Ok, here's something I'm thinking about, but never done. I have several used (but sound) single-coil HWCs. I want to stack one above the other to form, essentially, a twin-coil cyl.
The bottom cyl's coil will be fed from solar panels and the top cyl's coil from the CH boiler.
On one forum I read that simply connecting the output of the bottom cyl to the input of the top cyl would work - but I can foresee mixing problems in the link pipe and might end up with two cyls of lukewarm water, until the whole lot heats up. I don't know if proper stratification would be defeated or delayed with this method.
I thought about an Essex flange in the wall of the top cyl, taking the hot-ish output of the bottom in there, where it would be further heated. The displaced colder water in the top cyl would sink down the inlet pipe (as was) to a T in the inlet of the bottom cyl.
Any ideas, objections, prior experience?