I don't know yet if it should be tied into a future whole-house solar water heating or if the floor heating should be a small separate system. At this point we are just talking about a large bathroom, but I'd like to expand to an adjacent couple of rooms when we get ready to change those floors.

Any advice on such a piecemeal process?
If I do a separate solar water heating system for domestic water in the future (solar preheating of water before storage in existing electric tanks), couldn't the floor heating system be a very simple setup with a closed loop with no heat exchanger? Just the tubing going from the collectors to the floor and back? The collector would be very basic, just some pipes in a black-painted box.

My main question would then be, what elements would be required in such a loop?

I haven't finished researching this, but I'm thinking:
- air release valve and expansion tank at the highest point of the system.
- centrifugal circulator pump
- some kind of pump controller, thermostat or switch
- return manifold
- zone manifold to turn of a zone, for example if a towel warmer would be on a separate zone.
- 5/8" pex tubing with 10% polypropolene glycol in the water
- collector: Any advice on the materials or size?

Please advise if you think I'm missing something.

What to do about the system in summer? Can I just cover up the panel? I believe if the collector is mounted somewhat vertically, the summer heat gain would be neglectable, and the system could be left on year round.

I've understood that the water shouldn't be hotter than 150°F, if that happens, what should be done? Or is that what the expansion tank is for?