hi, i wanted to say a couple things about cold / warm climate thermal.
the potential to boil is usually a symptom of an atmospheric pressure system. whereas all closed-loop pressure systems will self-regulate via the pressure itself going up and creating a condition in which the boiling point conveniently goes up along with it. therefore, the higher the pressure, the higher the boiling point. the higher the heat, the higher the pressure, the higher the boiling point. you will rarely find liquid from a p.r.v. valve on the solar side of a correctly installed and functional closed loop solar hot water system.
i imagine oil could easily be a replacement in a year-round cold-climate closed-loop system. i don't think the performance would be much better than that of a glycol system. you can study the ratio of performance reduction per percentage of glycol verses water for your specific climate.
you can also build a drainback style system. the advantage in cold climates is that the collector sizing can be greater than that of your ideal storage. in california, 1 sq ft. collector area (high temp copper) to 2 gal of storage. this can be changed to say, 1.5 sq. ft. collector to 2 gal of storage. this can be problematic in a closed-loop system, but not in a drainback situation. during winter, a larger collection of energy is available. during summer a shorter cycle occurs. empty all other hours not requiring freeze protection nor having failures due to hot stagnant liquid problems... i have serviced distilled water drainback systems in freeze territory; they sound funny (literal), look funny, are installed out of plumb and square - but are still there. they work.
additionally, i would be diligent in studying the fitting of a rocket stove to heat water. it seems that the rocket stove is to ramp temperature high enough to cause all particulates to be burned and that the high temp is what causes the efficient fire to work and heat the mass. i would think the introduction of running cool liquid temperatures in an exchanger would cause the stove to have under-performance issues and clogging due to the loss of temperature from doing so? occasional use is probably not much of a problem. or just replace the solid mass with the liquid mass. i would like to experiment more with rocket stoves because of their great efficiency.
