The importance of a balanced domestic hot water system

The UK’s Legionella guidance, HSG 274 and the equivalent HTM0401 for healthcare sites, requires the testing of primary, secondary and tertiary loops in a domestic hot water system.

Both documents explain what a primary, secondary and tertiary loop is, putting this into practice in a complex system can be difficult however, but it is important.

The temperature of the hot water within the system is used as a control measure for Legionella bacteria and a poorly balanced system will undoubtedly lead to non-compliant temperatures in pats of the system.

This was investigated in a scientific report in 2017, ‘Investigative Approach to Improve Hot Water System Hydraulics through Temperature Monitoring to Reduce Building Environmental Quality Hazard Associated to Legionella | Elsevier Enhanced Reader’. Accessed 12 October 2019.

The abstract of this report summarised the findings as:

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>55oC. 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 55oC. 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 step wise 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 55oC within 2 min increased from 11% to 74% and L. pneumophila prevalence decreased from 93.1% to 46.1% after 4 weeks.

It can be seen therefore that a poorly balanced system can lead to a significant increase in the level of Legionella bacteria found within a system and hence the importance of ensuring a system is properly balanced.

The report also suggests that having the correct flow velocity through the system had an impact of the level of Legionella positivity within the system.

This report shows how important an holistic approach to Legionella is and there is no simple “silver bullet” to managing the problem.

If you want to discuss the application of the information above to your systems then contact Collaton Consultancy via: or phone on +44 (0)7958 124563.

Collaton Consultancy Limited are expert Legionella consultants working for both water treatment companies and end users alike, Expert Witness services are also offered should a legal case arise. If you have any specific issues relating to the above you would like help with then contact Collaton Consultancy Limited

Collaton Consultancy Limited

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