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Thermal Mixing Valves and Legionella Risk

Biofilm in a shower hose
Biofilm in a shower hose
Biofilm in pipework
Biofilm in pipework

Thermal Mixing Valves and Legionella Risk

We always have the same comment when people tell us that they use thermal control for Legionella – what happens after the Thermal Mixing Valve?

Thermal treatment works very well before the TMV, but beyond this point the water temperature is often capped in the upper 40 degrees centigrade area. Whilst this is needed to prevent scald risk it also provides the perfect habitat for biofilm, Legionella and Pseudomonas.

This is exacerbated if, as is common, the outlet after the TMV is a shower and uses a flexible hose – the photo on the left (above) is an electron microscope view of a standard shower hose.  It shows the structure of the hose and the biofilm that has built up in the hose.  The second photo also shows a bacterial colony sampled from a shower hose.  If you combine these conditions with an occasionally used shower then you should expect to get biofilm and possibly even Legionella and Pseudomonas colonisation in the pipework.   

The HSE suggests that to control the risk the maximum pipe length between the TMV and the outlet is 2m – despite searching to see if there is a risk based derivation of this distance we have been unable to find any record of how 2m was deemed the ‘safe’ distance.  Indeed it appears to be an arbitrary number, most likely based on someone’s view of what the likely distance between a TMV and an outlet is in a new build water system.

Pseudomonas Aeruginosa in Taps

The problem is not just confined to showers, the photos to the right (above) are electron microscope scans of samples taken from taps. The authors of “Investigation of healthcare-acquired infections associated with Pseudomonas aeruginosa biofilms in taps in neonatal units in Northern Ireland”J.Walker et al (which is well worth reading - found that Pseudomonas was able to form biofilm and colonies within taps and flow straighteners – “P. aeruginosa was predominantly found in biofilms in flow straighteners and associated components in the tap outlets and was a possible source of the infections observed. Healthcare providers should be aware that water outlets can be a source of P. aeruginosa contamination and should take steps to reduce such contamination, monitor it and have strategies to minimize risk to susceptible patients.

Managing Risks Beyond the TMV's

This all points to the fact that thermal treatment is not effective beyond thermal mixing valves. It is important to be aware of this and to make sure that you address this point. Breaking down and cleaning the shower heads and flexible pipework and the taps can help, or alternatively you could undertake continuous treatment of the whole system right through the end user.  We would love to show you how the Aquadron can treat the water system all the way to the end user and also how it can help you to reduce your water temperature and save money – it’s a win / win situation! Click here to arrange a call back.

Click here for more details on the Aquadron Legionella System.

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