TMV Checks: What HSE Guidance Says About Thermostatic Mixing Valves and Legionella
TL;DR: TMV checks matter because a thermostatic mixing valve protects people from scalds and can also hold blended water that Legionella can multiply in. HSE’s HSG274 Part 2 says TMVs should be fitted only where a scalding risk assessment supports them, should sit as close to the outlet as possible, and need routine maintenance by competent people, with strainers and filters inspected, cleaned, descaled and disinfected annually or as the risk assessment says. Where vulnerable people are at risk, fail-safe type 3 TMVs are needed. Keep the hot feed at 50 degrees within one minute and the cold below 20 degrees.
Last updated: 22 October 2026
On this page
Key takeaways
- A thermostatic mixing valve, or TMV, blends hot and cold water to a safe temperature, typically between 38 and 46 degrees depending on outlet use, according to HSE’s HSG274 Part 2. The blended water downstream may provide an environment in which Legionella can multiply.
- HSE says the fitting of TMVs should be informed by a comparative assessment of scalding risk against the risk of infection. Where the scalding risk is insignificant, TMVs are not required.
- TMVs should be fitted at baths and showers, where the most serious scalding risk is whole body immersion, and wherever the risk assessment finds a significant scalding risk, for example for very young, very elderly or disabled users.
- Where TMVs are fitted, HSE says to put them as close to the point of use as possible, preferably in the tap itself, because a single valve serving several outlets can increase the risk.
- Strainers and filters on TMVs should be inspected, cleaned, descaled and disinfected annually or as the risk assessment defines, and the valves need routine maintenance by competent people to the manufacturer’s instructions.
- Where vulnerable people use baths or showers, type 3 TMVs that are pre-set and fail-safe are required, and they should be checked regularly to confirm they fail safe if the cold supply is interrupted.
A care home refurbishes a bathroom and fits a thermostatic mixing valve behind the panel, a long pipe run away from the shower. The shower is used twice a week. The water between the valve and the shower head sits in the pipe for days, at a temperature that is neither hot nor cold. Nobody checked it, because the valve was doing its job.
TMV checks are the part of Legionella control that is easy to forget, because a mixing valve protects people from scalds and looks like a safety feature. This guide explains what HSE’s HSG274 Part 2 says about when to fit TMVs, where to put them, what routine checks they need, and why the blended water downstream is a Legionella risk. It also shows what to monitor around each valve so that the checks leave evidence. It is not legal advice, so check your own risk assessment and the full HSE guidance.
What is a thermostatic mixing valve?
A thermostatic mixing valve is a valve that uses a temperature sensitive element to blend hot and cold water and deliver water at a safe, steady temperature. HSG274 Part 2 describes TMVs as producing water at a temperature that safeguards against the risk of scalding, typically between 38 and 46 degrees depending on outlet use.
The valve takes two feeds, one hot and one cold, and mixes them. If the cold supply fails, a fail-safe valve closes down so that scalding hot water does not reach the user. That protection is why TMVs are standard on baths and showers in care settings.
The same feature creates a Legionella issue. HSE’s technical guidance says the blended water downstream of TMVs may provide an environment in which Legionella can multiply, thus increasing the risks of exposure. Blended water sits in the warm band that Legionella favours, it is not hot enough to kill the bacteria and not cold enough to hold them back, and it can stand in the pipe between the valve and the outlet.
That makes the TMV a place where two risks meet: scalding on one side and infection on the other. The guidance asks you to weigh them, not to fit valves by habit.
Do you need TMVs?
You need TMVs where a scalding risk assessment says they are needed, and not elsewhere. HSE says the use and fitting of TMVs should be informed by a comparative assessment of scalding risk versus the risk of infection from Legionella, and that where the scalding risk is insignificant, TMVs are not required.
HSE is specific about where the scalding risk is highest. The most serious risk of scalding is where there is whole body immersion, such as with baths and showers, particularly for the very young and elderly, and TMVs should be fitted at these outlets. Where a risk assessment identifies a significant scalding risk, for example where there are very young, very elderly, infirm or significantly mentally or physically disabled people, or those with sensory loss, fitting TMVs at appropriate outlets such as hand washbasins and sinks is required.
For most people the risk at a washbasin is lower. HSG274 says the scalding risk is minimal where water is delivered up to 50 degrees at hand washbasins, and that hot water signs may be considered sufficient where a TMV is not fitted. It also notes that there is a risk of scalding where the water temperature at the outlet is above 44 degrees, especially for vulnerable people in whole body immersion.
The practical rule is that the risk assessment decides, and it should record why a valve is fitted or not. Our page on TMV temperature monitoring covers the monitoring side.
Which type of TMV does HSE expect?
HSE’s guidance distinguishes type 2 and type 3 valves. Where the scalding risk is low, such as healthy users immersing their whole body, type 2 TMVs that users can override are required by building regulations. Where the scalding risk is significant, type 3 TMVs that are pre-set and fail-safe should be provided, and they are required at healthcare premises.
A type 3 valve cannot be overridden by the user. HSG274 notes that TMV3 meets the requirements of the NHS Estates Model Engineering Specification for thermostatic mixing valves in healthcare premises. It also says that the chance of a severe scald from a washbasin tap is low, and that the need for a TMV3 on a hand washbasin should be assessed against the need for Legionella control.
HSE adds that type 3 valves should be checked regularly to make sure they fail safe if the cold water supply pressure is interrupted. That is a functional check, and it is the sort of test that belongs in a maintenance schedule with a date and a name against it.
The guidance also stresses that a documented maintenance schedule should be followed and the valves maintained to the standard the manufacturer recommends. A valve that nobody maintains is a valve that may not protect anyone.
Where should a TMV be fitted?
Fit the TMV as close to the point of use as possible, and preferably inside the tap. HSE says that where practicable TMVs should be incorporated directly in the tap fitting, with mixing at the point of outlet preferable, and that TMVs should be as close to the point of use as possible to minimise the storage of blended water.
HSG274 lists the factors that raise the risk. Where TMVs are fitted with low flow rate spray taps on hand washbasins, the risk is increased. Where a single TMV serves multiple tap outlets, the risk can be increased. And where TMVs are designed to supply both cold and blended water, an additional separate cold tap is rarely needed and may become a low use outlet.
The pattern is clear. The more blended water you store, and the less often it moves, the higher the risk. A remote valve with a long run of blended pipe to a rarely used shower is the worst case, and it is exactly the situation in the opening example.
If a remote valve cannot be moved, treat the downstream pipework as a dead leg. Flush it regularly, record the flush and consider monitoring.
What TMV checks does HSE expect?
HSE’s monitoring table sets out four actions for TMV checks. Risk assess whether the fitting is required, and if not, remove it. Where needed, inspect, clean, descale and disinfect the strainers and filters. Maintain the valve routinely through a competent person to the manufacturer’s instructions. And flush infrequently used outlets.
The table gives a frequency for the strainer and filter work: annually, or on a frequency defined by the risk assessment, taking account of any manufacturer’s recommendations. The guidance explains the reason in the same paragraph. TMVs require regular routine maintenance carried out by competent individuals in accordance with the manufacturer’s instructions to maintain protection against scald risk.
Flushing is the other half. HSG274 says infrequently used equipment, meaning equipment not used for a period equal to or greater than seven days, should be included on the flushing regime. The outlets should be flushed until the temperature at the outlet stabilises and is comparable to supply water, and purged to drain. For high risk populations such as healthcare and care homes, more frequent flushing may be required as indicated by the risk assessment, and the guidance gives twice weekly as an example for outlets in healthcare facilities with susceptible patients.
If there has been a lapse in the flushing regime, HSE says the stagnant and potentially contaminated water within the shower or tap and the associated dead leg should be purged to drain without discharge of aerosols before the appliance is used.
| Check | What HSG274 Part 2 says | How often |
|---|---|---|
| Is the TMV needed? | Risk assess scald risk against Legionella risk, and remove if not needed | At the risk assessment and on review |
| Strainers and filters | Inspect, clean, descale and disinfect | Annually, or as the risk assessment defines |
| The valve itself | Routine maintenance by a competent person to the manufacturer’s instructions | As the manufacturer recommends |
| Type 3 fail-safe | Check it fails safe if the cold supply is interrupted | Regularly |
| Little-used outlets | Include in the flushing regime if unused for seven days or more | Weekly, more often for susceptible users |
What temperatures should TMV checks cover?
Monitor the two feeds that go into the valve, and the outlet. HSG274 says hot water should be stored at a minimum of 60 degrees and distributed so that it reaches a minimum of 50 degrees, or 55 degrees in healthcare premises, within one minute at the outlets. Cold water should be kept below 20 degrees.
Those figures are the controls that keep Legionella in check on each side of the valve. If the hot feed to a TMV falls below 50 degrees, or the cold feed warms above 20 degrees, the valve is blending two supplies that are both already in the growth range. The valve may still deliver a comfortable temperature while the water behind it is a risk.
The blended outlet needs a different check. HSE notes there is a risk of scalding where the outlet water temperature is above 44 degrees, and that testing outlet temperatures with a thermometer can provide additional reassurance where vulnerable people are in whole body immersion. A periodic check of the blended temperature confirms the valve is holding its set point.
A sensible plan for TMV checks has three points. A hot feed sensor shows that the supply to the valve stays at 50 degrees or more. A cold feed sensor shows it stays below 20 degrees. And a record of flushing shows that the blended branch has moved water through it. Together they show whether the controls are working, not just on the day of the check.
How does monitoring help with TMV checks?
Monitoring turns monthly TMV checks into a continuous record. Wireless temperature sensors on the hot and cold feeds show whether the controls hold between visits, and alert when they drift, so a failing return or a warm cold feed is caught in hours and not at the next round.
AQUAIOT’s automated Legionella monitoring places wireless temperature sensors at tanks, calorifiers, loops and sentinel outlets, logs every reading to the cloud over cellular or LoRaWAN telemetry, and alerts by SMS, email or app when cold water exceeds 20 degrees or hot water falls below 50 degrees at the point of use. It also shows when a scheduled flush has been missed.
For TMV areas the useful placements are the hot and cold feeds close to the valves serving the highest risk outlets, such as baths and showers in care settings, and the furthest outlet on any long branch. Our guides to Legionella temperature monitoring and sentinel outlets explain where those points sit, and our page on Legionella flushing monitoring covers recording flushes.
Monitoring does not replace the maintenance. A sensor cannot clean a strainer or confirm a type 3 valve fails safe. It shows that the supplies around the valve are in range and that the checks you did are still holding true.
What mistakes do sites make with TMV checks?
The common mistakes with TMV checks are fitting TMVs by habit, placing them far from the outlet, serving many outlets from one valve, forgetting the strainer, never testing the fail-safe and leaving little-used outlets out of the flushing regime. Each one is avoidable with a short schedule.
- Fitting by habit. HSE asks for a comparative assessment of scald risk and infection risk, and says TMVs are not required where the scald risk is insignificant.
- Remote valves. A valve far from the outlet stores blended water. Move it close to the tap or include the run in flushing.
- One valve, many outlets. HSE says this can increase the risk.
- Forgotten strainers. The guidance says to inspect, clean, descale and disinfect them annually or as the risk assessment defines.
- No fail-safe test. Type 3 valves should be checked regularly to confirm they fail safe if the cold supply is interrupted.
- Little-used outlets missed. Anything unused for seven days or more belongs on the flushing regime.
Frequently asked questions
What are TMV checks?
TMV checks are the routine inspections and tests that keep a thermostatic mixing valve safe and effective. HSE’s guidance covers risk assessing whether the valve is needed, cleaning and descaling strainers, routine maintenance by a competent person, checking type 3 valves fail safe, and flushing little-used outlets.
How often should TMV strainers be cleaned?
HSG274 Part 2 says strainers and filters associated with TMVs should be inspected, cleaned, descaled and disinfected annually, or on a frequency defined by the risk assessment, taking account of any manufacturer’s recommendations.
Do all taps need a TMV?
No. HSE says where a risk assessment finds the scalding risk is insignificant, TMVs are not required. They should be fitted at baths and showers, and wherever the assessment finds a significant scalding risk, for example for very young, very elderly or disabled users.
What is a TMV3?
A TMV3 is a type 3 thermostatic mixing valve that is pre-set and fail-safe and cannot be overridden by the user. HSE says type 3 valves should be provided where the scalding risk is significant, they are required at healthcare premises, and they should be checked regularly to confirm they fail safe.
Why can a TMV be a Legionella risk?
Because the blended water downstream of the valve may provide an environment in which Legionella can multiply, according to HSG274. Blended water sits in a warm temperature band, and it can stand in the pipe between the valve and the outlet if the outlet is used rarely.
What temperature should water be at a TMV?
HSG274 says hot water should be stored at a minimum of 60 degrees and reach 50 degrees, or 55 degrees in healthcare premises, within one minute at outlets, and cold water should be below 20 degrees. The blended outlet is typically between 38 and 46 degrees, and above 44 degrees carries a scald risk.
Where to start
Pull your Legionella risk assessment, plan your TMV checks and list every TMV with its location, its type, the outlets it serves and the date its strainers were last cleaned. Mark any valve that is far from its outlet, serves several outlets or feeds a little-used shower. If you want the hot and cold feeds around those valves watched continuously, contact AQUAIOT with the number of buildings and valves, and we will suggest where temperature sensors should go.
For the guidance quoted here, read HSE’s Legionnaires disease technical guidance, HSG274 Part 2 and its page on scalding and burning in health services.