Short answer: HSE's HSG274 Part 2, Table 2.1, says to flush expansion vessels through and purge to drain where practical, monthly to six-monthly as the risk assessment indicates, and to change bladders as the manufacturer or risk assessment says. Vessels on steady systems can sit for long periods without exchanging water, which can aid microbial growth (para 2.38).
Why expansion vessels are a Legionella risk
HSE says: "Expansion vessels in systems operating at steady temperature and pressure may have long periods without exchanging any significant amount of water and therefore can be at risk of aiding microbial growth" (HSG274 Part 2, para 2.38). Their bladders or diaphragms "are often made of synthetic rubber such as EPDM and may support the growth of microorganisms including legionella", so HSE suggests checking they are approved against BS 6920 (Info box 2.1).
These checks are for vessels on hot and cold water services, the water people drink and wash in, not vessels on sealed heating circuits.
Not every vessel behaves the same way. A hydraulic accumulator on a booster set, correctly installed, "will partially fill and empty between each pump run and should exchange water at regular intervals, which will reduce the risk of stagnation". An expansion vessel on a hot water system "may not fill and empty where the system pressure and temperature remains steady" (Info box 2.1). Vessels with a flow-through design "should provide less opportunity for water to stagnate".
What HSE asks for
| Topic | What HSG274 says | Where |
|---|---|---|
| Flushing | Where practical, flush through and purge to drain | Table 2.1 |
| Frequency | Monthly to six-monthly, as indicated by the risk assessment | Table 2.1 |
| Bladders | Change according to the manufacturer's guidelines or as indicated by the risk assessment | Table 2.1 |
| Materials | Check bladders are approved against BS 6920 | Info box 2.1 |
| Installation | In cool areas on cold flowing pipes; as close to the incoming supply as possible; mounted vertically to limit trapped debris; with an isolation and drain valve for flushing and sampling; small retained volume; designed to stimulate flow through the vessel | Para 2.39 |
The range in Table 2.1 is wide on purpose. The risk assessment picks the frequency for each vessel, taking into account how much water it exchanges and who uses the system.
Flushing a vessel on the visit
- Find the vessel on the site's schematic and asset list. Note whether it has an isolation valve and a drain valve.
- Follow the written scheme and the vessel maker's instructions for isolating it and draining it. On an unvented hot water system, only someone competent to work on unvented systems should isolate and drain the vessel, following the cylinder maker's instructions too.
- Flush it through and purge to drain, as Table 2.1 says, where that is practical.
- Put the vessel back into service and check the system pressure is back to normal.
- Record what was done, and anything that stopped the flush, such as a missing drain valve.
On a booster set the vessel's gas precharge is checked as part of the pump service. Read booster set servicing for the manufacturers' precharge rules, and use the pump service checklist.
Worked example: two vessels in one plant room
A plant room has a booster set with a diaphragm vessel on its outlet, and an unvented hot water system with an expansion vessel on its cold feed.
- The hot water expansion vessel sits at steady pressure and temperature for long periods, close to the case HSE describes in Info box 2.1. The risk assessment sets a 3-month flush. It has no drain valve, so the contractor records that and the office quotes to fit one.
- The booster set vessel fills and empties as the pumps cycle. The risk assessment sets a 6-month flush, done on the booster set service visit along with the precharge check.
- Bladders: both are changed when the vessel makers' instructions or the risk assessment say so. The bladder material is checked against BS 6920 when parts are ordered.
Who does what
- The dutyholder owns the risk assessment, which sets the flushing frequency for each vessel (L8, para 28; HSG274 Part 2, Table 2.1).
- The maintenance contractor flushes the vessels, replaces bladders or vessels, and reports installation problems such as no isolation or drain valve, or a vessel in a warm space.
- The installer fits vessels as para 2.39 describes, and uses fittings and materials suitable for drinking water. L8 points to the Water Fittings and Materials Directory, which lists approved products (para 59).
What to record, and for how long
- Each vessel's location, make, size and whether it has an isolation and drain valve
- Date flushed, and any reason it could not be
- Bladder or vessel changes, with the part used
- On booster sets, precharge as found and as left
L8 says records of any monitoring, inspection, test or check, with the dates, are kept "for at least five years" (para 72).
Expansion vessels in Field Suite
- Equipment register: each vessel at its site, with make, model, serial number and service interval (size in the notes), its job history, and a QR label that opens its page.
- Contracts: a visit line that repeats every 1, 3 or 6 months, matching the risk assessment, creates the job to book a set number of days before it is due. Visits are set in whole months.
- Build your own form in the form builder: a repeating group per vessel with "flushed and purged" as pass/fail and "isolation and drain valve fitted" as yes/no. A fail answer alerts the office.
- Faults to quotes: a missing drain valve or a failed bladder goes in as a fault found on the visit, marked by urgency, and the office turns it into a quote.
See pump and water services software and planned maintenance software, or book a demo. For the other checks on the same system, read Legionella checks for hot and cold water systems.
Questions
How often should an expansion vessel be flushed?
Table 2.1 gives monthly to six-monthly, as the risk assessment indicates. The building's assessment picks the figure for each vessel.
Do booster set vessels need flushing too?
Table 2.1 has one row, for expansion vessels. HSE describes hydraulic accumulators on boosted supplies separately: correctly installed, they partly fill and empty between pump runs, which reduces the risk of stagnation (Info box 2.1). Whether and how often to flush one is for the risk assessment.
What is BS 6920?
A British Standard HSE refers to in Info box 2.1. Because bladders are often synthetic rubber that may support microbial growth, HSE suggests checking they are approved against BS 6920. Ask the vessel or bladder maker for the approval.
Field Suite is job management software, not a water hygiene consultancy. The building's own risk assessment and the vessel maker's instructions decide how often each vessel is flushed and when bladders are changed.
Sources
- HSE, Legionnaires' disease: Technical guidance, HSG274 Part 2 (second edition, 2024), paras 2.38 and 2.39, Info box 2.1 and Table 2.1. https://www.hse.gov.uk/pubns/priced/hsg274part2.pdf
- HSE, L8 Approved Code of Practice and guidance (fourth edition, 2013), paras 28, 59 and 72. https://www.hse.gov.uk/pubns/priced/l8.pdf
Last checked: 5 November 2026 against the sources above.
Field Suite is job management software for UK building services firms, not a consultancy. Your own risk assessments, contracts and advisers decide what applies to your work. Spotted a mistake? Email info@fieldsuiteapp.com and we correct it.