1.5.2 Solid state relay
Modifying the STC to provide output signals to drive solid state relays instead of the mechanical relays is also pretty straightforward. De-solder the relays (the points at the end of the red lines above). And bridge the connections shown (by the red lines). Now, the 12V signal that usually drive the relays are fed to the screw terminals for heating and cooling, and the input to the solid state relays can be directly connected to these.