Do You Need a Thermostatic Mixing Valve? TMV Rules Explained

What a thermostatic mixing valve actually does

A TMV takes a hot supply and a cold supply and blends them to a fixed outlet temperature, adjusting continuously as either supply changes. If somebody runs the kitchen tap and the cold pressure drops, the valve closes down the hot to hold the temperature. If the cold supply fails completely, a properly working TMV shuts the hot off rather than sending neat hot water through. That fail safe is the reason for fitting one.

This lets two things be true at once. The cylinder can store water at 60°C, which is what controls Legionella, while the bath receives water at a temperature that will not scald. Turning the cylinder down to make the taps safe gets the first half wrong. Our guide on what temperature should a hot water cylinder be set at explains why.

A thermostatic shower valve is already a thermostatic mixer built into the shower. A TMV is the separate, usually concealed valve that does the same job for a bath, basin or group of outlets.

When Building Regulations require one

Approved Document G, which covers sanitation and hot water safety in England, includes a requirement that the hot water supply to any fixed bath in a new dwelling must be designed and installed so the water delivered to that bath cannot exceed 48°C. It sits under requirement G3, the same part of the regulations that governs unvented hot water systems. The usual way to comply is an in line thermostatic mixing valve on the bath supply.

The requirement applies to new dwellings and to dwellings formed by a material change of use, such as a house converted to flats or offices converted to homes. Refitting a bathroom in an existing house is not caught by it in the same way. That does not make a TMV a bad idea in an older home; it just means it is your choice rather than a regulatory requirement.

• New build dwelling, fixed bath: TMV or equivalent required, 48°C maximum
• Flat created by conversion or change of use: same requirement
• Replacement bath in an existing home: not triggered by itself, still recommended
• Showers: normally covered by a thermostatic shower valve

TMV2 or TMV3: which type do you need?

TMV2 is the scheme for domestic thermostatic mixing valves, tested against the British and European standards for mixing valves on high and low pressure systems. If you are fitting a TMV to a bath in a house or flat, a TMV2 approved valve is the right product, and it should be approved for the pressure of your system, which on an unvented cylinder or combi means mains pressure.

TMV3 is a stricter performance specification written for healthcare, used in hospitals, care homes and similar settings where the people using the water are vulnerable and the outlets are part of a managed water safety regime. Those settings work to lower outlet temperatures and more frequent testing than a home. Many valves on the market carry both approvals.

For a family home, a TMV3 valve does no harm but offers no practical advantage over a good TMV2 valve. For supported living, care homes and some houses in multiple occupation where the residents are vulnerable, the operator will usually specify TMV3 as part of their own risk assessment.

Where the valve should be fitted

As close to the outlet as practical. The pipe between the TMV and the tap carries blended water at a temperature bacteria can live in, so the shorter that run, the better. Under the bath, behind an access panel, is the usual position. A TMV built into the tap body or bath filler is another option on some ranges.

Fitting one large blending valve at the cylinder and running the whole house on blended water is sometimes proposed as a cheaper alternative. It is not a good idea in most homes. Every hot pipe in the property then carries water in the Legionella growth range, and the temperature at a distant outlet is less predictable. Keep the distribution hot and blend at the point of use.

Whoever fits it needs isolation valves and access to both supplies, strainers on the inlets and space to service the cartridge. A TMV sealed behind tiles with no access panel cannot be tested or maintained, and will eventually become a tile removal job.

Why TMVs need servicing, especially in London

A TMV is a small mechanism doing a safety job continuously, and in hard water it scales up. The thermostatic element and the ports around it collect limescale, the strainers on the inlets block, and the check valves that stop hot and cold crossing over start to let by. The symptoms are water that will not get as warm as it used to, a temperature that drifts or hunts, or a bath that runs at a trickle.

The dangerous failure is the one you cannot see: a valve that no longer shuts off the hot when the cold fails. The only way to know is to test it, by measuring the mixed temperature and then isolating the cold supply to confirm the flow stops. Manufacturers recommend checking at least once a year, and care settings test more often. The same London water that scales cylinders is behind this, and hard water and hot water cylinders in London explains what it does to every hot surface in the system.

• Bath not as hot as it used to be: scaled element or blocked hot strainer
• Temperature swings when another tap runs: sticking thermostatic element
• Very low flow to the bath: blocked strainers
• Hot and cold crossing over elsewhere in the house: failed check valves
• No annual check in years: test the fail safe before relying on it

Landlords and houses in multiple occupation

There is no general rule requiring a TMV on every bath in an existing rented home. What there is, is a duty to keep the property free of serious hazards. Burns and scalds from hot water are one of the hazards councils assess under the Housing Health and Safety Rating System, and the risk is judged against who lives there. A bath fed at 60°C in a property let to a family with young children is the kind of thing that attracts attention.

For most landlords, a TMV on the bath is a cheap way to take that hazard off the table, provided it is then included in the annual checks alongside the cylinder. If the property has an unvented cylinder, the TMV test fits naturally into the same visit as the unvented cylinder service. The wider obligations are covered in landlord hot water responsibilities.

Scalding water is sometimes a cylinder fault, not a missing TMV

If the hot water has recently become much hotter than it used to be, a TMV is not the first thing to look at. A cylinder thermostat or immersion thermostat that has failed can let the stored water climb far above 60°C, and on an unvented cylinder that ends with the temperature and pressure relief valve discharging into the tundish. Switch off the immersion and the boiler and read our page on hot water cylinder overheating first. A TMV on the bath would protect that one outlet while the fault carries on everywhere else.

Steaming water at the taps, spitting or banging from the cylinder, or hot water running into the tundish means switch off the heat sources now and get the cylinder checked. That is a safety fault.

Getting a TMV fitted or checked

We fit and service thermostatic mixing valves as part of our hot water work across London, usually alongside an unvented cylinder installation, a service or a bathroom refit. On an unvented system, anything touching the sealed side of the cylinder needs a G3 qualified plumber, and the TMV and cylinder controls should be set together so the storage and outlet temperatures make sense as a pair.

Call 020 4652 5111 with the type of hot water system you have, the outlets you want protected and whether there is access under the bath. Our rates are on the pricing page.