In the present study, the thermal load of a residential building is determined in two sample cities. Karadj and Hamedan. Then, the suitable solar combi-system and the underfloor healing system are designed for the units of the buildings. Furthermore, in response to the building energy demand and also by studying the expense-efficiency diagram, the efficiency of solar combi-system and the number of flat-plate solar collectors are determined. In continue, regarding the effect of the shade of the collector rows and the restriction of the roof space, a hybrid system is suggested. Finally, the annual energy saving in fuel consumption and reduction in social costs are studied and the payback period is determined. © 2009 Asian Network for Scientific Information.