Thermostatic mixing valves

The conflict they resolve

Two requirements pull in opposite directions.

• Legionella control requires stored hot water to be hot enough to suppress bacterial growth. Storing cool is not a safe answer.
• Scald prevention requires water arriving at a tap or bath to be cool enough not to injure – and the temperature at which stored water causes a serious scald is reached in seconds, not minutes.

Turning the cylinder down solves the second by breaking the first. A thermostatic mixing valve solves both: store hot, blend at the point of use.

How they work

A thermal element inside the valve senses the outlet temperature and modulates the proportion of hot and cold to hold it at a set figure. If the cold supply fails, the element shuts the valve down rather than letting pure hot water through – which is the safety feature that distinguishes a proper TMV from a simple mixer.

Where required by regulation, G3 sets a maximum temperature for water delivered to a bath, which in practice means a TMV on the bath supply – see what Building Regulations Part G3 requires.

Where they are worth fitting

1. Baths, where it is a Building Regulations requirement for new installations and sensible everywhere.
2. Any outlet used by children, because a bath or basin filled at stored temperature is genuinely dangerous.
3. Where somebody has reduced mobility or reduced temperature sensation.
4. Care and healthcare settings, where it is expected practice – see healthcare.
5. Where you want to store hotter, which some households do for capacity reasons. Blending lets you store hot and deliver safely, and it effectively increases the usable volume of a cylinder.
6. Showers, where a thermostatic shower valve does the same job and also holds temperature when a tap runs elsewhere.

They need servicing, and almost none are

This is the part that matters and the part that gets neglected. A TMV is a mechanical device with a thermal element, sitting in hot water, frequently in a hard water area.

• They scale. In this county, scale on the element and the seats is the primary failure mode.
• They drift. An outlet temperature set correctly at installation can be several degrees out years later.
• They restrict flow as they scale, which shows up as a weak hot outlet – see hot water pressure problems.
• They fail in both directions. Too cool is an inconvenience; too hot is the scald risk the valve was fitted to prevent.
• The fail-safe needs verifying, not assumed. The check is to isolate the cold supply and confirm the valve shuts down rather than passing hot water.

Servicing means cleaning or replacing the element and strainers, verifying the outlet temperature with a thermometer, checking the fail-safe, and recording the result. A valve that has never been verified is providing protection nobody has confirmed.

Installing them properly

1. AccessibleA valve buried behind tiling with no access panel will never be serviced. This is the single most common installation failure.
2. With isolation and strainersServicing valves either side, and strainers on both inlets – because debris is what fouls the element.
3. Correct type for the applicationAnd correct orientation, which some valves specify.
4. Set with a thermometerAt the outlet, at the flow rate the outlet actually runs at – not by the marking on the cap.
5. Setting recordedSo the next person knows what it was set to and can tell whether it has drifted.
6. Fail-safe verifiedCold isolated, and the valve confirmed to shut down.
7. Not too far from the outletA long run between valve and tap is a dead leg holding blended water at a temperature that suits bacteria. Point-of-use is better than remote for this reason.