A detailed building energy analysis tool is used to model the performance of a forced-air system and a radiant floor system. These two systems use a low-temperature tank to store solar energy from a solar collector array. An electrically heated tank at a higher temperature is used to provide any additional heat needed to meet the space heating load. The simulation models developed are then used to compare the performance of the,forced-air and radiant floor systems based on maintaining the same operative temperature inside the space. It is found that the portion of the heating load that comes from solar energy is higher in the case of the radiant system. This portion from solar energy increases even further when the operating temperature of the radiant floor is lowered. The results also show that the energy performance of the radiant floor relative to the forced-air system improves for houses with higher envelope R-values and infiltration rates. Given the many different interacting factors that influence the performance of the solar-assisted systems studied, it is very important to use detailed simulation models to help assess which system is more energy efficient for a particular application.