Geothermal Heat Pumps: Using Underground Temperature for Home Heating and Cooling
Geothermal (ground-source) heat pumps exploit the fact that underground temperature remains roughly constant year-round (50-60°F / 10-16°C in most US locations). In winter, the system extracts heat from the ground; in summer, it dumps heat into the ground. Efficiency is 3-5x that of conventional systems (300-500% effective efficiency). Installation costs are high ($15,000-45,000) due to ground loop drilling, but operating costs are 40-60% lower than conventional HVAC with 20-25 year equipment life.
Geothermal heat pumps (also called ground-source heat pumps) use the stable temperature of the earth a few feet below the surface as both a heat source in winter and a heat sink in summer. Unlike air-source heat pumps that struggle in extreme temperatures, geothermal systems work against a relatively constant underground temperature of 50-60°F (10-16°C) year-round in most US locations. ## How It Works A ground loop — a series of pipes buried in the ground (either horizontally in trenches or vertically in boreholes) — circulates a fluid that exchanges heat with the surrounding earth. A heat pump inside the building concentrates that heat (in winter) or rejects it (in summer) to condition interior air. **In winter:** The ground is warmer than the air. The fluid in the ground loop absorbs heat from the earth, the heat pump concentrates it to usable temperature (100-120°F), and distributes it through the building's ductwork or radiant floor system. **In summer:** The process reverses. The heat pump extracts heat from indoor air and transfers it to the ground loop, which dissipates it into the cooler earth. ## Efficiency Geothermal systems achieve coefficients of performance (COP) of 3-5, meaning for every 1 kW of electricity consumed, they deliver 3-5 kW of heating or cooling. This is 300-500% effective efficiency — compared to electric resistance heating at 100% or conventional air-source heat pumps at 200-300% in moderate conditions. The consistency of underground temperature means efficiency doesn't degrade in extreme weather. ## Desuperheater Many geothermal systems include a desuperheater — a heat exchanger that captures waste heat from the compressor to preheat domestic hot water. This provides "free" hot water as a byproduct of heating/cooling operation, reducing water heating costs significantly. ## Economics Installation costs range from $15,000-45,000 depending on ground conditions and loop type (vertical boreholes cost more than horizontal trenches but require less land area). Operating costs are 40-60% lower than conventional HVAC. Equipment life is 20-25 years for the indoor unit and 50+ years for the ground loop. Federal tax credits of 30% (under the Inflation Reduction Act through 2032) reduce the effective installation cost. Net Zero Home: Lessons and Regrets After Two Years Off-Grid