Just an update on the project:
1.Cut down a bunch of trees as they were starting to block the collector going into fall. More solar heat & more firewood!
2. Integrated the hot water tank with the furnace plenum heat exchanger, so I've got solar space heating. I have various thermostats & controls to make it work, but basically it has it's own dedicated thermostat in the living space and a thermostat that turns on when the tank is above 130F or so. It really blows out a good amount of heat and nice warm air. I can provide more details if anyone's interested.
3. The reservoir overflow issue has happened once since my last modification and levels stayed barely above the pipes but I think I've narrowed it down. In all circumstances the tank either reached its max temp of over 150F or the system didn't have power. So almost certainly there was boiling in the collector due to stagnation pushing the fluid out of the reservoir until it cooled enough to condense leaving the reservoir sparse. I simply refilled the reservoir with the glycol I was going to swap the water out with anyways. So the boiling point of the fluid should be higher. Also, the space heating cutting in above 130F should keep the tank below the 155F max tank temp, so the system will keep running.
Overall quite pleased so far. My electric bill is $20-25 less than last year's bills and I'm looking forward to seeing how much longer the firewood & propane lasts this winter