A low-cost retrofit, i.e., shortening the secondary-air port area in the side near the furnace center, was performed in a 300-MWe down-fired furnace. This retrofit was expected to improve the severely asymmetric combustion and particularly poor burnout that were found in the furnace. The aim here is to demonstrate the validity of this retrofit by the presentation of improved flow characteristics and coal combustion within the furnace. Within a 1:15-scaled model of the furnace, cold airflow experiments were conducted initially with the original secondary-air port and then with the shortened one. After this modification, the original asymmetric distributions of the flow field, downward airflow decay, and airflow penetration depth all changed to relatively symmetric patterns. Industrial-size results confirmed the secondary-air port retrofit improved the asymmetric combustion pattern and particularly poor burnout. Experiences in this work suggest that for this type of down-fired furnaces, relatively small secondary-air ports should be equipped if symmetric combustion along with acceptable burnout needs to be established. (C) 2014 Elsevier Ltd. All rights reserved.