In England a thermostatic mixing valve at a bath is required when a dwelling is newly built, or created by a material change of use. Requirement G3(3), Schedule 1 of the Building Regulations 2010 states that the hot water supply to a fixed bath must be designed and installed so the water delivered to that bath cannot exceed 48°C. Replacing a bath in a home that already exists does not trigger that duty. A blending valve can still be fitted by choice, and it is worth raising if very young or very old people use the bath.
Do you need a thermostatic mixing valve on a bath in England?
A blending valve at the bath is required in England when a dwelling is newly built, or created by a material change of use. Requirement G3(3), Schedule 1 of the Building Regulations 2010 sets the limit: the hot water supply to a fixed bath must be designed and installed so the water that can be delivered to that bath does not exceed 48°C. A thermostatic mixing valve is the usual way to meet that. It blends hot water with cold before the water reaches the bath, and a stop caps the highest temperature it can pass.
The wording names the fixed bath. Showers, basins and kitchen taps sit outside G3(3). A bath in a new flat is covered by the limit. The same bath in a house built twenty years ago is not, unless the home itself is being created.
The trigger is the dwelling, not the bathroom. A new build, or a conversion that creates a home, brings the 48°C bath limit with it. A bathroom refit in an existing house does not.
New dwelling or existing home: when the 48°C bath valve is required
In England the 48°C limit at a fixed bath applies where a dwelling is built or created by a material change of use. The Approved Document G FAQ of March 2016 confirms that fitting a new bath in an existing home does not bring that duty with it.
| Criterion | New dwelling or material change of use | Existing home, bath replaced or added |
|---|---|---|
| What applies | 48°C limit at the fixed bath under Requirement G3(3), Schedule 1, Building Regulations 2010 | No duty to fit a blending valve at the bath, per the Approved Document G FAQ |
| What sets it off | A dwelling is built, or a building is converted into a dwelling | Swapping, repairing or adding a bath in a home that already exists |
| The valve | In-line blending valve on the hot supply, with a maximum temperature stop | Optional, and worth weighing up where very young or very old people bathe |
| Showing it is met | Approved Document G route: a valve to BS EN 1111 or BS EN 1287 that fails safe | Nothing to demonstrate; the decision sits with the household |
Choosing a valve in an existing home is a scald-risk decision, not a paperwork one. The Approved Document G FAQ suggests weighing up the safety benefit of a mixing valve, or another anti-scald measure, when a bath is added, replaced or repaired. It singles out households with very young or very old people, who are the main scald risk group. Ask for it when the hot pipework is already exposed, because that is when the valve is simplest to add.
Where does the bath mixing valve go: at the tap or on the hot supply?
The valve goes in-line on the hot water pipe feeding the bath, not inside the tap. Approved Document G asks for an in-line blending valve, or another suitable temperature control device, with a maximum temperature stop and pipework arranged to suit. The 48°C figure describes the water that can arrive at the bath. So the limiting device has to sit between the hot source and that bath, with nothing hotter able to bypass it.
Suitability is shown through BS EN 1111 or BS EN 1287, the standards for thermostatic mixing valves named in Approved Document G. They cover valves that cannot pass more than the set temperature in service and that fail safe. HSE guidance on hot and cold water systems adds a position rule: fit the mixing valve as close to the outlet as possible, with no more than 2 m of pipe after it. Blended water standing in a long run sits in the temperature band where legionella multiplies.
Why 48°C at the bath, and why HSE guidance mentions 44°C
48°C is the Building Regulations limit for a fixed bath in England, while 44°C comes from HSE scald-risk guidance and applies in a different setting. Approved Document G explains the number through scald times. At 55°C a partial thickness scald takes around 12 seconds. At 48°C the same injury takes around ten minutes. That gap is the reason the bath is singled out, because a person can be immersed and slow to get out.
HSE treats water leaving an outlet above 44°C as a scalding risk. Where vulnerable people have access to a bath or shower and the risk is judged significant, HSE guidance points to a type 3 thermostatic mixing valve that prevents delivery above 44°C. That sits inside a risk assessment in care and healthcare settings. It is not the building regulations duty that applies to a bath in a new home.
My cylinder already has a blending valve: does that meet the 48°C bath limit?
A factory-fitted mixing valve on a hot water cylinder is not proof that the bath is held to 48°C. Heatrae Sadia's Megaflo installation manuals describe a thermostatic mixing valve built into the hot water outlet and set at the factory. The cylinder thermostat has to be set above that valve setting for the blended temperature to be reached, and the manuals tell installers not to dismantle or alter the pre-set valve. That valve blends everything leaving the cylinder, so it is a distribution control rather than a bath limit.
Storage temperature is the other half of the picture. HSE guidance says to store hot water above 60°C and distribute it above 50°C, because legionella multiplies between 20°C and 45°C and does not survive above 60°C. The cylinder therefore stays hot, and the water is blended down near the outlet instead. Settings are covered in more detail in what temperature a hot water cylinder should be set to.
Who fits a bath blending valve, and does building control need telling?
Fitting an in-line blending valve on the hot supply to a bath is plumbing work, but work on an unvented cylinder is restricted and notifiable in England. Approved Document G says an unvented hot water storage system needs at least two independent safety devices besides the control thermostat. It also asks owners and occupiers to choose an installer with proven competence, registered under a competent person scheme for unvented hot water systems.
GOV.UK guidance on competent person schemes sets out the two routes. A registered installer self-certifies the work and gives the occupier a certificate of compliance within 30 days of completion. Where the installer is not registered, the work must be notified to the building control body before it starts. Who may carry out that work is set out in who can install an unvented hot water cylinder.
If the bath limit comes up alongside a new or replacement cylinder, raise it at the free survey. The hot run to the bath, the valve position and the cylinder setting can then be looked at together before you receive a written fixed price for Megaflo and unvented cylinder work.
Sources
- Schedule 1 to the Building Regulations 2010 (Requirement G3) — legislation.gov.uk
- Approved Document G with 2024 amendments — GOV.UK
- Approved Document G Frequently Asked Questions, March 2016 — GOV.UK
- Hot and cold water systems — HSE
- Competent person scheme: current schemes and how schemes are authorised — GOV.UK
- Megaflo Eco unvented cylinder installation manual — Heatrae Sadia