The Cost to Install Geothermal HVAC depends upon the size of the house. For example, small houses (1,000 to 1,500 sq. ft.) cost between $12,000 to $22,000, whereas medium-sized houses (1,500 to 2,500 sq. ft.) cost between $18,000 to $35,000. Large houses cost above $28,000 to $50,000.The size of the equipment determines the price because large houses need strong equipment along with a large ground loop.