This is a multi-piece shower control system consisting of a thermostatic in-wall valve which works to maintain a constant temperature of the water, with a cover plate and temperature dial, and a separate volume control which controls the amount of water volume you prefer.
You leave the ‘mixing’ of the hot and cold water up to the valve, so it arrives at the right temperature, and the ‘volume’ control actually allows you to change how much water is actually coming out of the shower head.
A thermostatic shower control system will also maintain the temperature of the water plus or minus 3-degrees from what the original setting is, to a greater degree of accuracy. A thermostatic valve allows a you to have greater control over the temperature of the water: you to pick the temperature before turning on the shower, then separately turn up the volume of the flow. Even in a shower with just one shower head, if there are multiple users, a Thermostatic Valve is an excellent investment. An example of an “integrated” Thermostat and Volume Control trim is shown here.
The Valve Trim, or face plate, includes a temperature dial to set the exact water temperature up to 104-degrees. An anti-scald feature prevents the temperature from being turned past 100-degrees accidentally by including a two part control, which requires the user to deliberately push and turn the valve to achieve higher water temperatures.
In a shower with a thermostatic system installed, one valve generally controls the temperature of all water outlets. Each outlet, including the shower head, tub spout, body sprays, or hand shower will each have its own volume control. BathPacks designs create a customized shower experience; all or one water outlet can be turned on at one time, with a specific temperature set, before you even enters the shower.
Thermostatic systems work well for homeowners who prefer complete control over their shower, and for showers being used by two people with different preferences in water temperature. (My husband sets his showers for 102°, while mine is 96°!)