This guidance document is intended to help minimise risks associated with legionella bacteria in water systems that are due to be restarted following a COVID-19 shutdown and follows on from our initial guidance titled “Pragmatic Solutions on de-commissioning and re-commissioning a property during periods of mothballing”

In compiling this document we have utilised the guidance of the Legionella Control Association on reopening buildings and authoritative legislation and guidance from the current versions of ACoP L8, HSG274 Part 2 and COSHH. No single set of guidance will be suitable to cover every circumstance and UK Law requires each building and situation to be risk assessed on its own merits and appropriate measures taken.

A large number of buildings across all sectors in the UK have encountered periods of complete or partial

Scientific Image  shutdown during the Covid-19 pandemic. This in turn brings about additional risks such as bacterial growth including legionella within water systems.  As the Government begins to lift lockdown restrictions, there are public health risks to be aware of regarding water quality. Further, it is widely accepted that COVID-19 increases risk of legionellosis as a secondary infection.

Simply reopening a building that has stood idle, without addressing the safety of its water system, is unacceptable and is likely to be in breach of the law. If dutyholders are not able to put in place a proper recommissioning process to use the water system safely, they should not reopen the building.

In addition to legionella risk, drinking water retained within buildings may no longer be potable following a period of prolonged stagnation. This is a serious consideration with respect to drinking water fountains and dispensers.

Buildings that have remained empty with static water systems, or those that have been subject to flushing that does not represent normal usage, are likely to require recommissioning. Those that have remained in normal use or where flushing has approximated normal usage (evidenced by water meter readings) may still require additional control measures. The requirements for recommissioning a building water system will vary and the level of work and investment should be appropriate for the risk.

For very simple buildings, flushing alone may be sufficient but for most buildings some form of disinfection is likely to be needed. In the worst cases, repeat disinfection and extensive cleansing flushing may be required to clear contamination.

Control Measures

The following will likely need to be considered as part of a control strategy, but there may be others.


System Flushing

Flushing a water system will change the bulk water and, depending on flow velocity, may have a shearing effect on biofilm and move other contaminants through the system.

Flushing activities need to take into account:

  • Sufficient volume of water change
  • Circulation of hot water service return pipework
  • Flushing deadlegs
  • Removal of fouling such as scale and corrosion products
  • Etc.

Individual risk assessments must determine control measures required. Some systems may be close to the definition of a low risk system and operators may choose flushing as a control measure based on their risk assessment.

Paragraph 59b of ACoP L8 – avoiding water stagnation that may encourage the growth of biofilm is a required control strategy and therefore flushing stagnant water from a system and the mains water pipework will always be needed as an absolute minimum control measure. More complex systems are likely to need further control measures. The recent advice in response to COVID-19 from RSPH, ESGLI and the department for education is to disinfect water systems before bringing them back into use6,7,8.

System Disinfection

A system disinfection will involve any Cold Water Storage Tanks (CWST) to undergo a full drain down, clean and disinfection prior to chemical disinfection of down services pipework. This involves introducing chemical into the CWST and drawing this down to all outlets within the building. The chemical will be left to sit in the system to allow disinfection to take place. The system will then be flushed to remove the disinfectant chemical.

After a period of prolonged stagnation it is possible for a single disinfection to be unsuccessful and the process may need to be repeated. Careful consideration should be had to areas with flexible hoses or tap tails, sections downstream of Thermostatic Mixer Valves or mixer taps which again highlights the importance of site-specific risk assessments.

Validation Sampling

Sampling & Analysis is the only real way of determining water quality within a system and determining the success of cleansing flushing and disinfections. Sampling ultimately provides evidence and reassurance that the system is safe to use. Samples should be taken 2-7 days after disinfection to avoid false negatives following residual chemical in a system following disinfection.

Any service provider should provide documented sampling plans and methodology to address any areas of a water system that are of concern and ultimately take into consideration the risk profile of each building on its own merits.

Long Term Measures

If susceptibility of those exposed to the water system has increased due to COVID-19 (or if more information comes to light from COVOD-19 precautions) the legionella risk assessment should be reviewed. If necessary, the risk assessment should be updated, the written scheme reviewed and risks addressed.

Written schemes of control for legionella should always have included start-up and shut-down procedures for water systems. COVID-19 has highlighted that many schemes do not includethis or that the process is unworkable or unsuitable. Written schemes of control should be reviewed to ensure any future emergency shutdown of a building includes safe decommissioning and recommissioning of the water systems.


Further Reading and References

  1. LCA Guidance on reopening Buildings -full article –
  2. Xing, Q et al., (2020) Precautions are Needed for COVID-19 Patients with Coinfection of Common Respiratory Pathogens –
  3. HSE Legionella Risks during the coronavirus outbreak –
  5. British Standards Institute Published Document PD855468:2015 ISBN 978 0 580 88692 8
  6. RSPH Webinar–the-art-of-water-sampling.html
  7. ESGLI Guidance
  8. Department of Education Guidance

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