Pressure-balance valve
Also called: anti-scald valve, shower mixing valve, thermostatic valve, rough-in valve, shower valve
A shower valve that holds the hot-cold mix steady when pressure changes elsewhere in the house — a flushed toilet or a running washer — so the shower can't suddenly scald. Required by plumbing code on every new shower. $150–$400 for rough-in and trim; a thermostatic valve is the step up.
Old two-handle showers had no protection: when someone flushed a toilet, cold pressure dropped, and the person in the shower got a blast of hot. Pressure-balance valves fix that with a piston or diaphragm that shifts to keep the hot-to-cold ratio constant as either side’s pressure moves. Plumbing codes have required them (or thermostatic valves) on new and replaced showers since the 1990s, which is why a tub-to-shower conversion gets a new valve even if the old one worked.
Two parts, sold separately or as a set: the rough-in valve, a brass body with supply inlets and a shower outlet that’s set in the wall on blocking at 38–48 in, with a plastic plaster guard that sets the depth for the finished wall; and the trim, the handle and escutcheon that fit that rough-in. Rough-in and trim must be the same manufacturer and series. Most valves have a temperature limit stop that’s set after installation so the handle can’t turn past a safe maximum — the inspector may ask to see it set.
Thermostatic valves ($300–$900 for rough-in and trim) hold an actual temperature you dial in, with volume on a separate control, and are the choice for rain heads, body sprays and anyone who wants the same temperature every morning. Both types are serviced from the front by pulling the cartridge, so an access panel is a convenience rather than a requirement — but leave one if the wall behind the valve is reachable.