The effects of CO-tubes insertion on NOx (nitrogen oxides) and CO emissions from a compact heat exchanger were numerically simulated using a quasi one-dimensional premixed flame model, and the results were compared to experiments with a laboratory-scale heat exchanger and a commercial compact condensing boiler with a heat exchanger installed. The "CO-tubes" means that pre-cooling tubes were inserted between the burner and the main heat exchanger to reduce CO concentration. In the numerical results, the outlet concentration of NO increased as the distance between the burner and the CO-tubes was increased up to 3 cm. However, NO concentration was little affected by the CO-tubes when the distance was greater than 3 cm. Additionally, the concentration of CO reached a minimum at the distance of 3 cm. CO concentration increased when the distance from the burner to the CO-tubes was increased or decreased between 2 cm and 3 cm. These simulated results showed good agreement with the experimental results using a laboratory-scale heat exchanger. However, the maximum and minimum distances for NO and CO concentrations were different. When the CO-tubes were subsequently applied in a working commercial boiler, CO concentration was dramatically reduced by the CO-tubes and NO concentration was also slightly decreased. (C) 2011 Elsevier Ltd. All rights reserved.