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Can't suggest anything specifically; but for long-term reliability passive thermo-siphon can't be beat for simplicity and having only a single moving part. You can make all kinds of fancy stuff with all kinds of controls and valves and pumps etc . . . . just more stuff to go wrong. I've looked at some various systems; they seem so overly complicated and yet don't seem to offer anything additional over very simple systems . . .

There was a collector called the Copper Cricket years ago that seemed so outstanding to me. Used an alcohol mix in a system which you pulled a small vacuum on . . . makes the mix boil at a lower temp. Phase change takes more heat than just heating the liquid. Anyway; it also is an "anti-freeze" which requires NO draining. Self-regulates as well; can be used way up north where it gets cold. One and only one "moving" part . . .. a one-way check valve to run the entire system. If you are interested e-mail me; I can send you a copy of the brochure that I have; it's old but the principles have not changed and it gives you some insight of how effective yet simple a good design can be . . . easily adaptable to DHW or radiant heat. Am considering building my own version of one as I too have radiant heat . . . good luck . . .

Bob