In this study, the thermal performance of a Tank-in-Tank solar combisystem is dynamically simulated to investigate the effect of various parameters such as collector type and area, storage Tank volume, building specifications, heat exchange terminal units, and climatic conditions on system performance. The results showed that by increasing the collector area, Tank volume and thickness of wall insulation, the solar fraction increases. It was also found that the use of floor heating instead of a radiator system improves the system performance. The solar fraction using the evacuated tube solar collector is 2. 3% higher than that using flat plate solar collector. The annual solar fraction of 45. 6%, 63. 4%, 41. 2%, 34%, 57. 3% and 88. 1% is obtained in Hot-Dry (Tehran), Hot-Dry (Yazd), Cold-Dry (Tabriz), Moderate-Humid (Rasht), Hot-semi Humid (Abadan), and Hot-Humid (Bandar Abbas), respectively. The environmental analysis indicates that using the proposed solar combisystem could save 2241. 3 m³ natural gas and offsetting 4731. 5 kg less CO2 emissions during a year. The life cycle cost analysis shows that the payback time of the proposed system for the economic conditions of Iran is 7 years.