What it is

The refrigeration side of every geo unit is basically the same. What changes from job to job is how the system gets its heat to and from the ground — the loop. There are two families:

  • Closed loop: a sealed circuit of buried pipe filled with a water/antifreeze solution that circulates over and over, never touching the ground directly. Heat moves through the pipe wall to/from the surrounding earth.
  • Open loop: the system pumps actual ground/well water through the heat exchanger and then discharges it (to a second well, a pond, or daylight). The water is used once for its heat and sent on.

Which one you're standing in front of changes everything about how you service it — closed loops have antifreeze and air to worry about; open loops have water chemistry, fouling, and well issues.

How it works

Closed loop. A continuous loop of HDPE pipe is buried and filled with a water + antifreeze solution (so it can't freeze if the loop runs cold in heating). A circulating pump (in a flow center) pushes the solution through the buried pipe and the unit's water-to-refrigerant heat exchanger. The same fluid circulates forever; heat conducts through the pipe wall into the soil. Closed loops come in configurations:

  • Horizontal: pipe buried in trenches a few feet down over a large area — cheap to install where you have land.
  • Vertical: pipe in deep boreholes — used where land is tight; more drilling cost.
  • Pond/lake: coils sunk in a body of water — cheapest when a suitable pond is available.

Open loop. A well pump draws ground water and sends it through the heat exchanger; after giving up or absorbing heat, the water is discharged — typically to a separate injection/return well, a pond, or to the surface. No antifreeze (it's just ground water at a stable temperature), and the loop side is "open" to the environment. Open loops can be very efficient because well water comes in at a steady ground temperature, but they live or die on water quantity and quality.

Flow center / pump: closed loops use a flow center (one or two circulators plus connections for purging); open loops use the well pump and controls to deliver water on demand. Either way the unit needs the right GPM through the exchanger.

In the field

Tell which one it is:

  • Closed loop: sealed pipe, a flow center with circulator(s), antifreeze in the loop, pressure/temperature ports, and no water going down a drain. You can purge/charge the loop.
  • Open loop: a well supply line in and a discharge line out, often a strainer/filter, sometimes a control valve or solenoid that opens on a call, and water actually flowing through and out. No antifreeze.

What to check, closed loop:

  • Loop flow and pressure at the flow center; air in the loop is the #1 enemy (kills flow, can freeze the exchanger in heating).
  • Antifreeze concentration — too weak and the loop can freeze in deep heating; verify the freeze protection.
  • Entering water temp and ΔT across the unit — confirms the loop and flow are healthy.

What to check, open loop:

  • Water flow (GPM) and whether the well delivers enough on a call — inadequate water = poor performance, and in heating, risk of freezing the exchanger.
  • Water chemistry / fouling — minerals (hard water, iron) scale and foul the heat exchanger over time, choking performance. Strainers clog. This is the classic open-loop maintenance item.
  • Discharge — is the return well/pond/daylight discharge actually accepting the water, or backing up?
  • Filtration/strainer condition.

Normal values & targets

  • Loop flow: roughly ~2.25-3 GPM per ton on many residential units — confirm the unit's spec. Both loop types need adequate flow; low flow tanks capacity and risks freezing the exchanger in heating.
  • Closed-loop antifreeze: mixed to protect below the coldest expected loop temperature with margin (so the loop can't freeze when running cold in heating). Verify the freeze point.
  • Open-loop entering water: comes in near the local ground-water temperature (often steady year-round, e.g., roughly ~50°F in many regions) — that steadiness is the efficiency advantage.
  • Water ΔT across the unit: a target rise/drop of several degrees through the heat exchanger; the unit's data gives the required GPM and ΔT.

Representative — confirm against the unit and the loop/well design.

Common faults & what they mean

  • Closed loop, weak performance — air in the loop (most common), low antifreeze/flow, failing flow-center pump, or clogged strainer. Purge the air and verify flow.
  • Closed loop, exchanger freezing in heating — low flow or weak antifreeze let the loop water drop below freezing at the exchanger. Dangerous to the exchanger; fix flow/antifreeze.
  • Open loop, dropping performance over time — scaling/fouling of the heat exchanger from water minerals/iron, or a clogged strainer. Performance fades gradually. Clean/descale the exchanger; service filtration.
  • Open loop, intermittent poor performance — well not delivering enough water on a call (low yield, failing well pump, restricted intake), or discharge backing up.
  • Either loop, high head (cooling)/low suction (heating) tied to the water side — inadequate heat exchange from low flow or fouling rather than a refrigerant fault.

Tech tips & gotchas

  • Closed-loop problems are usually air or flow; open-loop problems are usually water quality or quantity. That single distinction routes most geo loop-side diagnostics.
  • Purge air aggressively on closed loops. An air-bound geo loop behaves like an air-bound hydronic system — flow dies even with a good pump. Flow centers exist partly to make purging possible; use the purge ports.
  • Open-loop heat exchangers scale up. Hard or iron-rich water fouls the coaxial/plate exchanger, and performance fades slowly enough that the customer just thinks "it's not as good as it used to be." Periodic descaling/flushing is normal open-loop maintenance.
  • Verify antifreeze freeze protection on closed loops before assuming the refrigeration side is at fault for an exchanger that froze. Weak antifreeze + low flow freezes the water side.
  • Open-loop discharge matters. If the return well or discharge can't accept the water, flow drops and performance suffers — check the whole water path, not just the supply.
  • Don't expect a defrost cycle on either — geo doesn't defrost. "Icing" on geo means the water-side exchanger froze, which is a flow/antifreeze fault to fix, not a normal cycle.

Safety / code notes

  • Open-loop discharge and well use are regulated — water withdrawal, injection wells, and surface discharge must follow local well and environmental codes; you can't just dump water anywhere.
  • Closed-loop antifreeze selection and handling follow environmental rules; some fluids have specific requirements. Dispose of loop fluid properly.
  • Refrigerant work follows EPA Section 608 — recover, don't vent.
  • Electrical: well pumps and compressors are line-voltage loads — lock out/verify dead; confirm disconnect per NEC §440.
  • Buried/scalding/pressurized loop fluid can be hot or cold and under pressure — relieve and verify before opening a flow center.