How should a plumbing system that contains underfloor heating and a back boiler be designed?
That's overkill.  All you need is a heat exchanger.
It's vital to separate the system that can boil from the system that can't boil.   PWR/VVER nuclear reactors have a similar heat exchanger separating the &quot;must not boil&quot; section from the &quot;OK to boil&quot; section.
A gas water heater avoids the problem of boiling by simply regulating the gas flow when the water gets too hot.  A wood stove can't do that, because wood isn't &quot;throttleable&quot;. So it's possible for the heater/&quot;boiler&quot; to get more heat than it can use, and then, boiling is unavoidable.
The floor-heat temperature can be &quot;throttled&quot; by regulating the water flow coming from the wood stove.
A thermal store is a big tank of water kept hot.  In that setup, the wood stove's water loop interchanges into the thermal store, and then, the floor loop draws heat from the store.   This provides a large thermal buffer, which is nice but not strictly part of your requirement.