Earlier quoted context omitted.
All thermostatic valves have cartridges that eventually wear off and need to be replaced. Cartridge is accessible from outside by removing the handle held by the side screw (handle can be stuck and hard to remove) and then unscrewing the cap. Remove the cartridge, look at the part number and buy the replacements at Amazon. Obviously cut off water to the shower first. Also handle has a small button on the bottom which…
Some even have integrated shutoff valves, so you can replace the cartridge without shutting off the water anywhere else! What a marvelous world we live in.
Wow, I wish the last one I encountered had that. Plumber hooked up the shower valve fixture backward at my dad's new place. Also didn't even properly tighten the screws. Thermostatic valves aren't reversible, so it just didn't work right at all (basically got to choose full-hot or full-cold). They neglected to provide a shutoff for the shower, and he wasn't sure about draining whole system (it's an over-engineered water system with well-draw, sump pump, water softener, multiple redundant filters, gas water heater, array of in-floor heating, hydronic air furnace, and who knows what else). Access around/behind the fixture plate was non-existant.
In retrospect probably should have just clamped the soft lines upstream to effect the repair.
