The thermal performance of a passive solar heating system has been investigated via simultaneous temperature, velocity and flux measurements and their acquisition. The sensor locations inserted on the full scale model of a passive heating system, a single zone with a thermal wall, were on the walls and at the vents. The thermal wall of the zone is similar to the so-called Trombe wall in the literature, but it has no storage property. Experimental data showed that the wall black surface is sensitive to fluctuations in the solar intensity rate. Evaluation of the data using the definitions of the modified Rayleigh numbers concluded the existence of two extreme points. One of them represents the situation of the maximum amount of heat transfer to the room (test room). The second extreme situation is the beginning of the heat loss to the environment due to the reverse circulation process, namely night time cooling. (C) 2002 Elsevier Science Ltd. All rights reserved.