This paper examines the optimisation of a PV/T air collector with a fixed length and width (L = 5 m, W = 3.6 m) linked to the mechanical air distribution system of a typical residential building. A methodology is developed for the full optimisation of the rate of effective thermal output ((Q) over dot(eff)) across a wide range of the air mass flow rate per collector unit areas ((m) over dot/A(c) = 0-0.07 kg/s m(2)). For the optimisation methodology the air temperature rise (Delta T) was not restricted and several values of the weighting ratio of heat to electricity were compared. The results show that optimally designed PV/T systems can deliver essentially the same amount of net energy across a wide range of air mass flow rates suitable for a wide range of end-use applications with varying temperature rise requirements. In comparison when an exergy optimisation method is used, the resulting PV/T air system design delivers a low air flow rate and a very high air temperature rise (Delta T), but delivers low amounts of energy. Therefore the exergy method is not recommended for the design of a PV/T air system where the end-use is space heating. In addition a sensitivity analysis of (Q) over dot(eff) and Delta T values was carried out across a wider range of the air mass flow rate per collector unit areas ((m) over dot/A(c) = 0.01-0.2 kg/s m(2)) and various values of the air channel depth (D = 0.01 - 0.09 m). It was found that a PV/T air system design with a smaller collector depth offers a good performance at higher Delta T but this design is very sensitive to changes in the air mass flow rate per unit collector area ((m) over dot/A(c)). On the other hand, a PV/T air system with a larger collector depth can deliver a better overall energy output but a lower temperature rise is achieved. However, the system is less sensitive to variations in the air mass flow rate. (C) 2016 Elsevier Ltd. All rights reserved.