What it is

A ground-source heat pump (GSHP), aka geothermal, is a heat pump that swaps heat with the earth instead of the outdoor air. The refrigeration cycle and the reversing valve are exactly what you already know from air-source heat pumps. The difference is on the outdoor side: instead of a coil and fan fighting 95°F summer air or 10°F winter air, a geo unit exchanges heat with a buried water loop that sees stable ground temperatures year-round. That stability is the whole reason geo is so efficient — the system always has a friendly "outdoors" to work against.

If you understand a heat pump, you understand 90% of geo. The new 10% is the water loop and the water-to-refrigerant heat exchanger that replace the outdoor coil.

How it works

A typical geo unit is water-to-air: it heats or cools the building air (like a normal indoor air handler) while exchanging heat with water on the loop side.

The cycle, in cooling:

  1. The compressor and reversing valve are set so the indoor air coil is the evaporator — it cools and dehumidifies house air.
  2. The heat pulled from the house (plus compressor heat) is rejected into the water-to-refrigerant heat exchanger (a coaxial or brazed-plate exchanger), which dumps it into the ground loop water.
  3. The loop carries that heat into the earth, which absorbs it.

In heating, the reversing valve flips:

  1. The water-to-refrigerant exchanger becomes the evaporator — refrigerant pulls heat out of the loop water (which is being warmed by the ground).
  2. The indoor air coil becomes the condenser — it heats the house air.
  3. The cooled loop water goes back into the earth to pick up more heat.

Because the ground sits at a moderate, stable temperature, the heat pump never has to work against brutal extremes. In winter it pulls from ~45-55°F ground water instead of frigid air; in summer it rejects to that same moderate ground instead of blazing-hot air. Less lift = high efficiency (high COP) and no defrost cycle needed (there's no frosting outdoor coil).

Variations you'll meet:

  • Water-to-air (most common residential) — heats/cools air via a duct system.
  • Water-to-water — makes hot/chilled water for radiant floors, fan coils, or hydronic systems.
  • The loop side is either a closed loop (sealed buried pipe with antifreeze solution) or an open loop (well water pumped through and discharged) — covered in the open-vs-closed article.

In the field

Orient the same way every time:

  • It's a heat pump — find the compressor, reversing valve, metering device(s), the indoor air coil, and the water-to-refrigerant heat exchanger (the "outdoor coil" equivalent). Most geo units are a single cabinet indoors (basement/mechanical room), not a split.
  • Find the loop side — supply and return water connections, the loop pump(s) or flow center, and whether it's closed (antifreeze, sealed) or open (well water in, discharge out).
  • Reversing valve convention — confirm which mode energizes the valve for the specific unit before diagnosing heating/cooling.
  • Two "splits" to read — the air-side temperature split (across the indoor coil) AND the loop water temperature change (entering vs leaving water on the heat exchanger). Both tell you how the machine is doing.

A geo unit that's "not heating/cooling well" is usually either a refrigeration-side issue (like any heat pump) OR a loop/water-side problem (low flow, air in the loop, fouled exchanger, wrong loop temps) — and the water side is the part that's new to most techs.

Normal values & targets

  • Loop water temperatures (closed loop): entering water to the unit commonly runs somewhere around the mid-30s to ~50°F in heating season and warmer (e.g., ~70-90°F) in cooling season, depending on climate, loop design, and run time. Stable compared to air.
  • Water temperature rise/drop across the unit: the loop water typically changes on the order of several degrees through the heat exchanger; manufacturers specify a target water ΔT and a required GPM.
  • Flow rate: sized roughly around ~2.25-3 GPM per ton on many residential systems — confirm the unit's spec. Too little flow tanks performance and can freeze the exchanger in heating.
  • COP / EER: high — geo's efficiency advantage comes from the moderate ground temperatures (less lift than air-source). No defrost penalty in heating.
  • Air-side split: like any system, a sensible supply-to-return air temperature difference appropriate to mode and airflow.

These are representative; always verify against the unit's performance data and the loop design.

Common faults & what they mean

  • Weak heating/cooling with normal refrigerant readings — water-side problem: low loop flow, air in the loop, fouled heat exchanger, or wrong entering-water temp (undersized/short loop). Check water flow and ΔT.
  • Low loop flow — air-bound loop, failing loop pump/flow center, clogged strainer, or a kinked/restricted line. Performance falls off and heating-mode can risk freezing the exchanger.
  • Refrigeration faults — same as any heat pump: low/overcharge, reversing valve, TXV/metering, compressor, controls.
  • High head (cooling) / low suction (heating) tied to the loop — poor heat rejection (cooling) or poor heat absorption (heating) from the loop, often water-flow or loop-temp related rather than a refrigerant problem.
  • Open-loop fouling/scaling — well water fouls the exchanger over time (covered in the open-vs-closed article).

Tech tips & gotchas

  • Treat the water-to-refrigerant exchanger like the "outdoor coil." When a normal heat pump would have an outdoor-air problem (dirty coil, low airflow), the geo equivalent is a water-side problem (low flow, fouling, air). Read the loop water ΔT the way you'd read an air split.
  • No defrost on geo. There's no frosting outdoor coil, so don't go looking for a defrost cycle. If a geo "ices," it's the heat exchanger freezing from low loop flow/temp in heating — a real problem, not a normal defrost.
  • Always check water flow and loop temps before condemning the refrigeration side. Geo's most common "it doesn't heat right" calls trace to flow and air in the loop, not the compressor.
  • Know the reversing-valve convention for the unit. Geo controls and valve energizing can differ; confirm before you chase a "wrong mode" ghost.
  • The ground side is stable, which is the point. If entering water temps are swinging wildly or trending the wrong way over a season, the loop may be undersized or short-circuiting — a design/installation issue, not a quick fix.

Safety / code notes

  • Loop antifreeze solutions and well-water discharge are regulated — closed-loop fluids and open-loop discharge must follow the applicable environmental/plumbing code and local well rules.
  • Refrigerant work follows EPA Section 608 — recover, don't vent.
  • Electrical: geo units are line-voltage equipment with heavy compressor loads — lock out/verify dead; confirm disconnect per NEC §440.
  • Backup/auxiliary electric heat (if equipped) draws heavy current — verify the circuit and sequencing.