Legionella and Winter Shutdowns

Legionella Control During Winter Shutdowns: Managing Little-Used Outlets and Safe Recommissioning

When a building closes, or part of it falls quiet, water stops moving. Stagnant water in pipework drifts towards the 20 to 45°C range where Legionella bacteria multiply, and biofilm builds on pipe surfaces. HSE guidance expects any outlet that is not used regularly to be flushed at least weekly, and a closed building to have a plan for safe recommissioning before it reopens.

Half term, Christmas and the quieter winter months change how buildings are used. Schools close for weeks at a time, offices run on skeleton occupancy, hotels shut seasonal wings, and leisure sites reduce hours. Each of those changes affects how water moves through hot and cold systems, and that is where legionella risk builds.

Legionella is often thought of as a summer problem. It is not. In March 2026, the UK Health Security Agency reported a Legionnaires' disease outbreak in London with a case count it described as unusual for that time of year (UKHSA Health Protection Report). Water systems need managing all year round, and periods of low use need specific attention.

Why does a winter shutdown increase legionella risk?

A shutdown increases risk because Legionella control in hot and cold water systems depends on two things: keeping water moving and keeping it at the right temperature. A closure undermines both.

• Stagnation. Water sitting still in pipes, tanks and outlets gives bacteria time to multiply and allows sediment to settle.

• Temperature drift. Cold water warms up in heated buildings, and hot water cools when heating and hot water plants are set back to save energy. Both can end up in the 20 to 45°C growth range.

• Biofilm. Slow-moving water encourages biofilm on internal pipe surfaces, which shelters bacteria from heat and disinfectant.

• Dead legs. Sections of pipework with little or no flow effectively become dead legs for the duration of the closure.

  
  

These conditions do not need weeks to develop. HSE's technical guidance treats an outlet as little used once it has gone seven days without use, which is shorter than most school holidays.

What counts as a little-used outlet?

A little-used outlet is any tap, shower or other water outlet that is not used for a period of seven days or more, as defined in HSE technical guidance HSG274 Part 2. During a closure, that can be almost every outlet in the building.

Outside of closures, the outlets most likely to slip through are those nobody owns day-to-day:

• Showers in changing rooms, spare bedrooms or rarely used accessible bathrooms

• Taps in closed classrooms, meeting rooms or unoccupied office floors

• Emergency showers and eyewash stations

• Outlets in empty let units or vacant flats

• External hose points and cleaners' sinks

• Hot and cold feeds to equipment that has been removed, leaving a capped pipe behind

  
  

The last example matters. Removing a sink or appliance without cutting the pipework back to the main supply leaves a blind end, which HSG274 Part 2 identifies as a stagnation risk in its own right.

How often should little-used outlets be flushed?

Little-used outlets should be flushed at least weekly, unless your legionella risk assessment sets a different frequency (HSE). Flush to drain, avoid releasing spray or aerosol, and record every flush.

Flushing on its own is not enough to demonstrate control. It sits alongside routine temperature checks, and both should continue through a closure. The table below sets out the core checks from HSE's published guidance for hot and cold water systems.

Check	Frequency	Target
Flush little-used outlets	Weekly	Purge to drain, without releasing aerosol
Cold water sentinel outlets	Monthly	Below 20°C after running for 2 minutes
Hot water sentinel outlets	Monthly	At least 50°C within 1 minute
Calorifier outgoing water	Monthly	At least 60°C
Showerheads and hoses	Quarterly, or per risk assessment	Dismantled, cleaned and descaled

Source: HSE, hot and cold water systems.

If an outlet is genuinely not needed, removing it can be a better long-term answer than flushing it forever. The pipework must be cut back as close to the main supply as possible so that no dead leg is left behind.

Should you drain down the system or keep it running?

For most hot and cold water systems, keeping the system live and maintaining a weekly flushing regime is easier to control than a full drain down. Draining a system rarely removes all the water. Residual water left in low points, valves and fittings still stagnates, and a dry system can corrode or dislodge debris when it is refilled.

Draining down can be appropriate for a building that will be empty for a long period and where nobody is available to flush. That decision should be made in advance, recorded, and reflected in the legionella risk assessment, not taken by whoever locks up on the last day.

If you do keep systems live, HSG274 Part 2 also advises that buildings taken temporarily out of use should be clearly signed and secured so that nobody uses the water without knowing the system has been out of service.

How do you safely recommission a water system after a shutdown?

Recommissioning should follow a written plan, agreed before the building closes. HSG274 Part 2 (paragraphs 2.50 to 2.52) recommends that buildings that have been temporarily shut down have a plan in place for recommissioning each water system. As a minimum, small and simple systems should be thoroughly flushed through with fresh mains water before normal use.

A practical recommissioning sequence for hot and cold systems looks like this:

1. Restore temperatures first. Bring calorifiers back to at least 60°C and allow the hot water circuit to reach temperature before flushing.

2. Flush every outlet. Work systematically, usually from the outlet nearest the supply to the furthest, flushing to drain without creating spray.

3. Check temperatures at sentinel outlets. Hot should reach at least 50°C within 1 minute and cold should be below 20°C within 2 minutes.

4. Decide whether cleaning and disinfection is needed. For longer closures, larger systems or sites with vulnerable occupants, flushing alone may not be sufficient.

5. Consider sampling. Where disinfection is carried out or the risk is higher, Legionella sampling to BS 7592 can confirm the system is back under control. Samples should not be taken immediately after disinfection; industry guidance based on HSG274 Part 2 recommends waiting 2 to 7 days.

6. Record everything. Flushing logs, temperatures, disinfection certificates and sample results all form part of your evidence of control.

   
   

Anyone carrying out flushing after a long closure should be protected. The water in a system that has stood unused may carry a higher bacterial load than normal, so minimising aerosol is essential.

Who is responsible for legionella control while the building is closed?

Responsibility does not pause when the doors close. Under the HSE Approved Code of Practice L8, the duty holder remains responsible for controlling legionella risk, and the appointed responsible person should continue to manage the control scheme throughout the closure.

The most common gap is practical rather than legal: holiday cover. Before a closure, confirm in writing:

• Who will carry out weekly flushing, and on which day

• Who will cover if that person is on leave or unwell

• How they will access the building and every outlet

• Where records will be kept and who will check them

  
  

Does it differ by sector?

The principles are the same everywhere, but the pressure points differ.

• Education. Christmas and half-term closures are predictable, which makes them easy to plan for. Our school summer checklist covers similar steps for the longer summer break.

• Offices and commercial property. Hybrid working leaves whole floors quiet for long periods, even when the building is technically open. Landlords and managing agents should agree with tenants who flush outlets in let areas.

• Hotels and leisure. Closed seasonal wings, spa facilities and changing rooms need the same weekly regime as the rest of the building.

• Healthcare and care settings. These buildings rarely close, but empty rooms and beds create little-used outlets, and the occupants are among the most vulnerable to Legionnaires' disease.

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For sites in Ireland, the same principles apply under the HPSC National Guidelines for the Control of Legionellosis (2009).

Planning Your Shutdown Before the Doors Close

The safest shutdowns are planned weeks in advance. Before your building closes, work through these five steps:

1. List every outlet that will go unused, including those in plant rooms, external areas and empty units.

2. Name who will flush each outlet, when, and who will cover holidays.

3. Set out how flushing and temperature checks will be recorded.

4. Agree your recommissioning steps, including when cleaning, disinfection or sampling will be triggered.

5. Review your legionella risk assessment if the way the building is used has changed.

   
   

ISHEM is an independent consultancy. We assess and advise, and we have no financial interest in flushing devices, disinfection or remedial works. Our team includes members and registrants of the Legionella Control Association, the Chartered Institution of Water and Environmental Management and the Water Management Society, and we operate in line with the LCA Code of Conduct for Service Providers.

We can review your shutdown and recommissioning plan, support your responsible person through our compliance monitoring service, or deliver legionella awareness training for the people doing the flushing. Read more about our wider water management services or our guidance on the HSG274 Part 1 update.

Book an independent compliance review

Frequently asked questions

Does turning off the hot water over Christmas increase Legionella risk?

Yes. When hot water plant is switched off or set back, stored and circulating water cools into the 20 to 45°C range where legionella multiplies. If you plan to switch off hot water, agree in advance how the system will be brought back to at least 60°C and flushed before anyone uses it.

How long can water stand in pipes before it becomes a Legionella risk?

HSE technical guidance treats an outlet as little used if it has not been used for seven days or more. From that point, weekly flushing should be in place, unless your risk assessment specifies a different frequency.

Do I need to test for Legionella after a building shutdown?

Not always. Small, simple systems after a short closure may only need thorough flushing and temperature checks. Sampling to BS 7592 is worth considering after longer closures, after disinfection, or where occupants are vulnerable

Is it better to remove little-used outlets altogether?

Often, yes. Removing an outlet that is no longer needed eliminates the flushing burden, but only if the pipework is cut back to the main supply. Leaving a capped pipe creates a blind end, which is a stagnation risk in itself.

Does the weekly flushing rule apply in Ireland?

In Ireland, legionella control is guided by the HPSC National Guidelines for the Control of Legionellosis (2009). Regular flushing of infrequently used outlets is equally expected, and the frequency should be set by your site's risk assessment.