Ground-Source (Geothermal) Heat Pumps: Principles, Loop Types, and Service Differences

Ground-Source Heat Pumps: How They Work

A ground-source heat pump (GSHP), often called a geothermal heat pump, moves heat between a building and the earth rather than between a building and outdoor air. Because ground temperatures below the frost line remain relatively stable year-round—typically in the 45–75 °F range depending on geographic location—a GSHP avoids the efficiency penalty an air-source unit suffers during extreme cold or hot weather. The coefficient of performance (COP) of a ground-source system is generally higher and more consistent across seasons than that of a comparable air-source system.

Inside the cabinet the refrigeration cycle is the same: compression, condensation, expansion, and evaporation. The key difference is that instead of an outdoor coil exchanging heat with ambient air, a GSHP uses a water-to-refrigerant heat exchanger (also called a coaxial or brazed-plate heat exchanger) that transfers heat between the refrigerant and the loop fluid circulating through the ground. This single design change eliminates defrost cycles, reduces fan noise, and removes the unit's dependence on outdoor air temperature.

Major Loop Configurations

Horizontal closed loops are buried in trenches, typically 4–6 feet deep. Multiple pipe runs are laid out side by side or in a slinky coil pattern. This configuration requires significant land area and is best suited for new construction on open lots where excavation equipment has access. Installation cost per foot of trench is lower than drilling, but total land disturbance can be large.

Vertical closed loops use boreholes drilled 150–400 feet deep, with U-bend pipe inserted into each borehole and grouted in place. Vertical loops are chosen when surface area is limited—commercial sites, established neighborhoods, or rocky terrain where trenching is impractical. Drilling cost per foot is higher, but fewer square feet of surface are disturbed.

Pond or lake loops (surface water loops) consist of coiled pipe sunk to the bottom of a body of water of adequate depth and surface area. Water provides excellent heat exchange. Installation is typically the least expensive closed-loop option when a suitable water body is available, but local regulations must be checked before installation.

Open loops draw groundwater directly from a well, pass it through the heat exchanger, and discharge it to a pond, stream, or return well. Water quality (mineral content, iron, pH) must be tested; fouling or scaling of the heat exchanger is a real risk. Permitting requirements vary by state and locality.

Key Components Inside a GSHP Unit

• Water-to-refrigerant heat exchanger: Replaces the outdoor coil. Loop fluid flows on one side; refrigerant on the other. Scaling, biofilm, or low flow reduces its effectiveness.
• Desuperheater: An auxiliary heat exchanger that captures superheat from the compressor discharge to pre-heat domestic hot water. It is active only when the compressor runs, so it supplements—but does not replace—a water heater.
• Flow center: A factory-mounted or field-installed assembly containing the circulating pump(s), isolation valves, and purge ports that move loop fluid through the ground loop. It is the geothermal equivalent of the outdoor fan—if the flow center fails, heat exchange stops.
• Expansion tank: Accommodates fluid volume changes as loop fluid temperature changes. An undersized or waterlogged expansion tank leads to pressure swings and nuisance high-pressure lockouts.
• Antifreeze solution: In climates where loop fluid can fall below freezing, propylene glycol or methanol is added to the loop water. Concentration must be correct for the design loop temperature; too weak allows freezing, too strong reduces heat transfer capacity.

Loop Fluid and Antifreeze Safety

Always check the Safety Data Sheet (SDS) before handling antifreeze concentrate. Wear chemical-resistant gloves and safety glasses. Methanol is flammable and toxic; propylene glycol is far less hazardous and is the preferred choice in most residential applications. Dispose of loop fluid per local regulations—do not discharge glycol solutions to storm drains.

When opening any part of the loop, be aware that the system may be pressurized. Relieve pressure through the purge port before breaking any fitting. Lockout/tagout the flow center power before servicing the pump or any electrical component inside the unit.

Unique Service Considerations

Troubleshooting a GSHP that fails to reach setpoint in winter requires checks that simply do not apply to air-source systems:

1. Loop entering water temperature (EWT): Measure at the supply connection to the unit. If EWT is abnormally low, the ground loop may be undersized, the soil may be depleted from continuous run time, or a pipe is damaged.

2. Loop flow rate: Use pressure-drop charts from the manufacturer to verify flow is within the design range for the unit. Low flow reduces heat transfer and can trigger freeze-stat lockout.

3. Antifreeze concentration: Use a refractometer calibrated for the specific fluid (glycol or methanol). A concentration that is too low risks ice formation inside the water-to-refrigerant heat exchanger—a costly failure.

4. Flow center operation: Confirm the pump is running and not air-locked. Purge air if pressure readings or flow calculations are inconsistent.

Checks shared with air-source troubleshooting include: refrigerant charge, compressor operation, reversing valve function, filter condition, and indoor airflow verification.

Credential Relevance

EPA 608 certification is required whenever you recover, recycle, or charge refrigerant in the GSHP cabinet—the refrigerant circuit follows the same rules as any other system. NATE, HVAC Excellence, and NCCER HVAC all include ground-source content in their study materials; understanding loop configurations and unique service steps is directly tested.