Geothermal heating in HVAC is a type of heating system that uses the natural heat from the ground to heat a building. It works by circulating a fluid, usually water, through a loop of pipes buried underground. The fluid absorbs the heat from the ground and carries it back to the building, where it is used to heat the air. This type of heating system is highly efficient and cost-effective, as it uses the natural heat from the ground to heat the building, rather than relying on electricity or gas. Additionally, it is environmentally friendly, as it does not produce any emissions.