This paper describes the modelling of flow rate in a photovoltaic (PV) driven, roof slate based solar system for preheating ventilation air in cold climates. The system consists of a photovoltaic driven, attic mounted fan, which draws air through the spaces between the warm slates and delivers it through a metallic flexible duct into a house. A model for predicting the flow rate of air as a function of irradiance and ambient temperature is developed based on the measured performance of the different components of the system. Considering all experimental sources of error, the model predicts the flow rate of air with a maximum error of 12%. The model is validated for different combinations of components in a roof section constructed at Napier University in Edinburgh. The predicted flow rates are within 10% of the measured values. The model is extended so that it can be applied for different locations and different roof tilts and orientations. A future paper will make use of the model developed herein for system optimisation based on maximum monthly volume of preheated ventilation air delivered. The model will also be used to investigate the effectiveness of PV driven, roof slate based systems as solar air heaters. (c) 2005 Elsevier Ltd. All rights reserved.