[Greenbuilding] thermosiphon questions for the solar DHW experts

[Reuben Deumling]

I am piggybacking onto an existing solar DHW system that features two
Thermomax collectors, a glycol loop and pump, and an 80 gallon storage tank
with electric elements that are basically never used. The apartment I'm
carving off the house with this system has a 20 gallon storage tank in the
attic, which I'd like to connect to the existing system to share the
copious solar heated water. The bottom of the attic-located tank is 40"
higher than the top of the main storage tank. I have installed a 3/4" PEX
tubing loop that ties the *tops *of both tanks together as well as the
*bottoms* of both tanks.

(1) Does it seem reasonable to assume that this system will adequately
thermosiphon itself without the need of a pump? We have a spare pump but
would prefer not to use it.

Outside of the summer, our solar insolation here in the cloudy PNW is
pathetic and the system provides no useful DHW, so I will be isolating both
systems and each will rely on wood-heated water for those months instead.
(2) My question is whether I should install shutoff valves at both ends of
the two 35' runs connecting the two tanks, or just at one end, and if just
at one, which one? My thinking is that with 70' of  3/4" that is a lot of
potential hot water we might as well not be heating.

Thanks for all suggestions and ideas.

Reuben
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