Thermostatic mixing valves

On a mixer like this the thermostatic part is built in behind the dial, and the diverter is a separate control that sends water to the spout or the hose. Do not confuse the two. A separate valve on the pipework, as a plumber may fit under a bath, does the limiting where the bather cannot reach it. Look for the maker's markings and ask the plumber which standard the valve meets: Approved Document G names BS EN 1111 and BS EN 1287.¹

Small mosaic tiles mean lots of grout, and grout around a valve is where water gets in behind the wall. Look for cracks, gaps or a dark line where the plate meets the tiles, and for any drip from the dial after a shower. Wiping the outside is fine; taking it apart is for a plumber. A valve is a safety device, and the HSE expects those in health settings to be monitored and maintained.²

White crust on the outside is a sign of hard water, and the same scale can form inside, where moving parts sit. The result can be a dial that goes stiff, a flow that surges, or water that runs hotter or colder than it used to. If that happens, stop using the bath for anyone vulnerable and have a competent plumber look at it. Valves exist because they can stop children being scalded within seconds, the Child Accident Prevention Trust says.³ Ask the maker how often to service.

The numbers round this dial, 10 to 50, suggest a temperature scale, but a photograph can't show what the valve does. A central dial can set a temperature, or it can be a diverter or switch, so read the markings and any paperwork. If you cannot tell whether the bath has any protection, ask a plumber to check. A new bathroom in an existing English home need not have a valve by law, so older bathrooms often will not.⁴ The cross handles are also hard on weak hands; see lever taps.

Scaled or worn nozzles change how the water flows, and a change in flow can change what the person under it feels. Clean or replace a heavily scaled head following the maker's advice. If the temperature swings when someone runs a tap elsewhere, that is a sign to call a plumber rather than to get used to it. Repeated heavy scale is also a reason to ask about the water supply itself; the stores below are in the US and are for comparison only.