This paper presents a study on the improvement of a system for measuring solar heat gain through fenestrations. The system allows the quantification of the thermal fraction that penetrates into a room through a fenestration due to solar radiation, and also the determination of the Solar Factor. To test and improve the functioning of the system, some field measurements and calibration of sensors (heat flux meters) were carried out. Initial measurements demonstrated that there was an error in the Solar Factor obtained and also that there was an increase in the temperature of the heat flux meter support plate. Currently the equipment uses a heat dissipation system with fans that insufflate air directly onto the rear side of the heat flux meter support plate. The increase in the temperature difference between the plate and the external air reduces the heat flow through the sensors, causing errors in the measurements. New tests were carried out using a cooling system with water circulation in a thermostatic bath. The decrease in the temperature difference between the heat flux meter support plate and the air provided good results, demonstrating the necessity for modifications to the currently used cooling system. (C) 2006 Elsevier B.V. All rights reserved.