On furnaces without forced air draft this is a major consideration. Heating of the water takes a long time and it is important that a steady supply is always present. Small tanks in units without forced air draft just don?t hold enough hot water between burn cycles. Extra in-house storage tanks can be added. In forced air models the replacement of hot water is very quick and efficient.
Unburned wood in a forced air system will instantly produce heat when the draft fan kicks on. These systems are more efficient and burn less wood. However there still needs to be sufficient water around the firebox to reduce the number of burn cycles. Even on small furnaces having forced air systems the absolute minimum for efficiency is 125 gallons capacity, increasing to 300 gallons on large furnaces.