Combustion of wood logs continues to improve despite the diversification and standardization of wood-derived fuels. Gasification boilers— featuring separate gasification and combustion chambers—are widely used to meet stringent environmental requirements, yet they remain underrepresented in the engineering and scientific literature. To bridge this gap, 146 simulations were conducted to assess how refractory size, position, inclination angle, producer gas quality, and gas-inlet location affect flue-gas residence time, temperature fields, and heat transfer to boiler water. An 18-kW boiler is experimentally characterized with respect to producer-gas composition and temperature under varying gasification conditions. These data are used as boundary conditions for a CFD model that features non-premixed combustion and is described using the standard k–ε turbulence model, the Discrete Ordinates radiation model, and a domain-based weighted-sum-of-gray-gases model for radiative properties. Refractories not only protect metal surfaces, enable complete combustion, and aid particulate removal, but also enhance heat transfer. In total, 8 different combustion chamber designs are analyzed. Compared to the best refractory-free case, a combustion chamber with a U-shaped flue-gas flow path and two additional refractory-coated surfaces achieves 22% higher heat transfer despite a 17% smaller
heat-exchange area. Refractories that create a U-shaped flow path extend flue-gas residence time, span at least half the chamber length, and feature an asymmetric channel height that favors a smaller flue-gas cross-section at the exit. Finally, combustion chamber design depends on upstream gasification performance—specifically, producer-gas composition and temperature, and air-preheating temperature.