Water safety compliance does not begin and end with temperature checks, logbooks or routine inspections.
Effective control of Legionella, scalding risk and waterborne pathogens relies on informed, structured risk assessments. Thermostatic Mixing Valves [TMV, including TMT – Thermostatic Mixed Tap] management plays a central and often underestimated role in this.
A safety balancing act
TMVs are frequently treated as standard fixtures rather than critical safety devices requiring justification, monitoring and governance. In reality, the decision to install, retain, remove or modify a TMV should always be driven by risk assessment, balancing scalding risk with microbiological risk.
Across healthcare, education and public or commercial estates, preventable Legionella risks rarely stem from major system failures. More often, they develop gradually through overlooked design decisions, redundant outlets, poorly maintained TMVs and low use outlets or facilities that introduce stagnation into otherwise compliant systems.
This position is consistent with UK HSE guidance, where Legionella outbreaks are rarely caused by infrastructure failure. Instead, they are typically the result of water systems that are not managed correctly, with the accumulation of unmanaged or poorly governed risk within systems that remain mechanically operational.
A minor oversight today can quickly become an incident tomorrow.
Why TMV management matters
TMVs are primarily installed to mitigate scalding risk, particularly in environments serving vulnerable users such as healthcare settings, schools and care facilities.
By blending hot and cold water to a controlled outlet temperature, they reduce the likelihood of thermal injury at the point of use.
However, from a microbiological perspective, TMVs introduce additional complexity:
- Mixed temperature zones within the valve body
- Internal components such as strainers and check valves prone to fouling
- Reduced temperatures downstream of the blending point
- Increased maintenance and inspection requirements
Where servicing, fail-safe testing and performance verification are inconsistent, TMVs can become areas of temperature instability and biofilm accumulation.
For this reason, TMVs must be understood as dual risk control devices, not a standalone scalding solution. Their use must always be justified through a combination of scalding risk assessment and water safety/Legionella risk assessment.
Installation by default, without structured justification, increases system complexity without necessarily reducing overall risk.
When TMVs are not required
TMVs are often installed as a precaution rather than through risk-based decision making.
They may not be required where:
- The outlet is staff-only
- Scald risk is low
- Temperatures are already safely controlled centrally [potentially with a supplementary control method, i.e. online dosing]
- Risk assessment supports alternative control measures.
Removing TMVs: A structured approach
Where a TMV is deemed unnecessary, removal must be controlled and documented:
- Review and update the Legionella Risk Assessment
- Reassess and document scalding risk
- Remove the TMV via a competent person
- Reconfigure pipework to avoid deadlegs or stagnation
- Update schematics, asset registers and the Water Safety Plan
Removal should reduce system complexity and eliminate risk, not introduce new stagnation points or risk.
When TMVs are required
There are clear scenarios where TMVs are essential, particularly in schools and nurseries, where vulnerable users are present.
This does not mean that TMVs are required throughout these settings – the requirement for risk assessment remains the same.
In these settings, TMVs must be:
- Correctly specified [e.g. TMV2 or TMV3 depending on the environment]
- Installed as close to the point of use as possible
- Commissioned to safe outlet temperatures [typically 39–43°C, outlet / facility dependent]
- Included within routine servicing, calibration and fail-safe testing programmes
A TMV that is installed but not maintained is not a control, it is an unmanaged risk.
Managing underused cold taps
A commonly identified configuration during Legionella risk assessments is a wash hand basin supplied via a TMV-controlled blended outlet [often labelled ‘hot’] alongside a separate cold tap that is rarely or never used.
From a scalding perspective, this may appear appropriate. However, from a water safety standpoint, the rarely used cold outlet presents a potential stagnation point.
Low-use pipework can allow water to remain static for prolonged periods, increasing the likelihood of:
- Temperature drift into the 20–45°C range
- Biofilm development along internal surfaces
- Reduced disinfectant residual
- Amplification of waterborne pathogens
In this context, ‘no use’ does not equate to ‘no risk’. It represents unmanaged stagnation within a configuration that may no longer reflect operational reality.
So, what should happen to an unused cold tap? There are three risk-based approaches, listed below:
1. Flushing regime [short-term control]
Where removal is not immediately possible:
- Implement routine flushing [typically weekly, minimum 2 minutes]
- Record and monitor compliance
- Document clearly within the Legionella Risk Assessment
Short-term controls such as flushing can mitigate risk, but they are dependent on consistent implementation and documented oversight.
2. Remove the redundant outlet [preferred risk elimination]
Where the tap is not required:
- Remove the cold tap and associated pipework
- Cap pipework as close to the branch connection as possible or piece through
- Eliminate deadlegs in line with HSG274 guidance
This removes stagnation risk at source.
3. Redesign as a single blended outlet [best practice solution]
This is a more robust, long-term solution:
- Install a single TMV-blended outlet
- Remove redundant hot/cold configurations
- Improve system turnover through regular use
Longer-term, risk-based solutions may include removal of redundant outlets, capping pipework close to the branch connection to minimise deadlegs or redesigning the outlet as a single blended point to improve turnover.
These decisions should always be driven by risk assessment rather than assumption.
Integration with Legionella Risk assessments
Legionella Risk Assessments must do more than identify hazards; they must drive action and reflect real world usage.
They should:
- Justify whether TMVs are required or not
- Identify underused or redundant outlets
- Define flushing regimes where necessary
- Recommend removal of deadlegs and unnecessary pipework
- Align system design with operational behaviour
All remedial actions must be:
- Clearly documented
- Assigned to responsible competent persons
- Tracked through to completion
The role of the Water Safety Plan
The Water Safety Plan is the mechanism through which risk assessment decisions are embedded into operational control.
It should include:
- TMV inventory and maintenance schedules
- Records of TMV installation, removal or modification
- Flushing protocols for low use outlets
- Updated schematics reflecting actual installations
- Evidence of monitoring, servicing and compliance
If an unused cold tap remains, it’s management must be clearly defined, not assumed.
Reactive compliance vs engineered risk reduction
A recurring issue across many estates is the shift toward reactive compliance, where:
- TMVs are installed without reviewing actual need
- Low-use outlets remain in place despite stagnation risk
- Deadlegs are assumed insignificant
- Maintenance regimes do not reflect system complexity
Over time, these individually manageable issues combine to increase overall system risk.
Effective water safety management requires:
- Alignment between Legionella and scalding risk assessments
- Accurate asset registers and up-to-date schematics
- Routine TMV servicing, calibration and fail-safe testing
- Regular review of outlet necessity and usage
- Removal of redundant assets where reasonably practicable
Governance must be proportionate, documented and reflective of real-world use – not historic design assumptions.
Long-term risk control through intentional TMV strategy
TMV management is not solely about preventing scalding; Legionella control is not solely about monitoring temperature. Both require a coordinated, risk-based approach that considers:
- Stagnation
- System design and complexity
- Hydraulic performance and turnover
- Long-term maintainability
Many enforcement actions and costly remediation programmes originate from cumulative oversight, an un-serviced TMV, an infrequently or unused tap, or a poor configuration that was never revisited.
The most resilient estates are those that regularly challenge legacy arrangements and ensure every outlet/facility and every TMV remains justified.
Pause for thought
A wash hand basin with a TMV controlled outlet and an unused cold tap is not a minor maintenance issue, it is a clear test of governance, design intent and risk ownership.
The key question is not “Is this compliant today?”. It’s “Has this TMV and outlet configuration been risk assessed to control both scalding and Legionella risk over the long term?”.
Water safety is not defined by the absence of failure; it is defined by the strength of governance.
Jim Perry is a Senior Consultant and Authorising Engineer (Water) at the Water Hygiene Centre.
