What is the Difference Between Standard Mixer Valve, Thermostatic Valve and Pressure-Balance Valve?
Standard Mixer Valve: It is able to adjust water temperature, water on/off and volume. But it doesn't have scald prevention feature.
Thermostatic Valve: Thermostatic valve features scald prevention. It has separate volume control and water temperature control. This is because thermostatic valves react to the temperature, not the pressure of the water. With this valve, you can change the flow volume without affecting the temperature. When water exceeds the maximum set temperature, the element expands to reduce the flow of hot water, and allow more cold into the mix. Should either hot or cold supply fail, the valve will close off flow from the other side. The advantage with thermostatic valves is the direct control over output temperature, while your water heater can be set to for example 60℃/140℉ to protect against bacterial contamination, the water coming out of your shower head can always be for example 37.8℃/100℉ (a maximum output temperature is set on the valve).
Pressure-Balance Valve: Pressure-balance valve features scald prevention. It has just one control for both volume and temperature, namely you control both volume and temperature simultaneously, as well as a dial or set-screw that sets the stop-point for the handle (the maximum ratio of hot to cold). Pressure is balanced by way of either a sliding disc on a piston, or a spool, that react to changes and maintain the pressure ratio. When some cruel soul flushes the toilet while you're busy rinsing, cold water is sent to the toilet tank, reducing the cold water pressure arriving at the shower valve. The mechanism inside the pressure-balance valve will move to reduce (or cut off) the hot water, maintaining the balance between hot and cold flows: temperature should not fluctuate more than a few degrees. Always keep in mind that a pressure-balance valve doesn't pay attention to temperature, so with the valve set at maximum temperature, and the handle turned all the way to "hot", the water coming out of the shower will be as hot as the water heater has to offer (which could be even higher than the setting on the thermostat).