Geothermal Heat Pump Basics
A geothermal system is still a vapor-compression heat pump — it just trades the outdoor coil for the ground. Here is how ground loops and water-source units work, and what actually changes when you have to charge and service one.
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In This Guide
What Makes Geo Different
A ground-source (geothermal) heat pump runs the exact same refrigeration cycle as the air-source unit you service every day: compressor, reversing valve, metering device, and two heat exchangers. The difference is where it rejects and absorbs heat. Instead of blowing across an outdoor coil into 5°F winter air or 100°F summer air, it exchanges heat with a fluid loop buried in the earth — where the temperature stays remarkably stable at roughly 45–70°F year round depending on your region and depth.
That stable source is why geo posts higher efficiency than air-source equipment. A well-designed system commonly runs a heating COP of 3.5–5.0 and cooling EER in the high teens to low twenties, because the compressor never has to lift refrigerant across a brutal outdoor temperature. Two field consequences fall right out of that:
- No defrost cycle. There is no outdoor coil to frost up, so the reversing-valve/defrost-board dance you know from air-source heat pumps simply does not exist on the source side.
- You now own a water problem too. Flow rate, entering water temperature, antifreeze, and air in the loop become first-class service items alongside superheat and subcooling.
Water-to-air vs water-to-water. Most residential geo is water-to-air — the loop fluid feeds a refrigerant-to-water coaxial heat exchanger, and an air coil conditions the ductwork. Water-to-water units make hot or chilled water instead, feeding radiant floors, fan coils, or a buffer tank. The diagnostics below apply to both.
Ground Loop Types
The loop is the ground-side heat exchanger, and its type dictates how you diagnose flow and temperature problems. Closed loops circulate a sealed antifreeze solution; open loops pump actual well or surface water through the unit and discharge it.
| Loop Type | Configuration | Field Notes |
|---|---|---|
| Closed — Vertical | U-bend HDPE pipe in 150–400 ft bores | Small footprint; most stable temps; check loop static pressure and purge for air |
| Closed — Horizontal | Pipe in trenches 4–6 ft deep | Cheaper to install; more seasonal temperature swing near the surface |
| Closed — Pond/Lake | Coiled pipe submerged and weighted | Low cost where water is available; verify minimum depth so it never freezes solid |
| Open Loop | Well water in, discharge to a second well or surface | No antifreeze, but watch fouling, scaling, and iron/hardness on the coax |
Antifreeze matters to your math. Closed loops in cold climates carry propylene glycol, methanol, or ethanol solutions rated to protect roughly 15–20°F below the lowest expected entering water temperature. That fluid has a lower specific heat and higher viscosity than water, which reduces the 500 factor in the heat-transfer formula and raises pumping head — both things you account for when reading a system.
How the System Actually Works
In heating, the loop fluid is warmer than the refrigerant. The coaxial heat exchanger acts as the evaporator: refrigerant boils and pulls heat out of the loop (this is your heat of extraction), the compressor adds work, and the air coil dumps that combined heat into the house. The loop water leaves colder than it entered — a typical source-side split of about 5–8°F.
In cooling, the reversing valve swaps functions. The coax becomes the condenser and rejects heat into the loop (your heat of rejection), so the water leaves warmer than it entered — usually a 9–12°F rise because you are also dumping the compressor heat. Many units add a desuperheater: a small refrigerant-to-water coil that skims superheat off the discharge line to preheat domestic hot water, essentially free during cooling season.
The two energy balances
Heating capacity = Heat of Extraction + Compressor Heat
Cooling capacity = Heat of Rejection − Compressor Heat
Roughly, the source-side heat transfer runs about 0.75× capacity in heating and about 1.25× capacity in cooling once compressor work is included.
Entering Water Temperature & Flow
Two loop measurements drive almost every geo diagnosis: entering water temperature (EWT) and flow rate. EWT is the single biggest lever on capacity and pressures — a geo unit will read completely different suction and head pressures at 30°F EWT versus 70°F EWT, and both can be perfectly normal. Always pull up the equipment's performance table for the actual EWT before you judge a charge.
- Design flow: most manufacturers spec roughly 2.25–3 GPM per ton. A common field target is 3 GPM/ton at full load, and never less than about 1.5 GPM/ton.
- Heating EWT: closed loops commonly land in the 25–50°F range in deep winter; open loops track the well temperature, often 45–60°F.
- Cooling EWT: closed loops often 60–90°F; higher EWT means higher head pressure and lower capacity.
Low flow is the classic geo killer
In heating, inadequate loop flow drives suction pressure down until the coax approaches freezing. That risks freezing the water in the heat exchanger and cracking it. If you see low suction and a large source-side temperature split, suspect flow — air in the loop, a weak circulator, or a clogged strainer — before you touch the refrigerant charge.
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Worked Example: Heat of Extraction
Problem:
A 3-ton water-to-air geo unit is in heating. You measure loop flow at 9 GPM, EWT of 50°F, and LWT of 44°F. The loop carries a propylene-glycol antifreeze. How much heat is the loop giving up, and does it look right?
Step 1: Check the flow rate against tonnage
9 GPM ÷ 3 tons = 3 GPM/ton — right on the common design target.
Step 2: Find the source-side temperature split
ΔT = EWT − LWT = 50°F − 44°F = 6°F
Step 3: Apply the water heat-transfer formula (antifreeze factor)
HE = 485 × GPM × ΔT = 485 × 9 × 6
Step 4: Solve for heat of extraction
HE = 26,190 BTU/hr pulled from the loop
Step 5: Sanity-check against capacity
3 tons ≈ 36,000 BTU/hr. Extraction is about 0.73× capacity, which is exactly what you expect once you add compressor heat back in.
Result: Loop is performing normally
Flow, split, and extraction all line up with a healthy 3-ton unit at 50°F EWT. If the split had been 12°F with the same flow, that would signal too little flow or a starved evaporator — time to purge the loop and verify the circulator.
Note the 500 → 485 factor swap. Pure water uses Q = 500 × GPM × ΔT (500 = 8.33 lb/gal × 60 min/hr × 1.0 BTU/lb·°F). Antifreeze lowers the fluid's density and specific heat, so a 15–20% glycol mix runs closer to 485. Use pure 500 only on open-loop well water.
Charging & Servicing Geo
Most geo runs R-410A, with new equipment transitioning to the A2L refrigerant R-454B under the AIM Act HFC phasedown. Charge is still verified with superheat and subcooling, but with a critical twist: you read them against the manufacturer's chart at the measured EWT and flow, not against a generic target. A subcooling of 10°F might be perfect at 30°F EWT and overcharged at 70°F EWT.
- Get the loop to stable operating EWT and full flow before judging the charge.
- Pull the unit's performance table; enter with EWT, entering air, and flow to get target superheat, subcooling, and pressures.
- For A2L equipment, follow ASHRAE 15 handling, leak-detection, and ventilation requirements, and keep to your EPA 608 recovery and recordkeeping obligations on every recovery.
- Verify loop static pressure and purge air: circulate at high velocity (above roughly 2 ft/s) until the sight glass on the flow center runs bubble-free, then set static to the installer's cold-fill spec (commonly 40–70 psi).
Air in the loop looks exactly like a bad charge
Trapped air kills flow and mimics low refrigerant: low suction, poor capacity, wide source split. Always confirm the water side is purged and flowing before you connect gauges and start adding or recovering refrigerant.
Water-Side Troubleshooting
When a geo unit underperforms, work the source side first — it is the part air-source techs are least used to. A quick triage:
Low flow / air-bound loop
Symptoms: wide source ΔT, low heating suction, weak capacity. Fix: purge to remove air, check the circulator/pump module, clean the strainer, confirm valves are open.
Fouled / scaled coax (open loop)
Symptoms: rising approach temperature, climbing head in cooling. Fix: descale or acid-flush the coaxial heat exchanger; address hardness and iron with treatment or filtration.
Healthy heating signature
5–8°F source split, 3 GPM/ton, pressures matching the table at the measured EWT, warm supply air, tight superheat.
EWT drifting each cycle?
Slowly falling heating EWT over a season can mean an undersized loop or ground thermal imbalance — not a refrigerant fault. Document loop temps before condemning the compressor.
The habit that separates good geo techs: always write down flow, EWT, LWT, and the source split alongside your superheat and subcooling. Those four water numbers turn a confusing set of pressures into an obvious diagnosis.
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