Building shutdowns: flushing and monitoring during low use

An empty building feels like a paused risk. It isn’t. The moment people stop using a water system, the thing that kept Legionella in check — water moving through the pipes and being drawn off at temperature — stops with them. Stagnation starts the same day, any disinfectant residual fades, and warm dead legs drift into the range where the bacteria multiply. A closed wing, a mothballed floor or a two-week holiday shutdown is a growth window, not a quiet spell.

So a shutdown is something you manage, not something you wait out. What follows is a practical sequence for keeping a system controlled while it sits idle, and for proving it was safe before the first person turns a tap.

What changes the day the building empties

Three things shift at once. Throughput collapses, so fresh mains water stops displacing the old. Temperatures relax toward room temperature — cold water creeps up past the low twenties, hot water cools in long runs nobody is drawing on. And any disinfectant level decays with nothing to top it up. Together they quietly undo the controls your risk assessment was leaning on [1][2]. Stagnation is the root cause sitting underneath all of it; if that idea is new to you, on neglected water systems is worth ten minutes first.

Before anything else, treat the shutdown as a change worth recording. A real shift in how a building is used is exactly the trigger that should send you back to the risk assessment rather than running the old routine on autopilot [5]. You want two things in hand: an assessment that reflects the building being lightly used, and a written scheme that names who flushes what, how often, and who hears about it when a reading drifts.

Step 1 — Set the regime before the doors close

Decide, in writing, how the system will be run while it is idle: kept live and flushed, or partially drained for a long closure. Walk the building and mark every outlet that will see no use at all — those are your priority points.

The reason to do this now is simple: the decisions are far cheaper to make with the site staffed and the plant accessible than in a scramble the week before reopening. Done well, you finish Step 1 with a flushing schedule keyed to named outlets and a named responsible person, not a vague hope that “maintenance will sort it”. The common slip is assuming cleaners or a contractor will “probably” run the taps. Probably is not a record.

Step 2 — Keep the water moving

Flush the little-used outlets on a fixed routine so fresh water reaches every branch of the system. A common starting point in HSE guidance is weekly flushing of outlets that would otherwise sit unused, but the interval belongs to your risk assessment, not to a rule of thumb [2][4]. Run each outlet long enough to draw water through to its working temperature, work from the nearest fitting to the furthest, and open showers gently to limit spray.

You are done with a flushing pass when every mapped outlet has been run and logged with the date, who did it and any reading taken. The classic failure is flushing the easy ground-floor taps and forgetting the disused shower in a plant lobby — often the single highest-risk fitting in the place. If you are building this from scratch, on implementing a flushing programme covers how to structure and record it properly.

Step 3 — Hold the temperatures, not just the schedule

A flush that pushes lukewarm water through the pipes proves very little. Check that hot water still leaves storage hot and arrives hot, and that cold genuinely stays cold. The figures sit in HSE guidance and in your written scheme — hot water commonly stored at around 60°C and reaching about 50°C at the outlet, cold held below roughly 20°C — and they exist to keep water out of the band, broadly 20–45°C, where Legionella thrives [3][1].

You have passed this step when temperature checks at your sentinel points come back in range and are recorded next to the flush log. The quiet trap during a shutdown is a cold tank in a warm, shut-up plant room slowly warming all month, with nobody on site to notice the drift.

Step 4 — Flush, drain refill, clean or sample? Decide the reopening path

How you bring a system back depends on how well it was held while idle and how long it sat. Work down these branches before anyone returns:

• Was the system kept live and flushed on schedule, with temperatures in range throughout?
  • Yes → a full flush of every outlet to temperature, plus sentinel temperature checks, is usually enough. Record it and move to Step 5.
  • No, or you cannot prove it → treat it as uncontrolled and read on.
• Was the system drained down for the closure?
  • Yes → refill, then flush thoroughly; many schemes also call for disinfection before use after a drain-down, so confirm against your assessment before reopening.
• Was any part left stagnant beyond the interval in your risk assessment?
  • Yes → clean and disinfect the affected parts, then flush, before those outlets go back into service.
• After the above, does your assessment call for confirmatory sampling — for example a long closure, vulnerable occupants, or a previous problem on site?
  • Yes → sample to the scheme from representative points, and hold high-risk outlets out of use until the results support reopening.
  • No → flush, confirm temperatures, log it, reopen.

When a branch is genuinely unclear, take the more cautious path. The cost of an extra clean or sample is trivial against bringing an uncontrolled system back into use under occupants.

Step 5 — Recommission and verify before anyone returns

Whatever the tree pointed to, the final act is the same. Run every outlet through to temperature, record temperatures at your sentinel points, and write the sign-off — the evidence that the building was handed back under control, by name and date. Where the risk assessment required sampling, treat a clean result as supporting evidence, not as a stand-in for the flush-and-temperature work [4]; on sampling frequency explains how that frequency is set. You are finished when a reader of the logbook could see, without asking you, that the system was returned to a controlled state before reoccupation.

A word on the numbers here

The intervals and temperatures named above are starting points drawn from HSE guidance, not fixed rules for your building. A two-week void in a heated city-centre office behaves nothing like the same gap in an unheated rural depot, and only a competent, site-specific risk assessment can set the right flushing frequency, the right recommissioning steps, and whether sampling is warranted. Use this sequence to ask sharper questions of that assessment — not to replace it, and not as medical, legal or design advice.

FAQ

Is it safer to drain the system down while the building is closed?

Not automatically. A drained system still holds residual water in low points, traps and seals, and it has to be refilled, flushed and often disinfected before use, which can introduce its own risk if done carelessly. For shorter closures, keeping the system live and flushed is usually simpler to control and to prove. Let the risk assessment make the call for the length of closure you face [1][2].

How long can an outlet sit unused before a flush alone isn’t enough?

There is no single figure, because it depends on temperature, the fitting and the water’s recent history. Once a branch has sat warm and stagnant beyond the interval your risk assessment sets, the safe assumption is that it needs cleaning, disinfection or confirmatory sampling rather than one flush. In doubt, treat it as uncontrolled and verify before use [2][4].

Do we need a Legionella sample before staff come back?

Only if your risk assessment calls for one. Sampling is verification, not control — it describes specific points at a single moment and cannot make up for missed flushing or warm storage. After a long closure, with vulnerable occupants, or where there has been a previous problem, a scheme may require it; otherwise thorough flushing and in-range temperatures are the core evidence [4].

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [5] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1