Rich Whites

First question: Are you attempting to keep the well equipment from freezing? If yes,start by improving the thermal insuation around the equipment you want to protect. Consider a secondary enclosure to reduce the volume of the protected space. The enclosure needs to conserve the heat within it. Most people think only about conductive heat loss, but fail to do a good job seaing up air leaks, the last consideration is radiant IR losses. Then you can get a simple, single contact 120/240vac electric baseboard thermostat to control 12vdc from your battery. 12 volt lamps are available at any auto part store (Or Amazon). Lamps can fail. I would say get a 100 ohm, 10 watt wire wound resistor.

I was that solar nerd in the early 80s. I got involved in the energy sector during the first Arab oil embargo in the late 70s. The glycol based solar systems are effective and actually relatively easy to fix. Yes, parts wear out. Just looking at your photos I can see the system pressure is low, so either the expansion tank has lost its precharge of air, or the system has fluid leak. Next, the magenta Grunfos pump's shaft is located on a vertical axis. This a pump orientation which if air is gets into the system will result in the ceramic watercooled/ lubricated bearings failing. The C-30 controller was one of the best in its day. It uses a reverse temperture coeffecient 3k @70f thermistor at the panel and at the bottom of the storage tank. Issues here can easily checked with an ohmeter and some understanding of Ohm's law. I retired in 2023 and kind of enjoy these kind of things. send me a email. Best regards, Rich