Reopening a building after closure: getting the water safe

When a building shuts for a while, the water in its pipes and tanks doesn't go anywhere. It just sits there, slowly warming up or cooling down to room temperature, which is exactly what Legionella likes. Before people come back, that water needs dealing with.

Early summer is when a lot of buildings change pace. Seasonal venues open up, refurbished offices get handed back, and some sites that have been mothballed are brought back into use. So here's a practical run through what to do with the water before you open the doors.

Why closure is a Legionella problem

Legionella bacteria multiply best in water between roughly 20°C and 45°C, especially where it's stagnant and there's sludge, scale or biofilm in the system. A normal, busy building keeps water moving, keeps the hot side hot and turns over the cold water regularly. Close the building and all three of those stop.

• Hot water cools in the pipes, and sometimes in the cylinder too if the heating's been turned off.
• Cold water warms up, particularly in tanks in plant rooms and lofts, and in pipes running through warm ceiling voids.
• Biofilm gets a chance to build up on pipe walls, giving bacteria somewhere to shelter.

The longer the closure, and the warmer the weather, the bigger the risk. A long weekend isn't usually a worry. A few weeks or months is.

Ideally, plan before you close

The HSE's guidance is clear that duty holders should keep managing the risk while a building is closed, not just when it reopens. Your risk assessment and written scheme should cover what happens during a shutdown. Typically that means one of two approaches:

• Keep the system ticking over. Leave the hot water on at its normal temperature and flush outlets weekly, so the water never really stagnates.
• Drain it down. For a long closure, some systems are drained and left empty. This sounds simpler but it isn't always, because pipes rarely drain completely and you'll need to clean and disinfect before refilling.

If flushing was kept up and logged throughout, reopening is much more straightforward. If it wasn't, you'll need to do more before people return.

A reopening checklist

Every building is different, and the right steps depend on your system and how long it's been closed. But for a typical office, shop or school, the job usually looks something like this:

1. Read your risk assessment and scheme first. They should say what to do. If the building's use has changed while it was shut, they may need reviewing too.
2. Get the hot water back to temperature. Turn the heating plant back on and allow time for calorifiers and cylinders to reach at least 60°C throughout before you start running outlets.
3. Inspect the cold water tank. Check the lid's on, the water's clean and nothing's fallen in. If it's dirty or the water's been sitting a long time, it may need cleaning and disinfecting.
4. Flush every outlet. Work methodically through the building, usually starting nearest the tank or water heater and moving outwards. Run hot and cold until the water reaches its normal temperature.
5. Take off and clean shower heads. Descale and disinfect them, or swap them for new ones, before running the showers.
6. Don't forget the odd ones. Cleaners' sinks, outside taps, drinks machines, eyewash stations and any appliance with a water feed.
7. Check temperatures and write them down. Hot reaching 50°C or more and cold below 20°C at your sentinel outlets. If they don't, find out why before you open.
8. Consider disinfection and sampling. For long closures, or if checks show control hasn't been regained, a full system disinfection may be needed, followed by water samples to confirm it's worked.

Flushing safely

Flushing stagnant water is the one moment when you're most likely to release a burst of bacteria, so do it carefully:

• Let water run gently rather than at full blast, to keep spray down.
• For showers, take the head off if you can, or put a bag over it and run it into the tray. Keep your face well away.
• Close toilet lids before flushing cisterns.
• Keep the room ventilated and the door closed so spray doesn't drift.

Does the water need testing?

Not always. If the system was kept flushed and at temperature during the closure, and your temperature checks look good, sampling may not add much. It's worth considering after a long period of stagnation, when temperatures couldn't be maintained, after a disinfection, or where the building's users are vulnerable. Samples should be taken by someone trained and sent to a UKAS-accredited lab. Our water sampling page has more on how that works.

A typical scenario

Picture a small office that's been empty for three months during a refit. The builders have been using one kitchen tap and the toilets on the ground floor, but the upstairs showers and kitchenette haven't been touched. The water heater was switched off to save energy.

In that situation we'd usually get the heater back up to 60°C, inspect and possibly clean the tank, flush every outlet floor by floor, descale the showers and record temperatures throughout. If the refit added or moved any pipework, we'd check for new dead legs too, as builders sometimes leave capped-off pipes behind. Depending on what we find, we might suggest a disinfection and samples before staff move back in.

After you've reopened

Getting through the first week is only the start. Go back to your normal routine of monthly temperature checks and weekly flushing of anything that isn't used much. If parts of the building stay quiet, such as a floor that's only half occupied, keep flushing those outlets until they're back in daily use. Your assessment should also be reviewed if the way the building's used has changed. Our guide on how often to review a risk assessment covers the usual triggers.

If you'd rather hand the whole job over, we can flush, inspect, sort any remedial works and keep the checks going afterwards with our Legionella maintenance visits.