What is stagnation in a water system?
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Key points
- Stagnation is water that is not turned over: it sits in the same pipe, branch, or tank without being replaced.
- It is the starting condition for legionella growth, because still water drifts into the 20°C to 45°C range and lets biofilm establish.
- The working rule: anything used less than about once a week is treated as stagnant and flushed weekly.
- Stagnation has physical causes, dead legs, little-used outlets, oversized storage, closures, and control means removing causes or restoring turnover.
- A whole closed building stagnates end to end, which is why flushing before reoccupation is a standard control.
Stagnation in a water system is water that is not being turned over: it sits in the same length of pipe, branch, or tank without being drawn off and replaced by fresh water from the supply. It matters because legionella control is, at heart, the denial of stillness. Moving water carries temperature control with it and gives biofilm no time to establish. Water that stops moving loses both defences, drifts into the temperature range the bacteria grow in, and becomes a habitat. Stagnation is the condition underneath almost every legionella problem in a building.
The mechanism: what happens to water when it stops moving
Picture a branch of pipe feeding a tap nobody uses, and watch what the water inside it does over days:
- The temperature drifts. Water in a live hot pipe is kept hot by the flow behind it; cut off from that flow, it cools towards the temperature of the room or void around it. In a heated building that usually means landing somewhere between 20°C and 45°C, which is precisely the growth range. The same happens in reverse to cold water in a warm riser. Our guide to temperature control sets out the numbers.
- Biofilm gets its chance. Biofilm, the slime layer of microorganisms on wet surfaces, is constantly trying to form, and flowing water disrupts it. Still water lets it attach, mature, and thicken. Once established, biofilm shelters legionella from heat and disinfectant and seeds the water passing over it.
- Nutrients accumulate. Standing water collects scale particles, sediment, and organic debris, and disinfectant residuals decay to nothing. The still water becomes warmer, sheltered, and fed at the same time.
This is why the guidance treats turnover as a control measure in its own right. Temperature control holds the bulk of the system out of the growth range; movement stops the corners of it from drifting back in. The full chain is covered in what causes legionella.
Where stagnation hides
Stagnation is rarely in the middle of the system; it lives at the edges. The usual places:
- Dead legs. Branches and capped stubs where water has nowhere to go. A dead leg is engineered stagnation: still water plumbed permanently into the live system.
- Little-used outlets. Any tap, shower, or appliance drawn off less than about weekly. The branch feeding it is a dead leg with a working fitting on the end, and the control is a documented flush, as set out in flushing little-used outlets.
- Oversized or under-drawn storage. A cold water tank or calorifier sized for a building that no longer uses that much water turns its contents over too slowly, and the stored water ages and warms. Right-sizing storage is a stagnation control.
- Closed buildings. A whole premises standing empty, a holiday let between guests, a school over the summer, an office after a long closure, is stagnation at building scale, and the remedy is a full flush before reoccupation, covered in reopening after a shutdown.
Legionella control is the denial of stillness. Moving water carries temperature control and disrupts biofilm; stagnant water loses both and becomes a habitat. Every stagnation control is a way of keeping the water turning over.
How long before still water becomes a problem?
There is no fixed legal clock, and any page that quotes one is inventing it. The risk depends on temperature, surface area, and what is already growing in the system, so warm weather and warm buildings accelerate everything. The working rule the guidance settles on is behavioural rather than chemical: treat any outlet used less than about once a week as stagnant, and flush it weekly so the water in its branch is replaced before it can age. Longer closures are handled differently, with a whole-system flush or disinfection before reuse, because after weeks of stillness the problem is no longer confined to a few branches.
The controls: remove the causes, restore the turnover
Stagnation control runs on two tracks, and the risk assessment decides which applies where. The first track removes the physical causes: cut dead legs and redundant pipework back to the live run, right-size storage, and take out outlets that serve no purpose. The second manages what must stay: regular draw-off from every outlet, a documented weekly flush for the little-used ones to temperature rather than to a guessed time, and a recommissioning flush after any closure. Both tracks only count if they are recorded, because a flush you cannot evidence is, to an inspector, a flush that never happened. The temperature checker confirms a flush has actually pulled a branch back into range, and the log book keeps the record.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
What does stagnation mean in a water system?
Stagnation is water that is not being turned over: it sits in the same section of pipe, tank, or branch without being drawn off and replaced. Because it does not move, it drifts towards the temperature of its surroundings, usually landing in the 20°C to 45°C range legionella grows in, and it gives biofilm the undisturbed time it needs to establish on the pipe wall. Stagnation is the starting condition for almost every domestic legionella problem.
Why does stagnant water grow legionella?
Because stagnation delivers three of legionella's requirements at once. Still water loses the temperature control that moving water carries, so it settles into the growth range. Stillness lets a biofilm form and mature on surfaces, and biofilm shelters the bacteria from heat and disinfectant. And standing water accumulates the nutrients, scale, sediment, and organic material, that feed growth. Moving water denies all three; that is why turnover is a control measure.
How long can water sit before it stagnates?
There is no fixed legal time limit, because the risk depends on temperature and what the water is sitting in. The working rule used in practice is that any outlet drawn off less than about once a week is treated as stagnant and put on a weekly flush. In warm conditions or a warm building the process runs faster, which is why a void property in summer is a bigger concern than the same property in a cool spring. The risk assessment, not a calendar, sets the right interval for each part of the system.
How do you prevent water stagnation?
By designing stillness out and flushing what remains. Remove dead legs and redundant pipework back to the live run, right-size storage so water is turned over rather than held for days, draw off from every outlet regularly, and put little-used outlets on a documented weekly flush to temperature. After any closure, flush the whole system before reoccupation. Every one of these is a turnover measure: moving fresh water through before the standing water can become a habitat.
Is a dead leg the same as stagnation?
No, but one causes the other. Stagnation is the condition, water sitting still. A dead leg is a physical cause of it: a branch or stub where the water has nowhere to go. Other causes include little-used outlets, oversized storage tanks with low draw-off, and whole buildings standing empty. Control works on both levels: remove the physical causes where you can, and keep the water moving where you cannot.
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