Thermostatic Mixing Valves: TMV2, TMV3 & Where They're Required

Summary

Scalding is a serious injury risk from domestic hot water. Water at 65°C can cause full-thickness burns in less than 2 seconds; even water at 50°C causes a serious burn in around 2 minutes. Thermostatic mixing valves protect vulnerable users — young children, elderly people, and those with disabilities — by ensuring the water delivered at the outlet never exceeds a safe maximum temperature.

In the UK, Building Regulations Part G (Sanitation, Hot Water Safety and Water Efficiency) requires TMVs on new builds and major refurbishments. Specifically, Part G3 requires that hot water at baths, accessible showers, and basins in certain buildings (particularly those housing vulnerable people) is supplied at a temperature that doesn't cause scalding.

There are two standards: TMV2 for domestic applications and TMV3 for healthcare and care settings. TMV3 is a more stringent standard requiring the valve to fail safe (shut off the supply) if the cold water supply fails, preventing scalding hot water from reaching the outlet. This is critical in environments where patients or residents may have reduced sensation, mobility, or ability to escape hot water.

Key Facts

• TMV2 — Domestic standard; blends hot and cold; designed for residential use
• TMV3 — Healthcare/care standard; fail-safe on cold supply failure; required in hospitals, care homes, schools, nurseries
• Set temperature — bath — Maximum 44°C (Part G recommendation); most plumbers set at 44-46°C
• Set temperature — shower/basin — Maximum 41°C for healthcare; 44°C residential
• Storage temperature — Hot water cylinder must be stored at minimum 60°C (preferably 65°C) to prevent Legionella growth
• Distribution temperature — Hot water should reach the outlet at 55°C+ within 1 minute (to prevent Legionella colonisation in pipes)
• TMV3 fail-safe — If cold supply fails, valve closes automatically; prevents full-temperature hot water reaching outlet
• TMV2 fail behaviour — Typically only limits temperature; does not shut off if cold fails; less appropriate for vulnerable users
• Building Regulations Part G — Requires scalding protection on baths in new dwellings (max 48°C hot water at bath outlet)
• Annual servicing — TMVs require annual inspection, testing, and calibration; particularly important for TMV3
• Cartridge replacement — Thermostat cartridges should be replaced every 3-5 years or per manufacturer recommendation
• WRAS approval — TMVs should be from the WRAS approved products list
• NHS Estates HTM 04-01 — Health Technical Memorandum governing safe water temperatures in healthcare premises; mandates TMV3
• Position — TMVs should be installed as close as possible to the outlet (within 2m recommended) to minimise the length of tepid water pipe that could harbour Legionella

Quick Reference Table

Detailed Guidance

How a Thermostatic Mixing Valve Works

A TMV contains a thermostat element (typically a wax or shape-memory alloy cartridge) that expands and contracts with temperature. This element controls a mixing shuttle between the hot and cold supplies:

1. Hot and cold water enter the valve through separate inlets
2. The thermostat element senses the mixed outlet temperature
3. If the outlet is too hot, the element expands and restricts the hot supply
4. If the outlet is too cold, the element contracts and allows more hot water through
5. The result is a stable blended outlet temperature regardless of fluctuations in supply temperatures and pressures

Important limitation: TMVs work within a pressure balance range. If the hot and cold supplies have very different pressures (more than 2-3:1 ratio), the valve may not blend correctly. Some TMVs include a pressure-balancing element; others rely on pressure being equalised by the system design.

TMV2 vs TMV3: Key Differences

For most domestic plumbing jobs (new builds, bathroom refurbishments in standard residential properties), a TMV2 is appropriate and compliant with Part G. For any installation in a setting where vulnerable people will use the facilities — including domestic installations for elderly or disabled occupants — TMV3 should be considered and may be required by the local authority or housing association.

Building Regulations Part G Requirements

Approved Document G (2015 edition) sets the following:

G3 — Hot water supply and systems:

• Domestic hot water must be stored at 60°C or above (Legionella prevention)
• Hot water at the draw-off from any bath must not exceed 48°C in new dwellings
• In practice, this means a TMV set to 44-46°C on bath outlets is standard in all new builds

Schedule 1, Part G3 (bath temperature): This directly requires that bath water temperature cannot exceed 48°C in new dwellings and material alterations. A TMV is the standard method of compliance.

Part G3 does not explicitly require TMVs on showers in domestic dwellings (though it's good practice). The requirement is temperature-based, not valve-based — you must demonstrate the outlet temperature doesn't exceed 48°C.

NHS HTM 04-01 (Healthcare)

NHS Estates Health Technical Memorandum 04-01 (Safe Water in Healthcare Premises) is the detailed guidance for water temperature management in healthcare settings. Key requirements:

• TMV3 valves on all patient-accessible hot water outlets
• Outlets to be labelled with the TMV3 compliance
• Annual TMV3 inspection and test records maintained
• All in-scope TMVs on an asset register
• Water temperature management log (regular temperature checks at sentinel outlets)

This is a contractual requirement for NHS work and most care home frameworks.

Installation Requirements

Position:

• Install the TMV as close to the outlet as possible (within 2m is best practice)
• This minimises the length of tepid-temperature water in the pipework (which can harbour Legionella)
• TMVs should NOT be installed in inaccessible voids — they require annual access for servicing

Commissioning:

• Set and record the outlet temperature
• Check cold-supply failsafe (TMV3): isolate the cold supply and verify the valve shuts off
• Check hot-supply failsafe (TMV3): isolate the hot supply and verify the valve shuts off
• Record in the water safety log

Access for servicing: TMVs must be accessible for inspection and maintenance. A service valve should be installed on both the hot and cold supplies to the TMV to allow isolation without draining the system.

Frequently Asked Questions

Do I need a TMV on every outlet in a new house?

Part G requires TMV protection (or equivalent) on bath outlets in all new dwellings — the outlet temperature must not exceed 48°C. For showers and basins, there's no mandatory requirement for TMVs in standard residential properties, but it's best practice and many builders include them throughout. If the property will be let as social housing or sheltered accommodation, additional requirements (often TMV3 on all outlets) will typically be specified in the housing association's technical standards.

Can I reuse an existing TMV when replacing a bathroom?

You can, but it should be inspected and serviced if it's more than 3 years old. Check the cartridge condition, verify the outlet temperature is correct, and test the failsafe function (TMV3). If in doubt, replace the cartridge or the whole valve — a new TMV cartridge is cheap compared to the cost of a scalding incident or a callback.

What's the difference between a TMV and a thermostatic bar valve?

A TMV (thermostatic mixing valve) blends hot and cold supplies to a set temperature and is designed to maintain that temperature despite supply fluctuations. A thermostatic bar valve (as used in some shower controls) may have a temperature-setting function but may not meet the TMV2 or TMV3 standard — check the product specification. For scalding prevention compliance, you need a valve that is tested and listed under the TMV2 or TMV3 scheme.

Why does my TMV keep going cold then hot?

This is usually caused by: (1) pressure imbalance between hot and cold supplies — if they're very different, the valve struggles to blend correctly; (2) a failed or worn thermostat cartridge; (3) a build-up of scale on the cartridge (particularly in hard water areas). Solution: check and balance the supply pressures, descale or replace the cartridge. Hard water areas may need servicing more frequently than the recommended 12 months.

Regulations & Standards

• Building Regulations Approved Document G (2015) — Hot water safety requirements including bath outlet temperature limits

• BS 7942:2000 — Thermostatic mixing valves (domestic) — domestic appliances; covers TMV2

• BS EN 1111 — Sanitary tapware; thermostatic mixing valves — technical specification (TMV3 basis)

• NHS HTM 04-01 — Safe water in healthcare premises; mandates TMV3 in all patient-accessible areas

• Water Supply (Water Fittings) Regulations 1999 — All fittings must be WRAS-approved

• Buildcert TMV Scheme — UK scheme for TMV2 and TMV3 product certification

• NHS Estates HTM 04-01 — Healthcare water safety guidance

• WRAS Approved Products List — Directory of Water Regulations-compliant plumbing products

• hot water systems — Cylinder and boiler systems that supply TMVs

• cold water storage — Cold water storage and Legionella management

• water regulations — Water Supply Regulations 1999 overview

• unvented cylinders — G3 unvented cylinder requirements