Water stores 3 to 4 times as many btu s per lb.
Thermal mass water storage tanks.
Water contained in low cost non pressurized cylinders has proven to be the most practical and effective approach to the capture and storage of thermal energy for space and hot water heating.
Warm and chilled water enters and exits the tank through diffusers located at the top and bottom of the tank.
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.
It is then sent to a hot storage tank.
What is water storage tank and thermal mass.
The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 c 1 051 f.
By definition thermal mass is equivalent to thermal capacitance heat capacity.
It also releases this heat energy faster when you really need it.
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.
We have tanks from 120 gallons up to 5 000 gallons for large solar heating systems of wood boiler system.
It is kept liquid at 288 c 550 f in an insulated cold storage tank.
With proper insulation of the tank the thermal energy can be usefully stored for up to a week.
As rock or masonry.
No water coils or piping logistics needed.