In geothermal heating systems, it is a secondary heat exchanger that transfers heat from the ground to the hot water heater to heat the water. In the summer, it uses the heat removed from the indoor space.
Using heat removed from a home's interior to heat its hot water makes the cost of hot water very inexpensive in the summer.
Costs vary greatly by region (and even by zip code). To get free estimates from local contractors, please indicate yours.