Exhaust gas recirculation is a technique that allows for controlling the combustion chamber temperature and reducing the NOx and particle matter emissions. Moreover, it helps to mitigate soot formation and ash agglomeration in combustion systems. The present study investigated the effect of exhaust gas recirculation on combustion temperatures of a 140 kW underfed stoker biomass boiler. To this purpose, a wide range of operating conditions were used, collecting data regarding flue gas and fixed bed temperatures. It turned out that the recirculating ratio has a significant effect on the temperatures in the primary combustion zone, affecting the thermal gradient and the main thermal zones of the biomass combusting bed. The obtained results can be useful for lumped parameter modeling, or CFD validation purposes.