top of page

Coming out of lockdown

What to do to protect yourself and your employees as well as the general public

This guidance is intended to provide information for businesses helping to minimise risks associated with legionella bacteria in water systems restarting post a COVID-19 shutdown. In no way does this guidance replace or supersede current versions of ACoP L8 or HSG274 Part 2. There are many guidance documents circulating that give guidance on how to reopen buildings and recommission water systems following a vacant period due to COVID-19. No guidance is going to be suitable for every circumstance and UK Law requires each situation to be risk assessed and appropriate measures taken.

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. 

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 may still require additional control measures due to the additional risk factors below. 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.

Additional Legionella Risk Factors - Note: This should not be considered an exhaustive list. During the COVID-19 period there may have been a number of factors that will increase both legionella risk in water systems and the risk of legionellosis. These may need to be considered in the risk assessment process:

  • Rushed planning for the shutdown, possibly a shortage of staff

  • Sub-optimal decisions on building closure

  • Lack of resource to maintain buildings while they are closed

  • Stagnation

  • Bacterial growth

  • Degradation of system components and build-up of by-products of that degradation particularly where oxidising biocides are continually dosed

  • Mechanical deterioration as a result of corrosion and settlement of suspended solids

  • Temperature increase within the building – facilitates bacterial growth and accelerates degradation of components

  • Circulating HWS leading to thermal gain in the CWS

  • Heating systems leading to thermal gain in the CWS

  • General thermal gain particularly in buildings with glass walls or a high percentage of windows and air conditioning switched off

  • Temperature increase in the water network – less use in non-residential areas (e.g. office parks) has led to some increase in the network temperature locally

  • Potentially flushing was less effective than the water meter readings may indicate

  • More difficult to maintain <20oC within buildings

  • Stand-alone or non-water system items may not have been considered during shut down

  • Ultrasonic humidifiers filled from the cold water outlets

  • Point of use water dispensers

  • Vending machines

  • COVID-19 increases risk of legionellosis as a secondary infection

  • Consider susceptibility for legionellosis before staff return

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

NB always minimise splashing and aerosolation with the use of a bag put over the fitting with the corner cut off. This will ensure the spray is contained in the bag. In healthcare a separate new bag is required for each fitting to prevent cross contamination.

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. Flow velocity is likely to be reduced by outlet fittings and flow restrictors. These will reduce the effectiveness of flushing and consideration should be given to temporarily removing these during the process. Flushing activities need to take into account:
• Sufficient volume of water change
• Circulation of hot water service return pipework
• Flushing deadlegs and deadends
• Removal of fouling such as scale and corrosion products
• Etc.
BSI document PD855468 differentiates between hygiene flushing (to prevent stagnation) and cleansing flushing (to remove debris or organic matter) and it states that it is important to include cleansing flushing as part of the recommissioning process rather than hygiene flushing. All valves should be operated in the fully open position so that any particulate matter can be flushed from the supply pipe. Of particular importance are float-operated or other restrictive valves which need to be induced to fully open to ensure clearing of particulates and prevent fouling of the valve. Where a
clearing velocity cannot be achieved, consideration should be given to removal of valves to enable the cleansing flush.

Cleansing flushing for recommissioning needs to be a considered process and is likely to be more involved than simple hygiene flushing to prevent stagnation. Individual risk assessments must determine control measures required but for low risk systems, as defined in HSG274 Part 2 info box 2.2, a cleansing flush may be sufficient to allow recommissioning of the system. 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 use.

System Disinfection
Disinfection of a water system can be achieved thermally (normally hot water systems only) or chemically. There is good guidance on this process in HSG274 Part 2 2.126-2.137 but note, the reference to BS8558 is no longer current. Readers should now refer to guidance in BSI PD855468 for flushing and disinfection. After a period of prolonged stagnation it is possible for a single disinfection to be unsuccessful and the process may need to be repeated. Some areas may require more attention during disinfection and maintaining a slow flow of disinfectant over these areas
for the full contact time may improve the chances of success. Areas with flexible hoses or tap tails, sections downstream of TMVs or mixer taps or other problem areas may benefit from this process.
In line with BSI PD855468 it may be helpful to flush the stagnant water from system prior to commencing the disinfection, to remove bulk contamination and reduce the likely demand on the disinfectant. Draining and refilling storage tanks (if present) may speed this process. Disinfection should be carried out in line with normal practice and impact on any building occupants should be considered.


If in doubt use a reputable water compliance specialist in Legionella Control. Risk Assessments will always provide the pathway to a good water system if carried out correctly by a competent Risk Assessor.

bottom of page