The aim of the research was to develop mathematical models describing the emission of selected pollutants correlated with the residual oxygen content in the flue gas. The correlation was made for low-temperature combustion of wood pellets in biomass boilers and furnaces. The developed models can be used in modern control systems of boilers, furnaces or for precise calculation of emission factors for the discussed group of heating devices. The description was made for devices with a stationary wood pellet combustion process with a heat output range from 12 kW to 30 kW. The obtained models, not currently used in this group of devices, will allow controlling the operation of heating boilers in a sustainable and ecological way, taking into account the environmental burden.