Legionella & Building Shutdowns: Reopening Water Systems Safely

This guide is for facilities managers, building owners, and employers responsible for any commercial, industrial, or institutional premises that has been shut down or substantially unoccupied for 7 or more days.

Why Building Shutdowns Create Legionella Risk

Under normal operating conditions, several factors keep Legionella under control in a well-managed water system: temperature (hot water above 60°C at storage, cold water below 20°C), regular throughput that prevents stagnation, and where necessary, chemical treatment. A building shutdown disrupts all of these. During a shutdown:

• 1 Hot water systems cool into the Legionella growth temperature range (20–45°C) as no hot water is drawn and storage temperature drops
• 2 Cold water warms above 20°C in pipework exposed to ambient building temperatures
• 3 Water in dead legs, little-used branch pipes, and infrequently used outlets becomes completely stagnant
• 4 Chlorine residual in mains water dissipates, leaving no chemical protection against bacterial growth
• 5 Biofilm in pipework, the habitat in which Legionella thrives, becomes more established

The longer the shutdown, the greater the risk. A building that has been unoccupied for several weeks or months can have very high levels of Legionella in the water system. The risk is not merely theoretical: multiple Legionnaires' disease outbreaks have been traced to water systems that were recommissioned following periods of stagnation without appropriate precautions.

Step-by-Step: Recommissioning Water Systems After a Shutdown

Step 1: Notify Your Water Treatment Specialist Before Reopening

Do not wait until the building has reopened to manage the Legionella risk. Contact Halpin & Hayward or your appointed water treatment specialist in advance of reopening. An experienced specialist will carry out a pre-opening risk review, advise on the flushing and disinfection programme required, and arrange post-recommissioning sampling to confirm the system is safe before occupants use the water.

Step 2: Review the Risk Assessment

Review the existing legionella risk assessment and log. Has the system been modified during the shutdown period? Have any new water systems been installed or removed? Has the building use changed? Any of these changes require the risk assessment to be updated before reopening.

Step 3: Check Temperatures Before Flushing

Before flushing begins, check the temperature of hot and cold water at key points. Cold water should be at or below 20°C at tank outlet; hot water should be at 60°C at calorifier. If temperatures are outside the target range, address the temperature control issue before proceeding, since flushing a system whose hot water is at 35°C will disperse, not destroy, Legionella.

Step 4: Systematic Flushing Programme

Carry out a systematic flushing of all outlets, starting at the furthest points from the tank and calorifier and working back. Each outlet should be flushed for at least 5 minutes, or until the water runs hot (from hot outlets) and cold (from cold outlets) and remains at the correct temperature. Do this before occupants are in the building, ideally when the building is still unoccupied. Points to include in the flushing programme:

• 1 All hot and cold taps, including those in rooms that are rarely used
• 2 All showers, the highest-risk outlet for aerosol creation
• 3 Any drinking water coolers, ice machines, or water dispensers
• 4 Any outlets connected to the hot water system that may have been isolated

Step 5: Consider Precautionary Disinfection

For longer shutdowns (4 weeks or more), or where the risk assessment indicates elevated risk, precautionary thermal or chemical disinfection of the system may be appropriate before the systematic flushing programme. Thermal disinfection involves raising the hot water temperature to 70°C or above and flushing all hot water outlets at that temperature. Chemical disinfection using chlorine dioxide or hyperchlorination can be used where thermal disinfection is not practical. Halpin & Hayward carries out precautionary disinfection for commercial and institutional premises before reopening following extended shutdowns.

Step 6: Post-Recommissioning Water Sampling

Following flushing and any disinfection, take water samples from sentinel outlets (hot and cold), tank outlets, and calorifier outlets in accordance with HSG274. Dispatch to an INAB-accredited laboratory for analysis. Do not allow unrestricted use of the water system until satisfactory results (< 100 cfu/L Legionella) have been confirmed.

Step 7: Update the Monitoring Log and Risk Assessment

Record all actions taken, including flushing dates, temperatures, disinfection activities, and sample results, in the monitoring log. Update the risk assessment to reflect the post-shutdown position of the system. Set the schedule for the next review.

Target Temperatures & Post-Recommissioning Checks

Use this table as a quick reference for the checks that should be carried out before, during, and after a recommissioning flush.

Do not allow unrestricted use of the water system until sentinel outlet, tank outlet, and calorifier samples all return satisfactory results.

Seasonal Buildings and Annual Shutdowns

Hotels, holiday parks, schools, and other seasonally operated buildings face this challenge every year. The recommissioning procedure outlined above should be incorporated into the annual opening checklist.