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Reverse piping an indirect HWT
I saw this diagram in an article by Siegenthaler, about reverse piping an indirect tank and using it as both an HWT and a buffer tank, but there's something I don't understand. It makes sense that if all the circs are operating, they would not interfere with each other due to the oversized headers, but it seems that if P1, the boiler circ, were operating alone, it would have to draw at least some flow through the circuits of P2 and P3. The indirect tank and the circuits of P2/P3 share the same oversized headers, so why would P1 draw only from the indirect tank?
Presumably there's a lot more head loss in P2 and P3 to overcome, compared to the indirect tank, but is that enough?
Here;s the original article: https://www.pmmag.com/articles/101984-john-siegenthaler-a-synergistic-system
Would appreciate any enlightenment. Thanks.