Fw: [Cob] cob/earth oven also heating hot tub intake? Boiler parts

[Dognyard]

I was watching a home renovation show, and the old hot water heating
system that they were replacing had a small tank within the system that
is kept only half full, so the heated water has room to expand into as
it is heated. So you could close off the system, and it wouldn't burst
the pipes. If you had this secondary place for the heated water to
expand into, couldn't you just then use a simple tap that could be
turned on or off to allow the hot water to circulate into the hot tub?

I might be missing the whole problem....

Karen