Can an expansion vessel harbour legionella?
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Key points
- An expansion vessel absorbs the few per cent by which water expands when heated, using a diaphragm with an air cushion on one side and system water on the other.
- Most vessels tee off the pipework, so the water inside them barely turns over: they behave as a dead leg with a large rubber surface.
- That pocket of still water cools toward the growth range and is in permanent contact with the diaphragm, a surface biofilm can colonise.
- Manage it by siting vessels on live flow paths or choosing flow-through designs, insisting on WRAS-approved materials, and naming the vessel in the written scheme.
- The annual pre-charge check is maintenance, not hygiene: a vessel that has lost its charge or its diaphragm is a larger pocket of static water than before.
An expansion vessel is a small tank that absorbs the expansion of water as it heats up. Water expands by a few per cent between cold and full hot-water temperature, and in a sealed system that extra volume has nowhere to go, so the vessel takes it in: a flexible diaphragm divides the tank, with an air or nitrogen cushion on one side and system water on the other. It is an ordinary, necessary component, and it is also one of the least-examined corners of the system from a legionella point of view, because the water inside it barely moves.
Why the vessel behaves like a dead leg
Picture how a typical vessel is connected: a tee off the main pipework, with the vessel hanging on the branch. As the system heats, expanded water pushes into the vessel; as it cools, the air cushion pushes it back. What never happens is exchange. The same water shuttles in and out of the vessel, rarely replaced with fresh water from the mains, because nothing in normal operation draws flow through it. By the definition on our dead legs page, that is a dead leg wearing a pressure rating: a volume of water that stands still, loses its chlorine residual, and drifts toward the temperature range legionella favours.
The diaphragm adds a second issue. It is a large flexible surface in permanent contact with static water, and elastomers that are not approved for potable use can leach nutrients that feed biofilm. This is why WRAS approval matters on any component touching drinking water: approved materials are tested so they do not support microbial growth. A cheap, unapproved vessel is a stagnation pocket lined with food.
Where vessels sit and why location matters
You will find expansion vessels on sealed heating circuits, on unvented hot water cylinders, and on boosted cold water sets. The hygiene concern is sharpest on the potable hot water side: a vessel teed off the pipework near an unvented cylinder holds water that can sit tepid, right in the growth range, indefinitely. The same component on a sealed heating circuit matters far less, because heating water does not feed taps.
The design answer exists and is increasingly specified: flow-through vessels, where the pipework runs through the vessel so ordinary draw-off keeps its contents turning over, and siting arrangements that put the vessel on a live flow path rather than a capped branch. If you are replacing a cylinder or refitting plant, that choice removes the stagnation rather than managing it forever.
An expansion vessel is the dead leg nobody flushes: same water in, same water out, a few per cent at a time. Either put it in the flow or put it on the checklist.
Assessing and maintaining the vessel
The practical regime has two halves. The maintenance half is the air pre-charge: annually, or at the interval the manufacturer sets, the vessel is isolated and drained and the pressure on the air valve is checked and topped up. A vessel that has lost its charge stops absorbing expansion, and one with a failed diaphragm is just a steel pocket of static water, worse than before. This check is a plant engineer's job, but it should appear in the monitoring schedule so it actually happens.
The hygiene half is to treat the vessel as what it is. Identify it in the risk assessment as a component that holds stagnant water, confirm the materials are WRAS-approved, drain and flush it where the design provides a drain point, and record each check in the log book. On a larger or higher-risk system, or where a vessel sits on a potable hot supply in a building with vulnerable occupants, it is worth asking a water-hygiene specialist whether a flow-through replacement is justified. Our risk assessment builder prompts for components like this precisely because they are easy to walk past.
Free legionella risk assessment template
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The wider lesson
Expansion vessels are a good example of why legionella control is a whole-system discipline rather than a checklist of famous components. Everyone looks at the tank and the calorifier; the guidance in HSG274 asks you to look at anywhere water can stagnate or sit warm, and a teed-off vessel qualifies on both counts. Walked past, it is a quiet pocket of old water connected to your hot supply. Named, sited well, made of approved materials and serviced on schedule, it is just another component doing its job.
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 an expansion vessel do?
Water expands as it is heated, by a few per cent between cold and full hot-water temperature. In a sealed hot water or heating system that expansion has nowhere to go, so an expansion vessel absorbs it. The vessel is a small tank split by a flexible diaphragm: one side holds air or nitrogen under pressure, the other side connects to the system water. As the water heats and expands it pushes into the vessel and compresses the air cushion; as it cools the cushion pushes the water back out.
Is an expansion vessel a legionella risk?
It can be, because of how it is usually connected. Most vessels tee off the pipework, so the water inside the vessel is a pocket that barely turns over: effectively a dead leg with a flexible rubber wall. That water sits still, cools toward the growth range, and the diaphragm itself is a large rubber surface in permanent contact with the water. The risk is managed by choosing flow-through designs or siting vessels so water moves through them, using vessels with WRAS-approved materials, and including the vessel in the risk assessment and flushing regime.
Is an expansion vessel a dead leg?
Functionally, yes. A dead leg is any length of pipework or component that holds stagnant water, and a teed-off expansion vessel fits that description precisely: water enters it when the system heats and returns when it cools, but it is rarely exchanged for fresh water from the mains. Flow-through vessel designs, where the system water passes through the vessel rather than sitting in a branch off it, exist specifically to remove this stagnation.
How often should an expansion vessel be serviced?
The routine check is the air pre-charge pressure, tested annually as part of normal plant maintenance or at the interval the manufacturer sets: isolate and drain the vessel, check the pressure on the air valve, and re-charge if it has dropped. A vessel that has lost its charge stops absorbing expansion, and a failed diaphragm leaves the whole vessel full of static water. From a water-hygiene view, the vessel should also be named in the written scheme so it is inspected, and drained and flushed where the design allows.
Can you flush an expansion vessel?
A standard teed-off vessel does not flush in normal operation, which is the core of the problem. Draining and re-charging the vessel during its annual service does exchange the water in it, and some vessels are fitted with a drain point for exactly this. The better long-term answer where a vessel is on a potable hot water system is a flow-through design or a location on a live flow path, so ordinary use keeps the water inside it turning over.
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