I think you would find it effective and easier to have separate thermal mass in contact with the water tanks and heated by the fire.
Thermal mass water storage tanks.
It is then sent to a hot storage tank.
A rocket mass heater will burn cleanly no creosote to condense inside flue channels and heat a masonry or cob mass warming the water while absorbing some heat from the water during the day.
If you check the specific thermal capacity value for various materials you will notice that water has a remarkably high thermal mass.
In other words it is the ability of a body to store thermal energy.
What is water storage tank and thermal mass.
It is kept liquid at 288 c 550 f in an insulated cold storage tank.
No water coils or piping logistics needed.
It also releases this heat energy faster when you really need it.
Thermal energy chilled water or hot water is produced during periods of off peak electrical demand or usage collected in a thermal energy storage tank then withdrawn and distributed to the facility during on peak periods.
We have tanks from 120 gallons up to 5 000 gallons for large solar heating systems of wood boiler system.
The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 c 1 051 f.
With proper insulation of the tank the thermal energy can be usefully stored for up to a week.
By definition thermal mass is equivalent to thermal capacitance heat capacity.
Thermal storage tanks are used in solar heating systems and wood boilers where you need to store the produced heat in water for later use.
Water stores 3 to 4 times as many btu s per lb.