Many plate fin and tube heat exchangers operate under low loads when the flow rate of air and water is low. The flow regime in tubes can change from laminar through transitional to turbulent. In this paper, much attention was paid to determine a semi-empirical correlation for the Nusselt number on the water-side in the transitional and turbulent range when the Reynolds number Re-w is higher than 2300. A new relationship for the friction factor in the transitional flow regime was proposed. The friction factor in the transitional flow range 2300 <= Re-w <= 3000 was obtained by linear interpolation of the friction factor for Re-w = 2300, and Re-w = 3000. The influence of the formula for determining the water-side friction factor in the transitional flow regime on the Nusselt number was shown. Based on experimental data, heat transfer correlations were determined for the air and water-side Nusselt numbers for the low velocity of air and water. The semi-empirical correlation for the Nusselt number on the water-side derived by integrating the Reynolds averaged energy conservation equation for turbulent flow agrees well with the empirical correlation for the Nusselt number. The heat flow rate from hot water to cold air was calculated as a function of the water flow rate using a numerical model of the heat exchanger with the correlation for the water-side Nusselt number developed in the paper to compare it with the results of the measurements. The results of the numerical simulation agree very well with the results of the measurements.