I agree with Jim. I think you can over do the storage concept. It all comes down to sizing of the components. How closely does the boiler output match the load of the building? If at design conditions the boiler matches or is close to the geat load there will be no need to store.
Also how often are you at design conditions where you live? If it gets cold and stays cold it is easier to have the fire match the load.
How many zones do you have and how often do you "micro-load" your system?
If you prefer to run the system hard and store energy for overnight, or possibly a day or so, buffer capacity can be useful. You need enough horsepower to both meet the load and have some power left to "load' the buffer.
Remember mother nature will take all that stored energy back, eventually. Even with 4" of foam, fairly tightly installed, I still lose heat from my tank to the room. Enough in fact that it will often heat the space without running the floor radiant.
I think there is an ideal storage capacity for every system. It's a moving target, of course
Ideally it is designed to your needs, wants, and desires. Also space, logistics and budget comes into play.
Really the best way and place to store your energy is in the un-burned chunk of wood. Once you combust it and turn it from chemical to thermal energy, it is easier to lose track of, and transfer and standby losses start chiseling away at your "stash"
Just like solar thermal, use it or lose it.
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