Short answer: HSE's HSG274 asks for TMV strainers and filters to be inspected, cleaned, descaled and disinfected annually or as the risk assessment says, with routine maintenance to the manufacturer's instructions. In Rada's TMV3 manual, the fail-safe test isolates the cold supply: the outlet should stop within 5 seconds or pass under 120 ml in 60 seconds, and settle no more than 2°C above the set temperature.
Where TMVs are needed
A thermostatic mixing valve blends hot and cold water to a temperature that protects against scalding, "typically between 38 °C and 46 °C depending on outlet use" (HSG274 Part 2, para 2.74). HSE also warns that the blended water downstream "may provide an environment in which legionella can multiply". So the decision to fit one is a balance between the two risks (para 2.75):
- where a risk assessment finds the scalding risk insignificant, TMVs are not required;
- the most serious risk is whole-body immersion in baths and showers, especially for the very young and elderly, and TMVs should be fitted there;
- where very young, very elderly, infirm or significantly disabled people, or people with sensory loss, are present, fitting TMVs at outlets such as washbasins and sinks "is required".
HSG274 Info box 2.3 separates two types. Where the scalding risk is low, "type 2 TMVs that can be overridden by the users are required by building regulations". Where it is significant, "type 3 TMVs that are pre-set and fail-safe should be provided", and they are required in healthcare premises. TMV3 valves meet the NHS Estates Model Engineering Specification D 08 and cannot be overridden by the user (Info box 2.11).
For new homes, Approved Document G limits the hot water to a fixed bath to 48°C, using an in-line blending valve or similar device with a maximum temperature stop that "will fail-safe" and is not easily altered by users (G3(4) and paras 3.65 and 3.66).
Set temperatures
Valve makers publish the mixed water temperatures for each TMV3 designation. Rada's TMV3 Requirements Manual (Kohler Mira, April 2019) gives these:
| Outlet | Mixed water temperature at the outlet | Maximum stabilised temperature in a site test |
|---|---|---|
| Bidet | 38°C | 40°C |
| Shower | 41°C | 43°C |
| Washbasin | 41°C | 43°C |
| Bath, 44°C fill | 44°C | 46°C |
| Bath, 46°C fill | 46°C | 48°C |
The same manual says bath fill temperatures above 44°C "should only be available when the bather is always under the supervision of a competent person". Its normal operating conditions for type 3 valves are hot supply 55 to 65°C and cold supply 5 to 20°C, with flow pressures of 1 to 5 bar (high pressure) or 0.2 to 1 bar (low pressure). Outside those, the valve "cannot be guaranteed to operate as" a type 3 valve.
The in-service test, step by step
From the in-service test procedure in Rada's manual. Use the same measuring equipment, or equipment to the same specification, as at commissioning.
- Record the temperature and pressure of the hot and cold supplies, and compare them with the last test. Supply conditions affect the result.
- Record the mixed water temperature at the maximum available flow.
- Record it again at about 50% of the available flow.
- Fail-safe test: isolate the cold supply to the valve and watch the outlet.
- If there is no flow after 5 seconds, restore the cold supply and check that the stabilised temperature is no more than 2°C above the reading in step 2.
- If water is still flowing after 5 seconds, collect it in a measuring vessel for 60 seconds. It must be less than 120 ml. Then restore the cold supply and make the same 2°C check.
- Record the highest temperature during step 4, the final stabilised temperature, and the thermometer used.
If the final temperature is above the maximum in the table, or the highest temperature is more than about 2°C above the previous result, the manual says service work is needed. If the mixed temperature has moved by more than 1°C since the last test, check that filters are clean, non-return valves work and isolating valves are fully open before re-setting.
How often
| Task | Frequency | Source |
|---|---|---|
| Inspect, clean, descale and disinfect strainers or filters; routine maintenance | Annually, or as the risk assessment says, taking account of the manufacturer's recommendations | HSG274 Part 2, Table 2.1 and para 2.165 |
| Check the blend setting, commercial sites | Every 6 months | Rada TMV3 Requirements Manual |
| In-service test, commercial sites | Every 12 months | Rada TMV3 Requirements Manual |
| In-service tests where no other instruction or guidance applies | 6 to 8 weeks and 12 to 15 weeks after commissioning, then at 18 to 21 or 24 to 28 weeks depending on results | Annex F of D 08, reproduced in the Rada manual |
| Replace critical components | At intervals of no more than 5 years, whatever the test results | Rada TMV3 Requirements Manual |
After the first two or three in-service tests, Annex F's principle is to set future intervals to what earlier tests have shown can be achieved "with no more than a small change in mixed water temperature".
Worked example: a care home
A care home has 20 washbasin TMV3s set at 41°C and 4 bath TMV3s set at 44°C, all fitted in a refurbishment. The residents are elderly, so the scalding risk is significant and the valves are needed.
- Commissioning. Each valve's supply temperatures, mixed temperatures at full and half flow, and fail-safe result are recorded as the reference.
- 6 to 8 weeks. 22 valves are within 1°C of commissioning. Two washbasin valves have drifted 1.5°C and are re-set.
- 12 to 15 weeks. All 24 are within 1°C. The 22 stable valves move to a test at 24 to 28 weeks; the two that drifted once also go to 24 to 28 weeks, as Annex F allows for a small change in only one period.
- Every visit. One bath valve passes 150 ml in 60 seconds on the fail-safe test. It fails, is taken out of use and serviced, and the home is told the same day.
- Every year. Strainers are cleaned, descaled and disinfected, and the risk assessment is checked for outlets where a TMV may no longer be needed.
Who does what
- The dutyholder (employer or person in control of the premises) decides, through the risk assessment, where TMVs are needed and how often they are tested.
- The maintenance contractor tests and maintains the valves to the manufacturer's instructions, with competent staff, and records every result.
- Site staff report outlets that run too hot or too cold, and in care settings check bath temperature before use: Rada notes that the user or attendant "must exercise diligence".
What to record
Rada's manual says "all results must be recorded in a log book". For each valve:
- location, outlet type, make, model and designation
- hot and cold supply temperatures and pressures
- mixed temperature at full flow and at about half flow
- fail-safe result: stopped within 5 seconds, or millilitres collected in 60 seconds
- highest and final stabilised temperature
- thermometer used
- strainer cleaned, descaled and disinfected
- adjustments, parts replaced and the date critical components were last changed
- next test date
Running TMV testing in Field Suite
- Your own TMV form: the form library has no TMV form today. Build one with the form builder: a repeatable group for each valve, number questions with units for temperatures, pressures and millilitres, and an answer that fails the form when the fail-safe test fails.
- Equipment register: each valve can sit at its site with its location and job history.
- Contracts: visits repeat in whole months, so 6-month blend checks and 12-month in-service tests fit directly. The Annex F weeks after commissioning are booked as separate jobs.
- Faults: a failed valve is recorded with its urgency and turned into a quote by the office.
Read our guides to Legionella checks and shower head descaling, see planned maintenance software, prices, or book a demo.
Field Suite is job management software. The site's risk assessment and the valve maker's instructions decide how TMVs are tested.
Questions
What temperature should a TMV be set to?
It depends on the outlet. For TMV3 valves, Rada's manual gives 41°C for washbasins and showers, 38°C for bidets and 44°C or 46°C for baths. HSG274 says "typically between 38 °C and 46 °C".
Can a TMV be removed?
HSG274 Table 2.1 says to risk assess whether a TMV is required and, if not, remove it. Where the scalding risk is significant, it is required.
Do we need TMV3 valves in an office?
HSG274 says type 3 valves should be provided where the scalding risk is significant, and they are required in healthcare premises. In an office the risk assessment decides: where the scalding risk is insignificant, no TMV is required (para 2.75).
Sources
- HSE, Legionnaires' disease: Technical guidance, HSG274 Part 2 (2024), paras 2.74 to 2.76, 2.163 to 2.165, Info boxes 2.3 and 2.11, Table 2.1. https://www.hse.gov.uk/pubns/priced/hsg274part2.pdf
- Rada (Kohler Mira), TMV3 Requirements Manual, 1386572-W2-A, April 2019. https://www.radacontrols.com/media/pv4d5qj0/rada-a-kohler-company-tmv3-requirements.pdf
- HM Government, Approved Document G (2015 edition incorporating 2016 and 2024 amendments), requirement G3(4) and paras 3.65 to 3.68. https://www.gov.uk/government/publications/sanitation-hot-water-safety-and-water-efficiency-approved-document-g
Last checked: 5 November 2026 against the sources above.
Checked in detail: HSE HSG274 Part 2 (second edition 2024) paras 2.74 to 2.76, 2.82, 2.163 to 2.165, Info boxes 2.3 and 2.11 and the TMV row of Table 2.1, PDF downloaded from hse.gov.uk; Rada TMV3 Requirements Manual 1386572-W2-A (Kohler Mira, April 2019) downloaded from radacontrols.com, Tables 1 to 3, in-service test, maintenance and Annex F pages; Approved Document G G3(4) and paras 3.65 to 3.68.
Field Suite is job management software for UK building services firms, not a consultancy. Your own risk assessments, contracts and advisers decide what applies to your work. Spotted a mistake? Email info@fieldsuiteapp.com and we correct it.