Abstract: A heating and cooling system comprises a first reservoir containing liquid at an elevated temperature, solar collecting means exposed to solar radiation for collecting solar energy and means disposed in thermal exchange relationship with said first reservoir for transmitting heat energy from said solar collecting means to said first reservoir, a second reservoir containing a liquid disposed in thermal exchange relationship with a portion of the earth, means for blending liquids derived respectively from thermal exchange relationship with said first and second reservoirs, a heat engine having first and second refrigerant coils, one of said refrigerant coils disposed in thermal exchange relationship with an atmosphere to be heated or cooled, and valve means responsive to the refrigerant pressure in said other coil for delivering blended liquid from said blending valve to thermal exchange relationship with said other refrigerant coil, whereby heat energy to be extracted from or delivered to said other refrigeran