It is a general purpose programmable microcontroller IC on a PC board that I designed. The thermostat is a temperature sensor with programmable turn-on and turn-off setpoints set in software. Valve is 1/2" with reducers to adapt to my line which is 3/16" (21 taps on a single line), but a 1/2" or larger mainline can be used too, with larger valve size if necessary.

Dave