Do TMVs still have a viable role in healthcare?

In healthcare environments, the balance between anti-scalding safety and the risk of Legionella infection is a perennial issue. Temperature control regimes have always been crucial for managing the risk of waterborne infection while mitigating the risk of scalding. Thermostatic mixing valves (that comply with NHS D08 performance specifications, i.e. are TMV3-approved) have typically played a significant role in protecting vulnerable users, but in recent years their reputation has become somewhat tarnished. Alternative mechanical solutions have been implemented to address the concerns raised by estates teams. So, will the thermostatic mixing valve (TMV) be consigned to the great scrap metal recycling heap, or does it still have a role to play in today’s healthcare environments?

Legionella control regimes in healthcare domestic hot water distribution systems mean that hot water at 55 °C is circulated right up to the point of use to prevent bacterial proliferation. However, this elevates the risk of scalding at the point of use. The Health & Safety Executive’s Technical Guidance for Legionnaire’s disease (HSG274 Part 2)¹ states that TMVs should be used where there is a high scalding risk, such as where there is full body immersion for baths and showers, and for hand washbasins and sinks where the risk is highest — i.e. for people who are infirm, elderly, very young, or significantly mentally or physically disabled, and also those with sensory loss.

Technology non-grata

TMVs provide optimal anti-scalding safety. A temperature-sensitive cell adjusts the incoming hot and cold water flow so that the mixed water output is controlled to a very high degree of sensitivity. Variations in the incoming water temperature and pressure are compensated for immediately, ensuring that the mixed water is delivered at a safe, constant temperature to the user. The thermostatic cell reacts instantly if there is a cold water supply failure, shutting off the hot water supply. Likewise, if the hot water supply fails, the cold supply shuts off too, meeting the TMV3 requirements for anti-scalding safety, i.e. failsafe. So, if user safety and comfort are preserved, and the risk of Legionella is mitigated, why have TMVs become technology non-grata?

For users most at risk from scalding, the potential for infection by Legionella bacteria is also high, and the solutions to overcome both risks involves an advanced technical solution. However, these engineered solutions have led to a rise in concerns about the role of TMVs in contributing to contamination episodes. These concerns centre on the possibility of a cold water deadleg downstream of the TMV, the complexity of the thermostatic mechanism as a factor in bacterial development, and the associated filters and non-return valves as potential contamination sources.

Cold water deadlegs

Location is everything when it comes to TMVs. A group TMV serving a run of showers and/or washbasins will inevitably result in a cold water deadleg downstream of the TMV, since mixed water is already blended to a safer, more comfortable temperature. The cold water draw-off is therefore infrequent, allowing biofilm time to establish significantly, and increasing the risk of bacterial contamination in the cold water pipework. In recent years there has been a determined switch to point-of-use thermostatic technology with TMV3 approved anti-scalding failsafe, which can operate with flow rates as low as 3 litres per minute.

The Health and Safety Executive guidelines¹ recognise this solution, advising that TMVs should be incorporated directly into the tap. Also referred to as thermostatic mixing taps (TMTs), blending occurs at the outlet itself. A TMT with a single sequential control will not only deliver potable cold water from the distribution system when the flow is first opened, but, as the lever turns, will also provide blended water with full anti-scalding safety at full flow. Since the hot and cold water is mixed at the point of use, any potential cold water deadleg is eliminated.

Complex mechanisms

Conventional TMVs can inadvertently create conditions favourable to bacterial proliferation. The mixing chamber on most TMVs and TMTs incorporates a highly sensitive mechanism which allows it to respond to any fluctuations in pressure and temperature with a high level of accuracy and reliability. In order to function correctly, most TMVs rely on mesh strainers to prevent large particles and debris from the system from damaging the sensitive elements within the mechanism. If these are not regularly checked and cleaned, they can become clogged, and scale can build up and inevitably provide a safe haven and source of nutrients for bacteria.

The thermostatic mechanism itself is also prone scale build-up, so to make sure that the anti-scalding failsafe is functioning correctly, it must be checked at least twice a year. This can be highly onerous and costly. Even with a TMT, access to shut off the water supply is often behind an IPS panel. Access can be limited and challenging, and removing the access panels can release dust and dirt into an environment where hygiene is paramount.

To complicate things further, thermostatic mixers require non-return valves to prevent interconnection between the hot and cold water. Issues arise if the mixing chamber is subject to pressure changes. Without non-return valves (NRVs), if there is a sudden drop in the cold water supply — for example when several toilets are flushed, then the imbalance in the system (due to the higher pressure on the hot water side) will cause the hot water to flow back into the cold water system. This inevitably changes the conditions in the cold water supply pipe, heating the water to above 20 °C — the temperature at which Legionella is no longer dormant, which in turn risks triggering its proliferation.

To prevent any cross-flow, NRVs are routinely installed on the water inlets. They only allow the incoming water to flow in one direction, operating as a check valve or backflow prevention device. Since NRVs are also very sensitive, they are also prone to being blocked by impurities. If they are not checked and serviced regularly, they can unexpectedly fail, resulting in cross-flow, and the associated challenges this entails — which adds a further responsibility for the Estates team.

In real terms, what does this burden look like? HSG274 Part 2¹ states that TMVs should:

‘…be checked regularly to ensure they are failsafe if the cold water supply pressure is interrupted.’ A risk assessment should define the frequency of inspection and monitoring, which can be as frequent as quarterly, depending on the rate of fouling or other risk factors, such as patient vulnerability. So, for example, in soft water areas, an anti-scald safety test must be carried out on average twice a year. This intervention generally requires access behind the IPS panel to isolate the cold water supply. If each safety check takes five minutes, then twice-yearly checks will take 10 minutes over the course of the year.

Now let’s add the filters and non-return valves. Currently, there is no legislative or regulatory guidance on servicing regimes and maintenance programmes for NRVs. However, if we assume that checks also take place twice yearly, and that each intervention takes 10 minutes, that equates to 20 minutes per year. A 10-minute failsafe check and 20-minute strainers check equates to 30 minutes per TMV/TMT per year.

Daily cross-flow risks

If we now extrapolate this to a healthcare facility with 500 beds, i.e. 500 hand washbasins each with a TMT for patient hygiene, 500 sinks with a TMT for healthcare professionals, plus 500 TMV3-approved shower mixers, this means 1,500 interventions at 30 minutes, or 750 hours per year. That equates to one engineer working a 40-hour week checking only TMVs for one third of the year. But if those checks are not undertaken, these 1,500 TMVs present 3,000 risks of cross-flow every day.

Given this level of infection risk, coupled with an onerous maintenance regime, it is hardly surprising that TMVs, and even TMTs, are no longer welcome in healthcare facilities. The risk of infection far outweighs the scalding risk. Indeed, following the deaths of six residents in a French care facility which were attributed to NRV failure, the French government took the infection risk very seriously, and updated its Medical Standard² to ban all NRVs in medical facilities.

The tendency within Estates teams is therefore to replace any point-of-use TMV with a mechanical mixer, especially where the scalding risk is low. Ideal for users who are familiar with the facility and use the same basin for regular handwashing, mechanical mixers with a maximum temperature limiter offer basic anti-scalding safety. The principle is simple: limit the amount of hot water that can enter the mixing chamber so that the pre-set maximum temperature is not exceeded — typically between 38 °C and 44 °C depending on the application. Maintenance staff can also override the temperature limiter to undertake thermal shocks, but the user is unable to override the limiter, avoiding accidental scalding in day-to-day use. But there is still a risk of scalding for those vulnerable cohorts identified earlier. So, what is the solution?

‘Next generation’ TMVs

Interestingly, the solution lies with the very same failsafe technology that enables TMVs to provide anti-scalding safety. For conventional TMVs the closing mechanism is located upstream of the mixing chamber. When the mixer is turned on, hot and cold water flow into the mixing chamber, where the thermostatic cell regulates pressure and temperature imbalances in the incoming water prior to delivery. The reason that cross-flow occurs is that the mixing chamber is subject to static pressure. If there is a sudden imbalance in pressure — for example due to simultaneous toilet flushes (as above), the imbalance will cause the mixed water to seek the path of least resistance, or lowest pressure. To guard against this, NRVs are installed on the inlets to prevent the pressure imbalance forcing hot water into the cold water supply pipes.

At least a decade prior to the French ban on NRVs, Delabie’s engineers developed sequential thermostatic technology, which ensures that the water flow is controlled upstream of the mixing chamber. This ensures that the mixing chamber is not subject to static pressure, so there can be no cross-flow due to pressure imbalances, eliminating the need for NRVs. The mixer opens with only cold water, and the hot water only begins to flow when the lever is in the vertical position and the thermostatic cell starts to blend the hot and cold, taking into account pressure and temperature variations in the system. The anti-scalding failsafe technology reacts immediately if there is a cold water supply failure, shutting off the hot water supply. Likewise, if the hot water supply fails, the cold shuts off too, meeting the TMV3 requirements for failsafe.

Delabie’s wall-mounted sequential TMTs also feature an often-overlooked component that has revolutionised hygiene in healthcare settings. Designed to isolate the water supply in front of the IPS panel, the Stop/Check connector allows the mixer to be removed for cleaning, descaling, and maintenance, without the inconvenience of removing the panels. Furthermore, the ability to isolate the water supply means that a failsafe check takes place in seconds.

If we return to our earlier example, a conventional mixer requires 30 minutes per year to check the failsafe and NRVs. The Delabie sequential TMV has no NRVs, and the failsafe check takes 10 seconds per mixer, a total of 20 seconds per year. These Stop/Check connectors are available on Delabie’s wall-mounted sequential thermostatic mixers and showers, offering improved user safety for patients and healthcare professionals wherever there is a scalding and infection risk, all while reducing the burden for over-stretched maintenance teams.

Conclusion

Once poster boys for anti-scalding safety, TMV mixers have fallen from grace. Increased infection risks and onerous maintenance regimes have seen Estates teams replace TMVs where the risk of scalding can be mitigated through mechanical means. However, next-generation thermostatic sequential technology offers a new solution when faced with the perennial challenge of balancing anti-scalding risks with Legionella infection risks. User safety is still guaranteed thanks to the familiar anti-scalding failsafe, but without arduous servicing and maintenance routines. Perhaps it’s time to let the next generation of thermostatic mixing valves show their mettle?

References

1 Health & Safety Executive: Legionnaire’s disease Technical guidance HSG274 Part 2, Published March 2024. https://tinyurl.com/4y5yuzc5

2 Association française de normalisation (AFNOR), NF Médical (DT077), I June 2017.