Carbon monoxide (CO), hydrocarbons (C_xH_y), nitric oxide (NO), nitrogen dioxide (NO₂), and total particulate matter (PM) concentrations from wood pellet combustion were studied in two types of furnaces frequently used in Poland – over-fed (channel) and under-fed (retort) – integrated into two heating boilers with a nominal heat output of 15 and 20 kW located in a heat station. The over-fed furnace was situated additionally in the 20 kW boiler. Pollutant emission indicators were estimated. The conditions resembled the ones existing during wood pellet burning in domestic boilers, cheap and old construction, working without water heat storage or its capacity is insufficient. These boilers work without any proper automatic device equipped with an oxygen probe (lambda sensor) located downstream the boiler for air stream regulation. They also lack a proper automatic device for fluent fuel stream regulation in relation to the value of boiler water temperature. Therefore, when boiler water temperature reaches its maximum value (e.g. 85ºC), the pellet screw feeder stops (fan is still operating) and reinitiates after a water temperature decrease of 5ºC (unstable boiler operation). This is why temperature in the combustion chamber and oxygen concentration decreases significantly and carbon monoxide concentration increases rapidly and significantly. The impact of temperature in the combustion chamber on pollutant concentrations was presented. A significant difference in CO concentration values between stable and unstable operating conditions of three wood pellet combustion devices was observed and compared. The following results of CO concentrations were obtained: over-fed furnace in the 15 kW boiler – 609 mg/m³ – stable operation, 19,000 mg/m³ – unstable operation; over-fed furnace in the 20 kW boiler – 78 mg/m³ – stable operation, 1,300 mg/m³ – unstable operation; under-fed furnace in the 20 kW boiler – 336 mg/m³ – stable operation, 2,000 mg/m³ – unstable operation (concentrations normalized to 10% oxygen concentration in flue gas). A higher increase of CO concentration during decrease of temperature in combustion chamber in unstable boiler operation compared with stable operation was a consequence of significant oxygen concentration increase in combustion chamber when pellet screw feeder stopped, but fan was still operating. To avoid a radical increase of carbon monoxide concentration, it is necessary to gradually reduce fuel stream as water temperature in the boiler approaches its maximum value. Such a reduction should be performed by an adequate regulation of operations/break time of the pellet conveyer that does not cause high emissions of carbon monoxide.