What is the 38 °C stop on a thermostatic shower mixer?

It is a mechanical detent, not a lock and not a safety device. The handle clicks at about 38 °C and goes no further unless you press and hold a button, which is a deliberate action anyone can perform.

What it is for

It stops the handle drifting past a comfortable temperature during ordinary use, and it gives you a repeatable starting point. That is genuinely useful, and it is all the manufacturers claim for it.

What it is not

It is not a scald-prevention device, and no thermostatic mixer sold on a general catalogue should be treated as one. Devices intended to limit temperature for vulnerable users are a separate class of product, specified against national standards, and they are outside what this site covers.

Where the authority actually is

The mixer’s own installation instructions and your national plumbing rules. Both describe what the valve does, what supply it needs and how it should be commissioned. This site reports what a manufacturer states; it does not interpret a rule.

A practical note

Some models let the detent be reset to a different temperature during installation. That is described in the manual, and it changes where the click is — not what the click is.

This does not replace the manufacturer's own instructions. Every dimension, thread size, flow rate, operating pressure, water seal depth and load figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A flow rate is reported with the pressure the manufacturer measured it at, and is never converted to another pressure, because no conversion exists. And the statement repeated rather than assumed: the 38 °C stop on a thermostatic mixer is a mechanical detent you can deliberately push past, not a scald-prevention device. What your installation needs is answered by the mixer's own instructions and by your national plumbing rules, and nothing here replaces them.