Photovoltaic thermal (PV/T) air collector design requires an accurate determination of key parameters such as the channel depth and the air mass flow rate. This paper focuses on PV/T air collectors linked to an air distribution system with the aim of optimizing the channel depth, the air mass flow rate per unit collector area and the air distribution duct diameter considering the whole system performance. A weighted effective thermal energy output which includes electrical and thermal energy generated and fan power usage was utilized to examine the energy performance of the whole system. This study models PV/T air collectors with various collector areas (A(c) = 10, 15, 25 and 30 m(2)) and different length to width ratios (L/W = 0.5, 1, 1.5 and 2) linked to an air distribution system of a typical residential building in a climate with a mild winter (e.g. Sydney). For a constant temperature rise (Delta T = 10 degrees C), and maximizing the rate of effective thermal energy output per unit collector area ((Q) over dot(eff)) delivered to the building, the optimum channel depth (D-opt), the air mass flow per unit collector area ((m) over dot/A(c)), and the air distribution duct diameter were optimized. The optimum value of (m) over dot/A(c) is almost constant and approximately equal to 0.021 kg/s m(2). The optimum depth (D-opt) value varies between 0.09 and 0.026 m and the optimum air distribution duct varies between 0.3 and 0.5 m. The optimum depth increases as the collector L/W ratio and the collector area (A(c)) increase. (C) 2014 Elsevier Ltd. All rights reserved.