Combating the risk of Legionella during an interruption in the use of water systems

If the building, water system or ship is closed for less than a month or if the building or ship has fewer users than normal, continuing with the usual control measures is recommended.

Further reading: Preventing the growth and health risks of Legionella

Below are recommended measures used if the building, water system or ship is closed for longer than a month but the aim is to be able to re-open without delay safely without a risk of Legionella.

• Maintain the usual control programme so that the hot water temperature in the storage tank, heat exchanger and inflow pipework is ≥ 60–65 °C and in the outflow piping is ≥ 50–55 °C.
• Ensure that the hot water temperature of ≥ 50–55 °C is achieved at all outlets within 1 minute of starting to run the water and the cold water temperature of ≤ 20 °C within 1 minute of starting to run the water (normal flow, without splashing). If you use biocides such as chlorinated chemicals, maintain the target levels of the active substance throughout the whole system.
• Flush (to reduce aerosols) all hot and cold outlets (showers and taps) at least weekly until they achieve the above temperatures. Flush all WC cisterns, urinals, by-passes and any other points on the network, including little-used outlets.
• Ensure drinking water storage tanks remain at 0.5–1.0 mg/L of free chlorine, for example on ships where chlorination is used.
• Monitor to verify that these temperature levels are achieved at all, including little-used, outlets.
• Document these checks, including reviewed and amended risk assessments, monitoring data and remedial actions, with evidence of who carried out the work, the time, date and signature.

• Check that the appropriate usage, monitoring and documentation practice is adhered to for other systems, such as cooling systems, swimming pools, and spa pools.
• When a pool is closed temporarily, the pool water heating can be turned off. Maintain the free chlorine level in the pool at 0.5–1.0 mg/L and the water flow rate at 50–60%. It is not recommended to drain pools completely, unless the safety of using them in future is secured, for example by shock chlorination. For more detailed instructions about interrupting pool use in English see, for example:
  Pool Water Treatment Advisory Group: Guidance on temporary pool closure
• Before opening, the pool water must meet the controlled values set out in the Finnish Ministry of Social Affairs and Health’s decree on swimming pool water (315/2002, in Finnish).
• Disinfect other systems before starting to use them again.
• Restart supplementary heating system (heat storage, solar panels, etc.) pumps weekly, if possible.

Below are recommended measures used if the building, water system or ship is closed for longer than a month but the aim is to be able to re-open without delay safely without a risk of Legionella.

• Maintain the usual control programme so that the hot water temperature in the storage tank, heat exchanger and inflow pipework is ≥ 60–65 °C and in the outflow piping is ≥ 50–55 °C.
• Ensure that the hot water temperature of ≥ 50–55 °C is achieved at all outlets within 1 minute of starting to run the water and the cold water temperature of ≤ 20 °C within 1 minute of starting to run the water (normal flow, without splashing). If you use biocides such as chlorinated chemicals, maintain the target levels of the active substance throughout the whole system.
• Flush (to reduce aerosols) all hot and cold outlets (showers and taps) at least weekly until they achieve the above temperatures. Flush all WC cisterns, urinals, by-passes and any other points on the network, including little-used outlets.
• Ensure drinking water storage tanks remain at 0.5–1.0 mg/L of free chlorine, for example on ships where chlorination is used.
• Monitor to verify that these temperature levels are achieved at all, including little-used, outlets.
• Document these checks, including reviewed and amended risk assessments, monitoring data and remedial actions, with evidence of who carried out the work, the time, date and signature.

• Check that the appropriate usage, monitoring and documentation practice is adhered to for other systems, such as cooling systems, swimming pools, and spa pools.
• When a pool is closed temporarily, the pool water heating can be turned off. Maintain the free chlorine level in the pool at 0.5–1.0 mg/L and the water flow rate at 50–60%. It is not recommended to drain pools completely, unless the safety of using them in future is secured, for example by shock chlorination. For more detailed instructions about interrupting pool use in English see, for example:
  Pool Water Treatment Advisory Group: Guidance on temporary pool closure
• Before opening, the pool water must meet the controlled values set out in the Finnish Ministry of Social Affairs and Health’s decree on swimming pool water (315/2002, in Finnish).
• Disinfect other systems before starting to use them again.
• Restart supplementary heating system (heat storage, solar panels, etc.) pumps weekly, if possible.

Below are recommended measures for situations where the water system is to be decommissioned for a longer period of time. Longer-term and thorough decommissioning may involve situations in which

• a building, water system or ship is closed for more than one month, or
• water is not heated due to energy saving, or
• hot water is not used.

• Turn off the hot water storage tank or heat exchanger, close the water inlet and drain the system.
• If the system has not been disinfected recently or there have been problems with temperature or biocide control, carry out a full system disinfection with 50 mg/L of free chlorine (1 hour) or the equivalent 10 mg/L chlorine level (5 hours), 5 mg/L chlorine level (10 hours) or other biocides such as peracetic acid. Free chlorine corrodes dirtier pipework faster. Therefore it is important to monitor the amount of chlorine that reaches the outlets. Especially when using the lowest level of chlorine, 5 mg/L, chlorine may not reach all outlets, so the cleaning effect is lower than desired.
• After disinfection, flush through and refill. Check the biocide is at the highest target normal operating level at the furthest outlets.

• Disinfect the entire water system by running water containing 5–50 mg/L of free chlorine or the equivalent biocide from all outlets. The target level should also be achieved at the furthest outlets. Leave to work for 1 hour if the level is 50 mg/L, 10 hours if the level is 5 mg/L and 5 hours if the level is 10 mg/L. Flush out the water and refill the system. Add chlorine to the water until the usual disinfectant level is achieved (the free chlorine level is usually at least 0.5–1.0 mg/L in tank water).
• Refill the hot water storage tank and heat the water to 60–65 °C. When the storage tank / storage water has been heated to 60–65 °C throughout, open the valves and flush through all outlets taking care to avoid any scalding risk.
• Monitor the temperatures and if necessary the biocide levels, monitor the situation and adjust as required for at least 48 hours.
• Then take Legionella analysis samples from the furthest outlets. Wait for 48 hours before taking samples, as samples taken before this may give false negative results.
• When you are satisfied the hot and cold water systems are under control you can reopen the building, water system or ship.
• Document amended risk assessments and monitoring data, with evidence of who carried out the work, time, date and signature.
• Comply with the instructions for other water systems and equipment (see “Other water systems”).

• All water systems that have been drained pose a risk when recommissioned, unless they are very small and simple and can be completely drained with certainty. There may still be remaining pockets of water and condensation which enable the growth of micro-organisms including Legionella.
• Carry out full system disinfection flushing through to all outlets to achieve 5–50 mg/L of free chlorine or equivalent biocide and leave to work for 1–10 hours, longer for lower levels of biocide. Drain the water system.
• Before opening the water system, follow the instructions for restarting the water system.