A repair is a deliberate disturbance of a system you spend the rest of the year keeping under control. To replace a calorifier, re-route a length of pipe or service a TMV, someone has to isolate, drain and open the system — and the moment they do, you have warm stagnant water standing in dead sections, disturbed biofilm, and very often swarf, flux or jointing compound left behind. Hand that straight back to users and the first person to take a shower breathes in whatever the works stirred up.

The restart is the risky bit, not the repair. A restart checklist exists to make the hand-back an evidence-based decision instead of a “plumber’s gone, turn it on” reflex.

When a repair actually needs a restart check

Not every job triggers this. Changing a washer at one tap is a different animal from replacing a section of incoming main, cleaning the cold water storage tank, or swapping a calorifier. The question to ask is whether the work disturbed water that people will later inhale as a fine mist.

Treat the full restart as necessary whenever the work drained a section, cut into pipework, opened a tank or vessel to the air, introduced new pipe, or sat part-built over a weekend. New pipework that ends up rarely used — a capped spur left for a future connection — is a fresh dead leg, and it belongs on the list too. If the alteration is significant, it should also prompt a review of the Legionella risk assessment rather than being treated as a like-for-like swap [6].

The post-repair restart checklist

Work through these in order. Each item is something you can tick and, more importantly, record against the job. The framework behind them is ordinary competent control — L8 sets the duty and HSG274 sets the technical expectations [1][2].

Before you refill

  • Confirm exactly what was done: which assets, which pipe runs, and the dates the system was drained or open. Write the scope down; it decides everything below.
  • Identify any new or redundant pipework created by the works. Flag genuine dead legs for removal rather than for a lifetime of flushing.
  • Confirm the temperature regime can be restored — that the heat source will bring hot water back up to its design temperature and that cold runs will return below the range where the bacteria multiply [3]. Lukewarm is the failure state to avoid, and it is the one a half-restored system drifts into.

Refill and flush

  • Refill slowly and vent trapped air, so you are flushing water rather than pushing air pockets around.
  • Flush every affected outlet fully open, working outward from the repair, until the water runs clear and the temperature holds steady. This is the core of post-repair flushing: it physically removes the stagnant water and debris the work left behind.
  • Flush showers and spray fittings with the head removed or bagged where you can, to clear the branch without filling the room with aerosol. Refit or replace the head afterwards.
  • Do not stop at the repaired section. Flush the low-use outlets downstream of it — the disabled wing, the cleaners’ sink, the end-of-run tap that the works fed through.

Disinfect and sample where the work calls for it

  • Decide with the competent person whether the job needs disinfection or whether thorough flushing is enough. New pipework, an opened storage tank, or any work that could have introduced contamination usually points to disinfection; see Cleaning and disinfection after remedial work: thermal and chemical approaches for thermal and chemical methods.
  • Where the risk assessment or the nature of the work calls for it, arrange commissioning or post-remedial sampling to a recognised method, with the laboratory and the sampling points agreed in advance [5]. Choosing the right lab matters here — Choosing a UKAS-accredited lab for Legionella testing covers what to look for.

Close out before hand-back

  • Record the restored temperatures, the flushing, any disinfection, and the sample references against the job — not as loose notes but in the logbook.
  • Update the asset register or schematic if the pipework layout changed.
  • Confirm who is signing the system back into service, and make sure users are told if an outlet was out of use.

Working through it on the day

Sequence is the point: isolate, refill, flush, verify temperatures, sample only if required, record, then hand back. Sampling before a proper flush tells you about the works, not about the water people will actually use, so it rarely earns its cost. Frequency and type of testing follow the system and the risk assessment, not a fixed calendar — HSE is explicit about that [4].

The habit worth building is to record the decision, not just the task. “New 22mm run to the third-floor showers; chemically disinfected, flushed to clear, two samples taken at 48 hours from the two new outlets, returned to service by the responsible person on hand-back” is a record that proves control. “Flushed — OK” is not. If you are still chasing those notes across paper sheets after every job, that friction is exactly the case Is a digital logbook worth it? Cost-benefit analysis makes for a digital logbook.

The steps that quietly get skipped

Most failed restarts are not technical. They are the four things people leave off the end.

The downstream outlet. The repaired riser gets flushed and the dead-end spur feeding the old store cupboard sink does not — and that is the one that sits unused until it sprays.

The shower head. The branch is flushed clean and the original contaminated head and hose go straight back on, so the next user gets an aerosol of exactly what you were trying to clear.

The weekend gap. A system left part-drained on Friday afternoon and nobody flushes it before Monday’s first use. Two warm, still days is plenty; treat the first Monday draw-off as a restart, not a normal use.

The paperwork. The work is done well and never closed out, so there is no evidence the outlet was safe when it went back into service. The duty holder owns that record and that decision; a contractor performs the task, but the accountability does not transfer with the job [7].

A fresh caveat for this one

This is general guidance, not a method statement for your building. How long to flush, whether a given repair needs disinfection, and when (or whether) to sample are site-specific calls that belong to the competent person who scoped the work and to your written control scheme. The temperatures and timings mentioned here are described in plain terms on purpose — confirm the exact figures and the post-works sampling approach against your own risk assessment before you rely on them, and never let a clear sample stand in for control you know was disturbed.

What to do next

Pull your last three repair or remedial jobs and check each one for a recorded restart: temperatures restored, outlets flushed, any disinfection and samples logged, and a named person who returned the system to service. Wherever that trail is missing, that is the gap to close — and the restart checklist above is the template for the next job, and the one after it.

FAQ

Do we have to disinfect after every repair, or is flushing usually enough?

Not after every repair. Thorough post-repair flushing clears the stagnant water and debris a job leaves behind and is often sufficient for minor work that did not open the system widely. Disinfection is the call where new pipework was installed, a storage tank was opened or cleaned, or the work could have introduced contamination. The competent person who scoped the job should decide, with your risk assessment setting the default.

How soon after the repair should we take a Legionella sample?

Only where the risk assessment or the nature of the work calls for a sample — it is not automatic. When it is needed, it is usually taken once the system is back in normal use rather than straight after flushing, so the result reflects real operating conditions, and to a recognised sampling method with the points agreed beforehand [5]. Testing frequency follows the system and the risk assessment, not a set interval [4], so confirm the timing with your competent person.

A contractor changed a tap over the weekend — does that need all of this?

Scale it to the work. A single tap swap that did not drain the wider system mainly needs a good flush of that outlet and a temperature check. But if the fitting sat unused or part-built across the weekend, treat its first use as a low-use restart and flush the stagnant water through before anyone draws from it.

Sources

[1] 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 [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [5] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [6] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1 [7] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm