One Response to “One tank or two, sir?”


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  1. I’ve been plumbing a Simple Drainback tank with standard gas hot water tanks using a thermosiphon design. This has proven to be an excellent proposition.

    The design is elegantly simple. Simply set the conventional tank up on a 20′ stand so the cleanout of the standard tank is slightly above the inlet to the Simple Drainback heat exchanger. I set a “t” in that line, with cold water feed at that location. I also place a check valve between the “T” and the backup tank to prevent reverse thermosiphoning.

    The balance of the plumbing is standard, with the heat exchanger pre-feeding the standard tank.

    What we find is thermosiphon action effectively moves cold from the bottom of the gas backup tank into the heat exchanger, the water is heated and then passes up into the top of the backup tank. This extends storage, allows the collectors to run more efficiently, and does so without pumps or controls.

    One note: We find evacuated tubes to be better for this application. I disagree with the notion that the SRCC efficiency curves correctly characterize evacuated tubes. The science used in testing is, in my opinion, based on a single irradiation point (high noon). In actuality, tubes run far hotter for longer periods of the day than flat plates. For thermosiphon design, tubes provide greater fluid heating, thus more effective thermosiphoning.

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