure means recommissioning its water system before people use it: flush every outlet in sequence, restore hot water storage to at least 60°C, verify that hot outlets reach 50°C and cold outlets fall below 20°C, clean the showers, and record each step. The longer the building stood empty, the more the response shifts from a thorough flush toward disinfection and sampling — a judgement the risk assessment makes, not a fixed number of days.
The problem a closure creates is simple. Water that moves daily stays fresh; water that stands for weeks warms into the 20°C to 45°C range in which legionella grows, and the biofilm that shelters it matures undisturbed. The day the building reopens, every tap and shower is used at once — the single highest-exposure moment the system will ever see. Recommissioning exists so that moment happens after the water has been turned over, not before.
The recommissioning sequence
The order matters, because each step depends on the one before:
- Inspect first. Walk the system before running anything. Look for leaks, a calorifier or boiler left off or turned down, a cold water tank with its lid displaced, and any section of pipework altered during the closure. Fixing a fault after flushing wastes the flush.
- Flush every outlet, nearest to farthest. Start at the outlet nearest the incoming supply and work outward, running each outlet until the temperature stabilises. This draws fresh water progressively through the pipework, as described in our guide to flushing little-used outlets.
- Restore and verify temperatures. Bring hot water storage back to at least 60°C, allow the system to circulate, then confirm hot outlets reach at least 50°C (55°C in healthcare) and cold outlets drop below 20°C. Any outlet that will not reach temperature is a fault to investigate, not a number to ignore — the temperature checker gives you somewhere to record the results.
- Clean the spray outlets. Descale and disinfect shower heads and spray taps before first use, following the method in cleaning shower heads. These are the outlets that turn contaminated water into breathable aerosol.
- Escalate where the closure warrants it. After a long shutdown, in buildings with storage tanks, or where occupants are vulnerable, add disinfection and sampling as set out in making a water system safe after it has stood unused and chlorination of water systems.
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.
Short closure, long closure: how the response scales
A week or two of closure in a well-maintained system — the office at Christmas, the shop over a refit — is normally managed by the flush-and-verify steps above. As the closure lengthens into months, or the system includes cold water storage tanks, long dead runs, or a history of temperature problems, flushing alone stops being a reasonable sole control. Biofilm that has had months to establish does not shift with one flush, and disinfection or sampling becomes the proportionate response. The risk assessment is what converts "how long was it closed?" into "what do we do about it?", which is why a building that may face closures should have that scenario written into its written scheme of control before it happens.
Reopening day is the highest-exposure moment a water system ever has: every outlet used at once after weeks of stagnation. The flush happens before the people arrive, not after.
The cheaper option: flush through the closure
If a closure is planned, there is an alternative to recommissioning at the end: keep the system turned over during it. A documented weekly flush of every outlet throughout the closure stops stagnation from ever establishing, so the reopening is a final flush and temperature check rather than a disinfection exercise. This is often the practical choice for schools, seasonal premises and buildings on reduced occupancy. Whether you flush through or recommission at the end, the record is what makes it count: dates, outlets, temperatures, and who did the work, kept in the log book and retained for at least five years.
Where this duty comes from
Nothing in ACOP L8 or HSG274 suspends the duty to control legionella because a building is closed — the duty holder's responsibility applies to the water system whether or not anyone is currently using it. The guidance treats stagnation as a foreseeable condition and recommissioning as the control. A closure that is managed, flushed and recorded is a nonevent; one that is ignored until the building fills up again is how systems end up in outbreak investigations.
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
What should you do to a water system before reopening a closed building?
Recommission it in sequence: inspect the system for obvious problems, flush every outlet starting nearest the incoming supply and working outward, bring the hot water storage back up to at least 60°C and check that hot outlets reach 50°C and cold outlets drop below 20°C, clean shower heads and spray taps, and only then reoccupy. After a long closure, or where the risk assessment requires it, add disinfection and sampling to that sequence rather than relying on flushing alone.
How long can a building be closed before the water system needs disinfecting?
There is no single fixed number of days in the guidance; the trigger is the risk assessment. As general practice, a short closure of a week or two in a well-maintained system is usually managed by thorough flushing, while a closure measured in months, a system with storage tanks, or premises with vulnerable occupants points toward disinfection and possibly sampling before reoccupation. The longer water stands in the 20°C to 45°C growth range, the more mature the biofilm a simple flush has to shift.
Do schools need to flush water systems after the summer holidays?
Yes. Six weeks of summer closure leaves every outlet in the building standing, and the first day of term is exactly the pattern of sudden full use that releases aerosol from showers, spray taps and drinks fountains. The control scheme should require a documented flush of every outlet before term starts, with particular attention to showers and any outlets that will not reach temperature, and the flush recorded in the log book.
Who can recommission a water system after a closure?
Routine flushing and temperature checks can be done by a competent member of staff who has been shown what to do and what to record. Disinfection of the system, investigation of outlets that will not reach temperature, and sampling should be done by someone with the training and equipment for the task, usually a water hygiene contractor for larger or higher-risk premises. The responsible person named under ACOP L8 decides which is which.
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