Thermoelectric generators (TEG) are based on Seebeck effect and produce electricity from a heat flow. Since they have no moving parts, they require low maintenance and provide quiet operation. On the other hand, they have low efficiency (what is less relevant when heat leaving the device at low temperature is used for other purposes) and they need suitable difference temperature (what requires careful thermal integration). In this framework, the integration of TEG in biomass boilers is a promising approach. It has been tested by including several TEG modules in a commercial pellet-fired 25 kWth boiler that has been modified and equipped with a monitoring system. The facility is described and some relevant experimental results are presented. For instance, the effect of TEG cooling is analyzed by comparing two options: i) tap water to be heated in order to produce sanitary hot water or ii) water from the heating circuit entering to boiler that is preheated while TEG cooling. Results show that more than 60W can be obtained by using 6 TEG and only a small fraction of heat released by biomass combustion. This power can contribute significantly to boiler’s ancillary consumption and would be significantly higher if more TEG were integrated, what would require additional boiler modifications.