SDMCF
I would agree heat losses to the interior is no problem but if the heated water is returned to the panel is not a good situation.
A simple solar hot water system would provide an excellent space heating solution for a weekend home. PV to power the pumps and be totally autonomic. Imagine your weekend retreat being warmed all week to be cozy warm on your arrival. FREE!!!!! Nirvana!
For a back-up source maybe wood or propane.
Tubing is quite inexpensive compared to a heat exchanger. A dedicated loop is the simplest, lowest cost and most effective. Drain-back system would also simplify and improve the efficiency. Glycol isn't cheap.
The insulated concrete is a huge heat battery that can be charged and stay relatively warm for a few days.
In our large installation I have noted a good amount of heat is returning to the solar panel. As with any system there is no heat-exchanger 100% efficient. This situation returns heated glycol solution to the panels and heat that could have been sent to the in-floor heating loop isn't harvested. Especially on a clear high energy day.
If I were to build again I would put two loops in the floor. One dedicated for the solar hot water collection and another for back-up heat.
Randen
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