The Water-to-Refrigerant Heat Exchanger in Geothermal

What it is

The water-to-refrigerant heat exchanger is the part of a geo system that takes the place of an air-source heat pump's outdoor coil. Instead of refrigerant trading heat with outdoor air, it trades heat with the loop water here. It's where the refrigerant either rejects heat into the loop (cooling) or pulls heat out of the loop (heating). Get comfortable with this component and the rest of geo is just a normal heat pump with ducts.

Two construction types dominate:

• Coaxial ("tube-in-tube"): a refrigerant tube inside a larger water tube (or vice versa), wound into a coil. Rugged and tolerant of so-so water.
• Brazed-plate: a stack of stamped stainless plates that pass water and refrigerant through alternating channels. Compact and very efficient, but the narrow channels are fussier about water quality and freezing.

How it works

It's a counterflow heat exchanger: loop water flows on one side, refrigerant on the other, and heat moves through the metal wall between them.

In cooling, the reversing valve makes this exchanger the condenser. Hot, high-pressure refrigerant from the compressor rejects its heat into the loop water; the refrigerant condenses to liquid, and the now-warmer loop water carries the heat out to the ground.

In heating, the reversing valve flips it to the evaporator. Refrigerant pulls heat out of the loop water (which the ground keeps warm), boiling the refrigerant to vapor; the now-cooler loop water heads back to the earth to reload. Because it's the evaporator in heating, the water side gets colder than the loop water — which is exactly why low flow or weak antifreeze can let it freeze.

The amount of heat it can move depends on water flow (GPM), the entering water temperature, and how clean the heat-transfer surfaces are. Foul the surfaces or starve the flow and the exchanger can't do its job, which shows up as poor capacity and abnormal refrigerant pressures.

In the field

Identify the type: a wound tube-in-tube assembly is coaxial; a compact stainless plate block is brazed-plate.

Read its performance through water and refrigerant:

• Water ΔT: measure entering vs leaving loop water temperature across the exchanger. Compare to the unit's target ΔT at the rated GPM. Too small a ΔT with low flow, too large a ΔT, or off-spec entering water all point to trouble.
• Refrigerant side: in cooling, it's the condenser — watch head pressure and the relationship between condensing temp and leaving water temp (an "approach"). In heating it's the evaporator — watch suction and superheat. Poor heat exchange (fouling/low flow) drives head up in cooling and suction down in heating.
• Flow: confirm GPM is in spec. Most water-side capacity problems are really flow or fouling problems.

Suspect fouling when capacity has faded gradually and water ΔT/approach has drifted — especially on open-loop (well water) systems where minerals and iron scale the surfaces. Brazed-plate exchangers are more sensitive to this than coaxial.

Suspect a freeze event when a heating-mode unit on a closed loop with low flow or weak antifreeze shows damage, a refrigerant-to-water leak, or has tripped a low-temperature/freeze protection. Freezing water in the channels can split a brazed-plate exchanger.

Normal values & targets

• Water flow: roughly ~2.25-3 GPM per ton on many residential units — confirm the unit spec. Adequate, in-spec flow is the single most important water-side number.
• Water ΔT across the exchanger: a target rise/drop of several degrees at rated flow; the unit's performance data gives the exact target ΔT and GPM.
• Entering water temp: stable, near ground/loop temperature — warmer in cooling, cooler in heating, but never the wild swings of outdoor air.
• Approach (cooling/condenser): condensing temperature runs some degrees above the leaving water temperature; a widening approach over time suggests fouling or low flow.
• Freeze margin (heating, closed loop): the antifreeze must protect below the coldest the exchanger water side reaches in deep heating — verify the freeze point.

Representative — confirm against the unit's performance tables and the loop design.

Common faults & what they mean

• Gradual capacity loss + drifting water ΔT/approach — fouling/scaling of the exchanger (classic on open-loop/well-water systems). Clean/descale it; service filtration.
• High head in cooling / low suction in heating tied to the water side — poor heat exchange from low flow (air in loop, weak pump, clogged strainer) or fouling, not necessarily a refrigerant fault. Check flow first.
• Brazed-plate split / refrigerant-to-water leak — usually a freeze event from low flow or weak antifreeze in heating, or severe scaling. Refrigerant crosses into the loop water (or loop water into the refrigerant circuit) — a serious failure.
• Low-temperature / freeze-protection lockout (heating) — the control caught the water side getting too cold (low flow/antifreeze) before it froze. Fix the flow/antifreeze; the lockout did its job.
• Refrigerant readings normal but capacity weak — almost always water side: flow, fouling, or entering-water temperature.

Tech tips & gotchas

• Most geo "refrigerant" problems are water problems. Before you condemn the charge or compressor, measure water flow and ΔT across the exchanger. Geo hides water-side faults behind refrigerant symptoms.
• Brazed-plate exchangers are efficient but freeze-fragile. Low flow or weak antifreeze in heating can freeze the thin channels and crack the plates — leading to a refrigerant/water cross-leak. Verify antifreeze and flow on closed loops; respect freeze-protection lockouts.
• Open-loop = expect scaling. Well water deposits minerals and iron on the surfaces; capacity fades slowly. Periodic descaling/flushing is routine maintenance, and brazed-plate units need it more than coaxial.
• Use water ΔT and approach like you'd use an air split and coil approach. They tell you how well the "outdoor coil" is exchanging heat without pulling the system apart.
• A refrigerant-to-water leak can contaminate the loop (or push loop water into the refrigerant circuit). If a closed loop keeps losing pressure or the refrigerant side shows water/non-condensables, suspect a breached exchanger.
• Coaxial is more forgiving of marginal water; brazed-plate trades that ruggedness for efficiency and compactness. Know which you have when judging fouling/freeze risk.

Safety / code notes

• Refrigerant work (and any exchanger replacement) follows EPA Section 608 — recover, don't vent.
• A breached exchanger can mix refrigerant and loop water — handle/dispose of contaminated loop fluid properly per environmental rules.
• Closed-loop antifreeze and open-loop discharge are regulated — follow the applicable environmental/plumbing code.
• Electrical: lock out/verify dead before servicing; geo units carry heavy compressor loads.
• Loop fluid can be hot or cold and under pressure — relieve and verify before opening the water side.