A cricket pavilion gets switched off at the consumer unit in September and nobody touches the showers again until the first net session in April. A holiday cottage sits between a half-term let and the Easter rush. A lido plant room idles through the winter. In each case the building feels safe because it is empty. That instinct is exactly backwards.
Legionella needs warm water that sits still, and a way to be breathed in as fine droplets. A closed seasonal building hands it the first condition for months at a time, then opening day supplies the second the moment someone turns on a shower that has not run since last summer. The risk is manufactured during the quiet months and delivered on the busy ones. So the controls that earn their keep are not the ones running while the building is full — they are the ones covering the shutdown and the reopening.
Most seasonal failures are not exotic. They are the same handful of avoidable mistakes, repeated every year because the closed season falls into a blind spot. Here are the ones worth designing out.
Where seasonal sites go wrong
Treating a closed building as a safe building
What it looks like: the site shuts, gets isolated, and the water system drops off everyone’s radar until the week before reopening.
Why it happens: an empty building reads as “no users, no risk”. But occupancy is not the driver — stagnation is. An unused system holds warm, still water in tanks and pipework for weeks, which is precisely the condition Legionella multiplies in. The off-season is not a pause in the risk; it is when the risk is being grown. Neglected water systems makes the point in full.
The fix: accept that the control duty does not switch off when the lights do. Either keep a minimal regime live through the closed period — turnover at outlets, temperature checks where the system stays energised — or mothball the system deliberately and on purpose. Both are defensible. Drift is not.
The opening-day blast
What it looks like: first morning of the season, staff go room to room throwing every shower and tap to full to “freshen things up” — often with cleaners, contractors or early arrivals already in the building.
Why it happens: it feels productive and it clears the discoloured water. But running a stagnant, aerosol-producing outlet at full flow is the single most efficient way to launch whatever grew over the winter straight into the air people are breathing. Legionnaires’ disease comes from inhaling contaminated droplets, not from the water sitting quietly in the pipe [1].
The fix: recommission before you reopen, not on the day. Bring the system back while the building is empty, and clear outlets in a way that keeps spray down — low flow, a hose run to drain, or the head removed. After a long shutdown your risk assessment may call for disinfection and a verification sample before anyone vulnerable walks in. A planned recommission, not a quick rinse.
Half-draining and hoping
What it looks like: to dodge frost damage and trim the energy bill, someone “drains down” — but a true drain-down is rarely complete. Water lingers in low points, traps, the base of the calorifier, capped branches and the outside tap.
Why it happens: a partly drained system feels safer than a full one left standing. In reality, a system that is neither properly live nor properly dry is often the worst of the three states: residual warm pockets, no turnover, and no monitoring because everyone believes “it’s drained.”
The fix: pick one strategy and carry it through. Keep the system live and managed, or mothball it fully — drain to the design points, isolate, label what has been done and why, and book the recommission date. A documented mothball beats a vague half-drain every season.
Temperature on the gauge, no water through the taps
What it looks like: the calorifier stays on all winter, the central readings look healthy, and everyone assumes the system is under control. Meanwhile the showers in the away-team changing room have not run since the last home fixture.
Why it happens: temperature control gets treated as the whole job. But heat at the cylinder does nothing for a dead leg at the far end of the building. Hot water that loses its heat along a long, unused run can land squarely in the growth range by the time it reaches the outlet.
The fix: pair temperature control with turnover. Low-use outlets need flushing through the quiet months so water does not stand — that is the backbone of any off-season regime, and How to implement a flushing programme covers how to build one. As general guidance many schemes flush low-use outlets at least weekly, but the right interval is whatever your risk assessment sets for your building and how long it sits idle [2].
Nobody owns the water while the building sleeps
What it looks like: the seasonal cleaner has gone home, the manager has moved to another site, and the flushing log has a three-month gap. Nobody actually decided who would keep it going.
Why it happens: the rota is built around the open season. When the building closes, the staffing closes with it — but the legal duty does not.
The fix: name a responsible person for the closed period specifically, with a short task list and a clear escalation route if something is missed or looks wrong. The duty holder stays accountable whether the building is heaving or deserted [3]. A pavilion nobody visits for four months needs a named person and a date in the diary more than any clever kit.
Forgetting the fittings that only exist in season
What it looks like: the assessment covers the indoor showers and the kitchen, but not the outdoor rinse-off shower at the campsite, the hose-down bay, the spray tap on the patio bar, the irrigation line or the mothballed second kitchen.
Why it happens: these fittings only appear when the season turns, so they slip out of the year-round mental model. They also tend to be textbook aerosol generators that sit bone dry, then run hard.
The fix: walk the whole site, in season and out, and list every aerosol-producing fitting — including the ones that only come alive in good weather. HSE’s guidance on which systems create foreseeable risk is a sensible checklist to walk the site against [4]. If it sprays and it sits idle, it belongs in the scheme.
The one correction that does the most work
If you fix only one thing, fix the handover between “closed” and “open”. Almost every seasonal failure lives in that gap: a system switched off with no plan to switch it safely back on, in front of the people who arrive on day one. Write a short seasonal protocol — what happens at shutdown, who does what through the closed months, and the recommissioning steps signed off before the doors open — and you close the hole the other mistakes fall through.
If you let a holiday property
Operators of holiday lets and self-catering cottages sometimes assume that because the building is “domestic”, none of this applies. It does. Letting accommodation to others brings a duty to assess and control the Legionella risk, and HSE sets out what that means for landlords [5]. For most small lets the response is proportionate, not heavy: a sensible assessment, simple controls, and a flush before each guest arrives after the property has stood empty.
Where this stops being general guidance
Nothing above sets a number for your building. How long a system can stand before it needs disinfection rather than simple flushing, what counts as low-use on your site, when a verification sample is worth taking, and whether to keep the system live or mothball it are decisions for a competent, site-specific risk assessment that weighs your water system, the length of your shutdown, and who walks back through the door on opening day [6]. Use this as a prompt for the right questions, not a substitute for the assessment that answers them.
FAQ
How long can a building sit empty before the water becomes a Legionella risk?
There is no single safe number, because growth depends on temperature and stagnation rather than the calendar. In warm, still conditions the risk can build over days to a few weeks, so your risk assessment — not a rule of thumb — should set the threshold for your system. Treat any extended closure as needing a managed shutdown and a managed reopening.
Should I drain the system down over the off-season or keep it running?
Both can be valid; the trap is the half-measure that does neither properly. Keeping the system live means lower recommissioning effort but a real flushing and temperature burden through the quiet months. A full, deliberate mothball saves energy but demands a proper drain-down and a planned recommission before reopening. Your assessment, the length of the closure and frost risk should decide which one fits.
Do holiday lets and self-catering cottages really need Legionella control?
Yes. Letting accommodation to others carries a duty to assess and manage the risk, and HSE publishes guidance aimed specifically at landlords [5]. A short risk assessment, basic controls, and a flush of the outlets after the property has stood empty usually cover it.
Sources
[1] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [5] HSE, “Legionella and landlords’ responsibilities”. https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm [6] 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