In my relentless pursuit of off grid hot tub perfection, I have gotten a heat exchanger so that I can run glycol through my evacuated tube array instead of water. Brominated or chlorinated water isn't good for the copper manifold. It is an extra hassle but the array wasn't cheap.
Of course, having a closed side to the system requires some kind of consideration for expansion and over pressure relief. So, my first thought on how to go about this is to get a automotive radiator cap and do it like they do in cars. Have a container for over flow and pressure release if needed, as well as allowing coolant back in when everything cools down and contracts.
Opinions? Think this will work? And if so, what is the easiest way to get or make a radiator cap seat? I don't have rigs with a brass neck any more and I can't seem to find one yet. EDIT: Found one easy peasy.