This paper presents a methodology to predict the performance of a natural-circulation solar air heater. The forcing functions are monthly mean hourly values, and both the diurnal and seasonal cycles are considered. Results are for Naples, Italy, 40.85°N latitude, for a single south-oriented vertical glass and for a winter period of October to March. The seasonal useful collected energy corresponds to an average value of 2.77 MJ/m² per day. The following heat-removal factor and overall heat-loss coefficient are suggested:

F_R = 0.56 +/- 5%, U_L = 4.6 +/- 5% W/m² x K