The residential dwellings and other buildings which will be part of the demonstration programs referred to in sections 5503, 5504, and 5507 of this title shall be located in a sufficient number of different geographic areas in the United States to assure a realistic and effective demonstration of the solar heating systems and combined solar heating and cooling systems involved, and of the dwellings and other buildings themselves, in both rural and urban locations and under climatic conditions which vary as much as possible.