I'm think of having both DHW and home heating from drainback solar
It seems to me that there's a lot of benifits in having not too large
a tank for domestic hot water, and there's a lot of benifits in having a
large thermal store for home heating.
So, perhaps multiple tanks of perhaps 40 or 50 gallons each.
I think you'd want to connect all the tanks near the top so that if
you overfilled one it would drain off to the next. I suppose you could
just drain into one tank and let the rest fill from that.
But what about the rest of the plumbing? Is it better to work one
tank at a time or some series connection.
Or is this just too much plumbing with too many heat exchangers? I'm
thinking on the fly again tonight... not always good!
I've reached that same pointing in my planning. Multiple tanks
help because one can be warmer (holding the heated water from
the panels) than the rest so the warmer tank would be used
for DHW/pre-heat but the entire battery of tanks can be used
for radiant heating.
As noted, this arrangement works very well for drainback
so it saves on initial costs and simplifies the design,
except for the design of balancing the water across tanks.
I'm considering using siphon tubes but I haven't convinced
myself that they are reliable enough.
So, my preferred arrangement is (keyboard-graphics)
[collector(s)] -- hot --> tank1 | tank2
Cold water is taken from tank2, heated by the
collectors and drained into tank1. When tank1
gets too full, its excess flows into tank2.
DHW is drawn from tank1.
Radiant draws from tank1 and returns to tank2.
To be really fancy, more than 2 tanks could be used.
NOTE: all this is only sketched out, it has not been
built and tested. Has anybody done something like this?