Investigative approach to improve hot water system hydraulics through temperature monitoring to reduce building environmental quality hazard associated to Legionella

•Unbalanced hydraulics resulted in temperature losses and increased Legionella risk.•Adding secondary recirculation pumps can lead to local flow inversions.•An investigative approach is proposed to improve hot water system hydraulics.•Selection of sampling points in unbalanced system should be based on temperature.•Legionella contamination significantly reduced after hydraulic corrective measures.

Several countries have promulgated control measures and design guidelines to limit the proliferation of Legionella within hot water distribution systems (HWDS). However, there is little information on how to assess and improve existing HWDS unable to maintain water temperatures ≥55 °C throughout the system. A 50-year old hot water system of a 10 story hospital was investigated in terms of temperature distribution and Legionella pneumophila prevalence. Concentrations of L. pneumophila were correlated with the maximum temperature reached at the tap, with a significant decrease observed at T ≥ 55 °C. Continuous temperature and flow monitoring was performed on the overall HWDS, characterizing the principal and secondary horizontal return loops for all 9 wings, and detailed investigations of the secondary vertical return loops was completed in Wing 3. Results indicated the system inability to systematically maintain desired operating temperatures of 55 °C. The deficient hydraulic distribution was the root cause of the poor temperature maintenance throughout the secondary loops, but defective devices were also identified as playing an important role in sectorial temperature failure. A simple stepwise investigative approach was developed to identify hydraulic deficiencies. The implementation of flow restrictions on identified recirculation loops and increased pumping efficiency was conducted within a short period of 2 months, with no major system upgrade. These corrective measures resulted in a balanced system with increased flow velocities (>0.2 m/s). As a result, the proportion of taps achieving 55 °C within 2 min increased from 11% to 74% and L. pneumophila prevalence decreased from 93.1% to 46.1% after 4 weeks.