I am trying to post a diagram of the setup design overview, as it is right now, but it was reduced to quite a small size, so I am trying again, in a different way.
Some text has been cropped in the diagram, as I have problems exporting from TAPPS (Windows XP!) to something readable.
DC1 and DC2 pumps will run if 1) electricity is received by PV panels and 2) S1 temperature probe is exceeding a set value (possible 75C to 80C, depends on liquid and certain other items). That way, those pumps will augment the existing pumps in case of AC power failure, controller failure, AC pump failure or other situation causing overheat condition.
As I am not always at the house to fix it, I would prefer some liquid which would not boil away in case of a failure, hence the oil mentioned previously sounds interesting (despite viscosity and heat capacity problems).
A liquid that never boils nor freezes, at any expected temperature whether there is system failure or not, is quite tempting to consider. Especially if it lasts a long time, and doesn't gel up.
