This article relates the results of a research project aiming at characterising the impact of load variations on wood boiler efficiency and emissions. It is based on a case study consisting in a wood fired district heating system in Geneva canton (Switzerland). The heat is supplied by two wood boiler (2 MW and 0.65 MW). The boilers efficiency and emissions were measured on the entire load range. The results given by the traditional indirect method (based on Siegert formula) were corroborated by direct measurements of the efficiency. Surprisingly, the efficiency found not to be influenced by the load: a constant value was observed for both boilers over the entire load range, even during the stand-by periods (83% for the 2 MW boiler, 90% for the 0.65 MW boiler). On the other hand, CO emissions are strongly deteriorated at low load because of the necessary increase in the excess air. NOx emissions remained low over the entire load range. The operation of the overall district heating system was also analysed during all year 2011. High distribution losses (>20%) due to the low linear density of the district heating system (0.9 MWh/m/yr) and a high oversizing factor of the wood boiler (>1.5) were highlighted.