The goal of the present study is to develop a 200,000 kcal/h-class wood pellet boiler. A series of numerical simulations regarding combustion load and heat transfer characteristics inside the combustion chamber of a wood pellet boiler has been conducted. We have considered the load from 56% to 130%. The results show that a high temperature zone is at the entrance of the 2nd chamber for all combustion loads considered. The flame is stretched to the top of the 2nd chamber. The high temperature zone at the 2nd chamber becomes wider and strong for a higher load. The flow is along the partition. The high velocity region is at the exit of chamber for all combustion loads considered. Compared to experimental data, numerical simulations predict the temperature distribution for partial load operation.