Thermostatic mixing valves

The purpose of a thermostatic mixing valve is to mix hot and cold water, supplied to the valve by two pipes, to a desired temperature. The valve is designed for constant and fixed values of the incoming water temperature and pressure but the thermostat compensates for variations. The valve has two handles, one adjusting the preset temperature and the other adjusting the flow. The thermostat is a mechanically fed-back temperature adjustment valve. The thermostat incorporates a temperature-sensing element. This element can be of various type, e.g. employing shape memory alloys, but the most common type is a wax actuator.

The heart of the thermostat is the wax actuator, see figure 2. The actuator piston position is a function of the actuator temperature. The actuator consists of a brass cup enclosing a wax and copper flake mixture. The size of the copper flakes is in the order of magnitude of 10 m. The largest part by weight is the copper. It is used both as a filler to allow a proper amount of wax as well as to conduct heat to equalise the temperature distribution. The temperature sensing is based on the volume expansion when the wax liquefies as its temperature is increased. This change in volume is amplified by a rubber cone and transferred to the piston. The wax and the rubber cone are separated by a rubber diaphragm.

The wax actuator cup acts on a valve spool supplied by springs, see figure 3. The actuator piston is connected to the temperature handle. The mixed water flows past the actuator cup and heats or cools the wax/copper mixture. When the water temperature drops below or rises above the preset temperature, the actuator detects the incorrect temperature and moves the spool to adjust the water temperature.