A three year study has been conducted concerned with the operation of convective heat diodes-passive solar heating devices which absorb and accumulate solar energy and then transport heat preferentially to the inside of a building. The study has been carried out in Gliwice, Poland (latitude 50 degrees north, longitude 19 degrees east), in a strongly industrialized region with a considerable level of air pollution. A module was built that consisted of four overlapping thermal diodes made of plastics. It has been found that the diodes, which constitute only 6% of the test cell surface area, lead to energy savings as high as 37% compared with the reference cells. This corresponds to some 700 MJ of energy per 1 m(2) of the diode per year. Based on the experimental results, the parameters have been estimated which appear in the energy balance equations for the diodes. It has been concluded that it is the thermal resistance of the insulation between the accumulator of one diode and the collector of another which has a crucial effect upon the efficiency of the diode, and appropriate improvements to present-day diode designs have been suggested.