[jeff5may]

I see a few unknowns in your post. Without knowing your targets, goals or main purpose of your solar thermal collector, it is difficult to lead you in the right direction. Rather than try to read between the lines, make assumptions, and take potshots at an answer, i'm going to ask more questions.

Sure, the collector is made to gather heat and store it, but for what purpose? Are you circulating potable water through it, to use for dhw? Or is it filled with brine or glycol with a separate heat exchange component? Are you also using it for space heating?

Another unknown facet is the interaction with the external store. How warm is the store getting while the sun is shining? Does it discharge quickly when drawn from? How is the circulation pump being operated now? How will the indirect hot water loop heat your potable water quickly enough to meet demand?

Feel free to provide as much or as little information as you are comfortable to divulge. The better you can describe your system and intents, the more spot-on your replies will be. There are as many ways to configure these systems as one can imagine.