Abstract An updraft gasifier-combustor was built to burn chunkwood (a random accumulation of wood particles 5–25 cm long, ranging from finger size up to 10 cm diameter discs) for on-farm heat production. Fuel charges of 30 and 45 kg, and various air injection rates were used for the combustion tests. An oil burner delivered a continuous flame into the secondary chamber. As judged by the stack gas CO/CO 2 ratio, the cleanest burn was achieved by injecting 50% stoichiometric air in the primary and 170% in the secondary. Calculated thermal efficiency of the combustor ranged from 32% for a 35% primary, 65% secondary test to 52% for the 50% primary, and 170% secondary test. The combustor produced heat energy equivalent to 11·3 litre h −1 kerosene at 40% efficiency, or in effect multiplied the fuel consumed in the oil burner by a factor of 7. At 52% efficiency the factor was 9. Estimated cost of the combustor and related equipment is $25 000, and at current fuel prices a user could afford to convert to chunkwood if his annual liquified petroleum gas consumption is 47 000 litres.