--- title: "Managing Legionella during shutdowns and reopening" source_url: https://legionella.io/articles/managing-legionella-during-shutdowns-and-reopening/ canonical_url: https://legionella.io/articles/managing-legionella-during-shutdowns-and-reopening/ pillar: "UK Legionella Law & Compliance" summary: "A closed building doesn't pause Legionella risk, it stores it. The shutdown and reopening mistakes UK duty holders make, and how to flush and reopen safely." primary_keyword: "Legionella during lockdown" date_published: 2025-11-21 date_reviewed: 2026-06-26 author: "Legionella.io editorial team (REMOTE TECH LTD)" reviewed_against: "HSE L8 and HSG274 guidance" region: "United Kingdom" license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url." --- # Managing Legionella during shutdowns and reopening A building that closes does not stop being a Legionella risk. It just stops telling you about one. While the doors are shut, water sits still in pipes designed to keep it moving, hot water drifts down toward room temperature, and the bacteria that thrive in warm, stagnant water multiply where nobody is looking. The harm arrives on day one of reopening, when someone turns on a shower that has not run for three weeks and breathes in the first cloud of mist. The mistakes that cause real trouble here are rarely technical. They are decisions made in a hurry: to save energy over a closure, to skip flushing while the place is empty, to reopen on the morning staff return rather than the week before. Legionella during lockdown made that lesson public, but the same pattern repeats with every Christmas shutdown, summer void, and refurbishment. L8 and HSG274 still treat a closed building as something you assess, control and record [1][2]; closing the doors does not pause the duty. Below are the mistakes worth designing out before your next shutdown, not after it. ## Where shutdowns and reopenings go wrong ### Treating "closed" as "no risk" What it looks like: the logbook simply stops on the day the building shuts and picks up again whenever it reopens, with a blank in between. Why it happens: an empty building feels safe. No occupants, no exposure, so monitoring seems pointless. The fix: a stagnant water system is usually a higher risk than a busy one, not a lower one, because stagnation is the single biggest driver of growth [7]. A shutdown should generate a documented decision about how the system will be kept under control while it sits idle, not silence in the records. If you remember one thing, make it this: closed is a state to be managed, not a holiday from the duty. See [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/) for why stagnation, not age or size, is what actually bites. ### Turning the heat down to save money What it looks like: the hot water setpoint gets dropped, or boilers and calorifiers switched off, to cut bills over a long closure. Why it happens: heating empty water seems wasteful, and energy budgets are real. The fix: letting stored and distributed hot water cool into the warm band is precisely the condition Legionella wants [3]. If the system is staying wet, it needs to stay hot where it should be hot and cold where it should be cold. Saving a few weeks of energy by parking the whole system at growth temperature is a false economy you pay back, with interest, on reopening. ### Stopping flushing because nobody is there What it looks like: the weekly flush of low-use outlets gets quietly paused for the duration of the closure. Why it happens: with zero occupants and reduced staff, flushing empty rooms feels like effort for nothing. The fix: flushing matters more when use drops to zero, not less. A skeleton regime that draws fresh water through every outlet on a defined interval is what stops the whole building turning into one large dead leg. The interval is set by your risk assessment and the system, not by who happens to be on site [2]. If you do not already have one, [How to implement a flushing programme for Legionella control](https://legionella.io/articles/how-to-implement-a-flushing-programme-for-legionella-control/) covers building a flushing programme that survives an empty building. ### Draining down badly, or for the wrong reasons What it looks like: a partial drain to "make it safe", which leaves water trapped in low points, dead legs, the base of tanks and the guts of fittings. Why it happens: drained reads as empty, and empty reads as safe. The fix: a botched or partial drain-down can be worse than a wet system you actively manage, because you end up with residual warm water, no flushing, and a false sense that the risk is gone. Whether to drain or keep circulating depends on how long you are closing and what you can guarantee on the way back in. It is a competent-person call to document, not a default to reach for. ### Reopening cold and flicking everything on at once What it looks like: the building goes live the morning people return. Showers, taps, spray fittings, the lot, straight into service. Why it happens: reopening is driven by logistics and diaries, and water is invisible until it sprays. The fix: bring the system back deliberately, before anyone arrives. Restore temperatures, flush thoroughly through every outlet, and where the risk assessment calls for it, disinfect and sample before fittings go back into use. This is the moment that matters most, because Legionnaires' disease comes from inhaling contaminated aerosol, and a long-idle shower is an aerosol machine [4][5]. Reopening is not the end of the shutdown; recommissioning is. ### Forgetting the quiet assets What it looks like: the obvious showers get flushed, but TMVs, spray taps, drinking water dispensers, ice machines, emergency eye-wash stations, any idled cooling tower, and the far wing you are reopening a fortnight later all get missed. Why it happens: out of sight, out of the flushing round. The fix: work from a list of every water-using asset, including the ones that do not look like taps. A phased reopening needs phased recommissioning, so each area is brought back the same way the first one was rather than just switched on because it is next. ## The one habit that prevents most of these Plan the reopening before you plan the shutdown. The day you decide to close is the day to write down how you will safely bring the system back, because almost every mistake above is really a reopening problem that was set in motion weeks earlier. A shutdown is also a change in use, which is itself a trigger to review the risk assessment, so the version you reopen against should be one a competent person has looked at since the building emptied, not a copy of last year's plan. One caveat specific to this situation: a closure changes the very things your assessment was built on, including how much water moves, how warm it sits, and who is exposed when the doors reopen. So the temperatures, flushing intervals, and the drain-versus-circulate decision are site-specific judgements for a competent person, not figures to lift from an article. Use this to ask sharper questions of whoever holds that competence. The concrete next step: before your next planned closure, even a bank-holiday weekend, add a short reopening section to the shutdown plan that names who restores temperatures, who flushes what and in what order, and who signs the system back into service. Then make sure the gap in the logbook can be explained by a decision, not a shrug. For the bigger picture on stitching this into your wider duties, [Integrating Legionella risk assessment into health and safety management](https://legionella.io/articles/integrating-legionella-risk-assessment-into-health-and-safety-management/) is the natural companion. ## FAQ ### How long can a building stay closed before the water becomes a Legionella risk? There is no single safe number. Risk starts building from the first days of stagnation and depends on temperatures, pipework layout, and what was done before the doors shut. Many duty holders treat anything beyond about a week of no use at an outlet as enough to warrant action, but the interval that counts is the one your risk assessment sets for your system, not a figure from a blog [2]. ### Should we drain the system down or keep it circulating during a long shutdown? Both can be valid and both can go wrong. A fully drained, properly dried system removes the water Legionella needs to grow, but a partial drain-down that leaves water sitting in low points and dead legs is worse than a wet system you keep under active control. The choice depends on how long you are closing and what you can guarantee on reopening, so make it a documented decision with a competent person rather than a reflex. ### Is flushing enough to reopen, or do we need to disinfect and sample first? For many short closures, restoring temperatures and thorough flushing before outlets go back into use is the core step. Longer closures, vulnerable occupants such as in healthcare or care settings, or any sign that routine controls lapsed can push you toward disinfection and verification sampling. Your risk assessment, reviewed after the shutdown rather than before it, is what decides which applies [6]. ## Related reading - [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/) - [How to implement a flushing programme for Legionella control](https://legionella.io/articles/how-to-implement-a-flushing-programme-for-legionella-control/) - [Integrating Legionella risk assessment into health and safety management](https://legionella.io/articles/integrating-legionella-risk-assessment-into-health-and-safety-management/) - [The cost of non-compliance: legal and business impacts](https://legionella.io/articles/the-cost-of-non-compliance-legal-and-business-impacts/) ## 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, "Legionnaires' disease - what you must do". https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [5] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html [6] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [7] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm