The use of biomass for district heating is a carbon neutral and efficient way to heat buildings. To ensure a sustainable use of the biomass, it is important to ensure a high thermal efficiency not only in combustion facilities of all sizes. In this paper, the thermal efficiency of a 3 MW combustion unit with recovery of waste flue gas energy has been evaluated, using process data from a full year. A decreased efficiency is observed at boiler loads below 1 MW, while the efficiency is more stable at higher load. The furnace and boiler efficiency are stable over different moisture contents of the fuel, while the efficiency including heat recovery is greatly enhanced at high moisture content. High return water temperature was linked to a decreased efficiency of the whole system due to decreased efficiency of the heat recovery unit.