Quiet buildings are not low-risk buildings
There’s an instinct that a closed-up building is a safe one — nobody’s there, so what’s the harm? For water systems, the opposite is true. Reduced occupancy means reduced water turnover, and reduced turnover is the single biggest driver of Legionella growth in an otherwise healthy system.
Summer makes it worse: incoming mains water is warmer, and cold systems drift more easily above the 20°C threshold where bacteria proliferate.
What stagnation actually does
When water stops moving, three things happen. Disinfectant residual drops. Temperature equalises towards the ambient of the building. And biofilm — the slime layer where Legionella lives and multiplies — gets the still, warm conditions it likes best.
A fortnight of an unused wing in July can undo months of good control. The system looks fine; the risk is invisible until someone samples it.
A simple holiday flushing plan
You don’t need anything elaborate. Identify every outlet that will go unused during the shutdown and flush it at least weekly — run it long enough to draw fresh water through to the outlet, hot and cold. Record who did it and when.
For larger or higher-risk sites, keep temperature monitoring running through the quiet period rather than pausing it. The shutdown is when you most need the data, not least.
Coming back online safely
Before a building repopulates, flush the whole system thoroughly and confirm temperatures are back in range — hot returning to 50°C and above, cold below 20°C. For sites that have stood idle for an extended period, or where vulnerable people are present, a disinfection and sampling regime before reoccupation is the prudent call.
Plan the reopening checks into the shutdown from day one. Discovering a problem on the morning everyone returns is the worst possible time to find it.