There are two reasons for this:

• For efficient combustion wood needs to be burned at a high temperature, and this requires a large firebox lined with refractory brick lining
• Fuel storage and handling is less simple than for gas or oil.

The fuel storage and handling system adds significantly to both the complexity and cost of a woodfuel heating system, costing perhaps as much as the boiler itself.

Various systems and designs exist, and stand alone pellet silos are available commercially.  For many small and medium sized installations, an auger screw feed is used to transfer wood chips or pellets from the store into the boiler.