The limitation of fossil fuels use (especially coal) as an energy source for space heating and hot water enhances the development of new energy sources in domestic applications. Within different sources of renewable energy, the application of biomass is recognized as the main way for this purpose. The use of solid renewable fuels (e.g. wood pellets) in domestic-scale boilers is especially common in places where the distribution of natural gas by network or other renewable energy sources is strongly limited (mountain and foothill regions). Moreover, the application of raw or processed pelletization biomass can be problematic due to specific conditions during fuel transportation and storage. Namely, high moisture (humidity) and limited ventilation have a destructive impact on fuel stability. Wood pellets are extremely moisture unstable and present significant problems during the transportation, storage, and handling stages. Thermal treatment of biomass (including torrefaction) is recognized as a possible way to enhance the wood pellets’ hardness under wet conditions. Nevertheless, there are a few investigations of thermally–treated combustion biomass in domestic-scale boilers. Thus, the lack of knowledge in this field is observed. In this work, the combustion performance and emission of main pollutants (NOx, SO2, CO) during combustion of torrefied biomass are analyzed. The experiments were carried out using two types of boilers, i.e. (1) coal-fired 25 kWth (retort burner) and (2) biomass-fired 10 kWth (pellets burner). The construction of boilers has been not modified to analyze the direct fuel substitution in commercially available boilers. The nominal boiler’s output was achieved, and the emission issues are strongly influenced by oxygen excess. The emission of CO dramatically increased at oxygen excess of less than 5 vol.%. An overall conclusion is that torrefied biomass can be used as a substitute fuel in coal-fired domestic boilers. Nevertheless, the boiler construction can be improved to obtain lower emissions of carbon monoxide.