In this study. natural convection heat transfer in a vertical flat-plate solar air heater of 2.5 m height and I in width, with one- and two-glass covers was studied experimentally. Totally six cases of airflow (two for air heater with one glass cover and four for air heater with two-glass covers) were considered. These cases included states that air Could flow within spaces between absorber plate and glass covers or air was enclosed in such spaces. Absorber plate temperature, back-plate temperature, glass cover temperatures. mass flow rates of air within channels and the solar radiation were measured. The following relations are suggested: For channels in which air could flow: Nu = 0.7362Ra(0.2579) For enclosures: Nu = 0.1377Ra(0.2229) In the first relation. the characteristic length in Ra and Nu is the height of the air heater and in the second relation this length is the distance between the two vertical plates. The efficiency of the air heater was determined in various cases. The maximum efficiency was found for when the air heater had two glass covers and air could flow in all channels. (c) 2008 Elsevier Ltd. All rights reserved.