The inverter is a 2,000/4,000W unit. Notice the two missing FETs and the wire soldered to the switch.
I learned as a child building model airplanes that there are always extra parts that aren't needed. The temperature probe is six silicon diodes in series. The globbing is hot melt glue. This assembly is inserted in some heat shrink tubing and when reheated makes a great watertight seal.
This gives about 10mV per C and makes the voltage ref less critical. The controller was depending on thermal delays and really needs a 30 minute delay before starting the next cycle.