In texas during the summer we often take showers with no hot water (or very little hot water).

A tankless water heater with a feeder pump, dedicated to bathroom use and with a bathroom-installed remote control, makes this trivial. It's probably also more efficient, easier to use, and (vs. thermostatic valves) about as easy and maybe also cheaper to implement than any fussing with valves.

A 30-year-old tank heater in a basement that needs a wrench to adjust a temperature that won't meaningfully change in a 2nd-floor bathroom for another half-hour, and also has to serve a dishwasher and clothes washer that might also run at higher temperatures at the same time, can't swing this as easily, or at all.

I’m not sure that’s a good idea. 120F is in the danger zone for Legionnaires' disease. https://en.wikipedia.org/wiki/Legionnaires%27_disease#Preven...:

“Keep water temperature either above or below the 20–50 °C (68–122 °F) range in which the Legionella bacterium thrives.”

Of course, you might say, why not put the thermostatic mixing valve at or in the shower, which also works pretty well!

That's clearly a case of conversion results being given unwarranted precision.

This is the one with one grip that lets you set the temperature, and the other sets the flow rate.

It'll (try to) hold the output temperature steady even if the input temperature varies a bit.

It seems to have had slower uptake in the US. Donald Norman’s ”The Design of Everyday Things”, 2013 edition, contains this passage in a list of different faucet designs (p. 151):

> Control temperature and rate of flow. Use two separate controls, one for water temperature, the other for flow rate. (I have never encountered this solution.)

I e-mailed Don about it a few years ago, and by then he had become aware of the concept.