The Cost to Install Geothermal HVAC depends on the size of the house and its energy consumption. Small houses ranging from 1,000 sq ft to 1,500 sq ft will require $12,000-$22,000, while medium-size houses (1,500 to 2,500 sq ft) usually require $18,000 to $35,000. In large houses, the cost could be in the region of $28,000 to $50,000 depending on the system capacity that is needed for such houses.