The efficiency of a finned coil type heat exchanger mounted in the solar collector air outlet duct of a space heating system for providing hot water in the spring, fall and summer months was determined with a series of equations and graphically represented. The equations were used to determine the effect of coil fin surface area on the hybrid system efficiency. From an equation calculating the air to water heat transfer rate, the heat balance for the air to water heat exchangers can be determined. Other thermal performance parameters were calculated prior to the development of a graphical solution to the evaluation. The graph can be applied to economic tradeoff studies for determining the optimum heat exchanger area relative to that of the collector. An experimental solar air heater test facility indicated the limitations of an air to water hybrid solar heating system.