The first experimental results are presented regarding the workings of a plant which uses interseasonal heat stored solely from the sun as an energy source. The results obtained over one year concern radiation collection, the behaviour of the storage system and the satisfaction of the thermal requirements of a building to be heated. The comparison of performance obtained with those predicted by a plant simulation model provided a confirmation of the reliability of the models used for collection and storage tank accumulation. For the latter, the validity of the method used for calculation of heat losses that is the basis for the design of this kind of plant is shown.