Thermostatic Mixing Valves & Water Hygiene

Authors: David Harper & Dr. Catherine Whapham

Contamination of Thermostatic Mixer Valve (TMV) Taps

In August 2022, Meda & Gentry from Frimley Park Hospital, published a letter in the Journal of Hospital Infection containing positive evidence of good water hygiene practice in action. 

In 2018, following the persistent presence (despite remediation, cleaning, disinfection, retesting) of Pseudomonas aeruginosa within TMV-taps in the neonatal intensive care unit (NICU), a targeted risk assessment was completed with the NICU matrons, and a decision was taken to replace all outlets with non-TMV taps.  Recognising that the risk of healthcare-acquired-infection posed by Pseudomonas aeruginosa contamination was significantly higher than the chance of scalding in this vulnerable patient group and then acting upon it is to be applauded.  Over the last 4-5 years, Frimley Park Hospital have followed a risk assessment approach and removed approximately one third of TMV-taps, replacing them with non-TMV taps.  This has been completed without any reported incidents of scalding. 

All TMV-taps in areas where scalding risk of vulnerable patient is considered low, such as staff only areas and augmented care wards, have been risk assessed and subsequently removed.

Hazards in Design Specification

In 2011 the National Health Service stated that “scalding by water used for washing/bathing” would be a “never event”.  At that time the considerable impact of waterborne pathogen contaminated componentry within TMV-taps was not fully understood, and following subsequent publications such as those from Walker et al., 2014; Moore et al., 2015;  Garvey et al., 2019, the NHS would certainly not wish to have vulnerable patients knowingly exposed to contaminated componentry or water. 

In reality many public buildings (healthcare, offices, leisure centres, shopping centres, residential apartment blocks etc) are opened with every water outlet under TMV control.  TMVs have become default fixtures in newbuild projects.  How did that happen?  Vigilance during the design, specification and commissioning process must be applied to ensure a realistic risk assessment is completed and that TMV-taps are used in areas where vulnerable users warrant them.  The burden of costs to maintain and service TMV-taps, plus repeat testing and remediation to try and keep them pathogen free is considerable.  They may be unacceptable resource burdens for most facilities when maintained correctly.

Risk Assessing for Removal of TMV Taps

There is often considerable angst when risk assessing for the removal of already installed TMV-taps.  Meda & Gentry’s recently published experience will help those responsible for water systems within buildings to re-risk assess their user population and remove TMV taps where they are not considered to be required or sensible, thus reducing the risk of harbouring and amplifying waterborne pathogens.   

Who is at Risk of Scalding?

According to National Institute for Health & Care Excellence (NICE), children under 5 years old and the elderly (> 70 years) are most at risk of burn injury. In the elderly group this is mainly due to:

  • reduced mobility
  • sensory impairment
  • slowed reaction times
  • thinner skin

There are approximately 250,000 burn injuries per year in the UK, with 175,000 cases attending A&E and 16,000 cases admitted to hospital.  Approximately 300 individuals die each year from their burn injury.  Burns are complex, and the seriousness of burn injury should not be underestimated.

Burns may be caused by:

  • flame
  • scalding with hot liquid or steam
  • contact with hot surfaces (e.g. iron, oven door, radiators)
  • electrical and/or chemical sources

How Common are Hot Water Scalding Events?

Scalds are the most common type of burn, mainly from spillage of hot drinks and liquids and sometimes from immersion in too hot a bath or shower.  NHS Digital report 668 hospital admissions from scald injuries between April 2021-March 2022.  Shields et al (2013) estimates hot tap water causes 25% of all scald burns in the US and leads to an estimated 1500 hospital admissions and 100 deaths per year.  The authors also note that tap water scalds primarily occur at home in residential kitchens and bathrooms.  Singer & co-workers (2022) have published that 92% of scalds in Australia and New Zealand are from the bathroom based on 650 individuals seeking hospital support over an 8 year period (approx. 80 per annum).

A recent German study by Schulz et al (2020) evaluated patients admitted to an adult burn intensive care unit over a 25 year period (1989-2014) and compared those with scalds from hot tap water to those with other scald types.  A total of 333 patients were enrolled in this retrospective study, including 78 patients (23.4%) with scalding associated with hot tap water – which represents 3 adults per annum admitted with hot water scalding at this burns unit. Other scalds were caused by hot drinks or soup, hot water released from machines, liquified metal or hot liquid from the car radiator.

The Role of Thermostatic Mixing Valves

Hot tap water is a hazard and has an established link with scalding.  Thermostatic Mixing Valves placed at the periphery of the water system, optimally positioned within the outlet itself, can reduce exposure to hot water temperatures likely to lead to scalding.  Hot water temperatures downstream of a TMV are restricted through automatic blending of mains hot water with cold water and are typically between 37-43 oC  at the outlet.   However, Thermostatic Mixing Valves are not risk-free.  Not all TMVs are the same construction or materials, and more modern versions may be made with less complexity, less microorganism-attractive materials and surfaces, and may be easier to fully disassemble and clean.   However, TMVs are more complex in construction and more expensive vs a non-TMV manual mixing outlet, they attract microorganisms which adhere and can lead to biofilm formation, and they require regular (annual or more frequent) service and maintenance.  TMVs are not “fit and forget” devices.  Additionally they do fail, so regular monitoring of temperatures downstream remains important.

Unwanted Pathogens in our Plumbed Water Systems Causing Infection

Opportunistic waterborne pathogens present in our plumbing systems lead to major and preventable healthcare-acquired infections (HAI).  Indeed, water distribution systems are likely the overlooked source of HAI. 

The rate of Legionnaires’ disease should not be the only waterborne infection that risk associated calculations are based upon.  HAI from pathogens such as Pseudomonas aeruginosa and non-tuberculous mycobacteria are the heavy-weights that should be included in any waterborne risk assessment, alongside Stenotrophomonas maltophilia, Burkholderia spp., Elizabethkingia, Cupriavidus, Ralstonia, Serratia marcescens, Klebsiella.  Whilst these latter organisms may not solely be transmitted from water outlets, it is a well-recognised transmission pathway and accounts for approximately 40% of all nosocomial cases of Pseudomonas aeruginosa for example.

There is substantial evidence to support removal of water outlets completely in high risk/ICU patient areas in order to reduce infection and antibiotic usage, and as a side-effect removal also prevents poor user habits at handwash basins such as stagnancy and inappropriate fluid disposal (De-Las-Casas-Cámara et al., 2022 & 2019; Catho et al., 2021; Kizny Gordon et al., 2017; Feng et al., 2020; Tracy et al., 2020; Shaw et al., 2018; Kearney et al., 2021;  Jung et al., 2020; Grabowski et al., 2018).  Low or no-use water outlets should be removed in order to reduce risk of waterborne infection.  This is a topic worthy of its own blog, so will embrace fully at that time.

Hot Water Temperature as a Main Pathogen Control Measure

Experience has shown that if temperature is to be used as an effective control measure to keep waterborne pathogens in check, the hot water needs to be hot.  Green initiatives raise the question regarding reducing hot water temperatures within buildings to save energy and help support Carbon Zero targets, however Totaro’s paper (2020) indicated that from 58 buildings utilising solar thermal systems for hot water production and where temperature was averaging 40 oC, Legionella spp were detected in 40% of such systems.   Indeed, the cold water was shown to also have an increased risk once rising above 19.1 oC, emphasising the need to keep cold water temperatures well below 20 oC.

Professor Borella (2016) made it clear in her excellent paper analysing 15 years of activity trying to control Legionella spp within buildings that systemic disinfection was generally not sufficient to control the risk of infection, and in performance ranking for Legionella control the highest rank was use of  0.2 micron point-of-use water filters, followed by hot water boilers continuously supporting high temperatures, then systemic disinfection specifically with monochloramine, and then with more limited results for chlorine dioxide and chlorine systemic disinfection.

This work has recently been supported by Girolamini et al., (2022), who have published their 7 years’ experience in controlling hospital water systems and  confirming the positive effective of temperature on Legionella control.  Where the maximum mean temperature recorded at the hospital outlets was 54.5 oC they saw control of both Legionella pneumophila serogroup 1 and Legionella anisa

Engineering Control by Design

Waterborne pathogen prevention must start with the correct design , construction and commissioning of an in-premise water network.  Of equal importance is the correct and continuous use and user behaviour at water outlets.  In combination this makes colonisation and amplification of unwanted bacteria less likely.  Sadly poor temperature control, stagnancy at the outlet through lack of use and remediation apathy is driving amplification of waterborne pathogens within our buildings.

What Can You Do?

Review all the outlets within your buildings and consider if they are at risk from contamination, if control measures are suitable and appropriate, and who are your user populations.  Read Meda & Gentry’s letter, and consider this helpful checklist to see if you can improve water hygiene practice within your facility:

  • If low/no use (less than 5 times per day, Whiley et al, 2019) review with the aim to remove the outlet and ensure all pipework is removed back to the main circuitry
  • If a low use outlet is required, ensure it is used multiple times per day and with extended flow volumes (Whiley et al., 2019). 
  • Use TMV protected outlets for vulnerable users where there is full body immersion e.g. showers and baths.  Use TMV protected outlets on handwash basins where vulnerable users (not staff or visitors) are very young or very elderly, and/or where the risk factors for reduced mobility, sensory impairment, slowed reaction times and thinner skin are considered significant
  • Use non-TMV manual mixer outlets where the user population are not vulnerable, have good sensory awareness and reaction times,  and are therefore at low risk of scalding

Be vigilant for scalding risks at the water outlet, but do not create plumbing environments suitable for waterborne pathogen amplification which carry a higher risk of infection to a much wider group of users.

Discuss this at your water safety group meeting or with your Responsible Person and let us know what problems you face with TMVs and contamination.  Harper Water are happy to help facilitate discussions and risk assessments on this topic


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Digital NHS–emergency-activity/2021-22

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