Store hot water at 60°C to kill legionella, then deliver it at 43°C so nobody gets scalded. Those two requirements sit in direct conflict with each other, and the thermostatic mixing valve is the device that resolves them. Understanding how it works and how to maintain it properly is not just good practice. In many settings, it is a legal obligation.

How It Works

Hot water enters one port, cold enters another. Inside the valve sits a thermostatic element, typically made from wax or bimetallic strip, that expands and contracts in response to temperature changes. As it moves, it slides a piston that opens one inlet and closes the other, continuously adjusting the blent to hold the outlet at the same temperature. 

The critical part is what happens when something goes wrong. If the cold supply fails, the element expands and shuts off the hot water supply entirely. The outlet stops flowing rather than delivering scalding water to the user. That fail safe behaviour is the whole reason TMVs are specified in care settings, and it is precisely why a TMV cannot be swapped out for an ordinary mixer tap without consequence. 

TMV2 and TMV3: Know the Difference

Not all thermostatic mixing valves are equal, and in regulated environments the distinction between specifications is not a matter of preference.

  • TMV2 is the domestic and general commercial specification, suitable for most standard installations.
  • TMV3 is the higher specification required in healthcare, care homes and any setting with vulnerable users. It carries tighter tolerances and a more demanding test regime.

If your setting carries a duty of care, fitting a TMV2 where a TMV3 is required is a compliance failure. We stock both specifications. Here is a selection of valves available now:

Gledhill Mixing Valve XC005

Zip Thermostatic Blending Valve AQ4

Santon ALK03 TMV2 Blending Valve 94970010

Heatrae Sadia Pack U3 Thermostatic Mixing Valve 95970354

Santon ALK07 TMV3 Blending Valve 94970037

Servicing: What It Actually Involves

TMVs require periodic servicing. In regulated settings, the interval and the record keeping are part of the compliance regime, not a matter of convenience or budget. Here is what the routine looks like in practice:

  1. Isolate both supplies before doing anything else.
  2. Check and clean the strainers. Most TMVs have inlet strainers, and debris in them is the most common cause of poor performance. Clearing them alone resolves a significant number of complaints.
  3. Check the non-return valves. Failed check valves allow hot water to cross into the cold supply or vice versa, producing symptoms that closely resemble a failed element.
  4. Inspect the cartridge or element. Scale is what kills them. In hard water areas, this is where the fault will almost always be found.
  5. Reassemble, then recommission. This is the step that gets skipped most often, and it is the one that matters most.
  6. Verify the outlet temperature with a calibrated thermometer and record the result. If you cannot show the record, the service did not happen as far as a regulator is concerned.

Adjusting a TMV

Most thermostatic mixing valves adjust via a cap or collar over the temperature spindle, sometimes located beneath a cover that needs removing first. Set the valve to the required outlet temperature for the application, then verify by measurement rather than by relying on the markings alone.

Always adjust with both supplies live and at their normal operating temperatures. Setting a TMV against a cylinder that has not yet reached its working temperature will give you a reading you will simply have to repeat once it has.

Descaling in Hard Water Areas

In hard water areas, scale build up on the thermostatic element is the most common failure mode. Some cartridges can be cleaned and descaled successfully. Many are designed to be replaced outright. Given the safety critical function of these valves, if there is any doubt whatsoever about the condition of a cartridge, replace it. The cost difference between a service and a new cartridge is negligible compared to the potential consequence of a valve that fails to perform.

When to Replace Rather Than Service

There are situations where servicing is not enough. Consider replacement when:

  • The outlet temperature will not hold steady even after a full service
  • The valve will not reach the set temperature at all
  • The fail safe does not operate correctly during a cold supply isolation test
  • The valve body is corroded or showing signs of leakage

Important Limitations

  • Specific service intervals depend on the setting and the applicable guidance. In healthcare and care home environments, follow the regime that applies to your sector.
  • Manufacturer instructions take precedence over any general guidance, including this article.
  • This article is not a substitute for a competent person. In regulated settings, TMV servicing and verification is a documented task that must be carried out by someone with the appropriate competence.

Not Sure Which Valve You Need?

Tell us the application, whether you require TMV2 or TMV3, and any relevant product details, and we will confirm the correct valve for you. Our team is available on 01934 253600, Monday to Friday, 8:30am to 5pm.