Bathrooms & Wet Rooms
Thermostatic mixer valve
TL;DR
A thermostatic mixer valve (TMV) blends hot and cold so the outlet is capped at a safe temperature — typically 43°C for showers, 38°C for basins in care settings. A standalone valve swap is £145–£245 fitted; a full install on a new shower is £245–£425. TMV3 (the higher spec) is a legal requirement in care homes, nurseries, and disabled-access showers. In your own home you don't legally need one, but they're the single best thing you can fit to protect kids and older parents from scald injuries.
A thermostatic mixer valve is the little disc of engineering that stops your shower going ice-cold when someone flushes the loo, and stops the hot going scalding when the cold mains dips. Modern showers usually have one built into the valve body. Standalone TMVs are used on baths, basin taps, care-home showers, and anywhere the water temperature needs a hard safe cap.
This page covers TMV2 vs TMV3, the compliance rules for landlords and care settings, honest install costs, and the questions to ask any fitter — because a TMV that isn't commissioned properly is legally the same as one that isn't fitted at all.
Fixed £50 diagnostic
Flat fee. No hourly meter running.
Fair quote or refund
Transparent pricing — refunded if we don't fix it.
Vetted UK engineers
DBS-checked, insured, real reviews.
AI-triaged in seconds
KJ chats first so the right engineer arrives.
TMV2 vs TMV3 — what's the difference?
TMV2 — the domestic standard. Fine for private homes, HMOs, hotels, offices. Blends hot and cold to a stable outlet temperature (typically 43°C max for shower, 46°C max for bath). TMV3 — the healthcare standard. Same blending, plus tighter tolerance and mandatory fail-safe: if the cold supply is lost, the valve shuts off within seconds so the hot can't scald. Legally required in NHS estates, care homes, nurseries, and disabled-access wet rooms.
- TMV2 → private homes, HMOs, hotels, gyms, offices
- TMV3 → care homes, nurseries, hospitals, sheltered accommodation, disabled bathrooms
- Both must be WRAS-approved and BSEN1287 / BSEN1287 compliant
- Both need annual commissioning and inspection in commercial / care settings
Where you legally need a TMV
England & Wales Building Regs Part G3 require a TMV on any bath tap fitted to a new build or full bathroom refit, with the bath outlet capped at 48°C max. Care Quality Commission (CQC) regulated settings must fit TMV3 on all baths, showers and washbasins used by service users, with annual commissioning records. Nurseries have similar duty of care under Ofsted. In private residential rented accommodation, TMVs are strongly recommended (and increasingly demanded by insurers) but not universally legally mandated — check your local council's HMO rules.
What a proper TMV install and commissioning includes
We size the TMV to the flow rate of the outlet it serves — an undersized valve can't blend at low draw and gives temperature-hunting. We fit isolator valves on hot and cold sides, non-return valves upstream (to stop hot back-feeding into the cold supply), and commission the valve at both maximum and minimum flow. On TMV3 we run the mandatory 60-second cold fail-safe test and issue a written commissioning certificate.
- Valve sized to outlet flow (2 L/min basin vs 12 L/min bath — different valves)
- Isolator valves fitted on hot and cold feeds
- Non-return check valves upstream of the TMV (WRAS requirement)
- Commissioned at max flow and min flow — both must hit setpoint ±2°C
- TMV3 cold-fail test: cold supply closed, outlet must shut within 5 seconds
- Written commissioning certificate for landlords, care homes, and insurance
- Annual recommissioning offered for care and commercial settings
Before you commit — questions to ask any plumber
TMVs are one of the easiest things to fit badly and one of the highest-consequence when it goes wrong. Ask these before you agree.
- Is the valve WRAS-approved and TMV2 or TMV3 (depending on my setting)?
- How are you sizing it to the outlet — flow rate measured?
- Are you fitting non-return valves upstream (WRAS mandatory)?
- Will you commission it at both max and min flow and give me a written certificate?
- For a TMV3 install — will you run the cold fail-safe shut-off test?
- Do you offer an annual recommissioning service? (Required in care settings.)
- What brand and model — Reliance, Honeywell, Mira, Thermorad?
Typical prices
Real-world ranges from recent Northwest jobs. Your final price is confirmed in writing before any work starts.
| Item | Typical price | Notes |
|---|---|---|
| TMV2 basin / shower — swap only | 145 – 225 | Existing valve, 60–90 mins |
| TMV2 — new install with isolators + non-returns | 245 – 385 | First-fit, 2–3 hours |
| TMV3 — care-home or nursery install | 285 – 425 | Includes commissioning cert |
| TMV3 — bath install with fail-safe test | 325 – 495 | Larger valve, longer test |
| Annual TMV3 recommissioning (per valve) | 45 – 75 | For CQC / Ofsted compliance |
| Multi-valve commissioning (care home, 5+) | 225 – 425 | Per site, batched |
| Written commissioning certificate | included | Per valve, per commission |
| TMV unit — brand-name WRAS | 45 – 165 | Supplied at cost |
Kit & brands we use
We fit Reliance Water Controls, Honeywell Braukmann, Thermorad and Mira Beat TMVs as our defaults — all WRAS-approved, all easy to source parts for, and all trusted by NHS estates and care providers. We do not fit no-brand TMVs from unverified suppliers.
TMV red flags
- Fitting a TMV2 in a care-home setting where TMV3 is legally required
- No non-return valves fitted upstream (WRAS breach)
- No commissioning certificate issued
- Not running the cold fail-safe test on a TMV3
- Undersized valve for the outlet (basin valve fitted on a bath supply)
- 'It doesn't need commissioning, it's factory set' — false, every TMV needs on-site commissioning
- No annual recommissioning offered to a care or rental client
Our promise on this job
Flat £50 diagnostic — deducted in full from any work booked. Fair, transparent quote before we start. If we can't fix the fault we diagnosed, we refund the diagnostic. All gas work is signed off by a Gas Safe registered engineer.
Talk to KJ about thermostatic mixer valve
Real triage in seconds. £50 diagnostic visit — refunded if we don't fix it.