Geothermal Loop Field Troubleshooting
Symptom
Customer reports that the geothermal system "isn't keeping up". The compressor is running long hours, the air-handler discharge feels lukewarm in heating mode (or barely cool in cooling), and the utility bill is double what the previous owner promised. The thermostat reads 68 F when set to 72 F. In severe cases the unit faults on low refrigerant pressure or high refrigerant pressure depending on the season and the loop type.
The reflex assumption is "compressor weakness" or "low refrigerant". In a properly installed geothermal system, both of those are far behind the actual root-cause distribution. IGSHPA training data and Climate Master service-call records both suggest that 60 to 70 percent of geothermal performance complaints trace to the loop field or the loop-side circulating pump, not the refrigerant circuit. The reason is that the loop is a closed system out of sight, while the refrigerant circuit is the part the technician knows from air-source work.
Causes ranked by frequency
| Rank | Cause | Diagnostic signature | Typical fix |
|---|---|---|---|
| 1 | Air entrained in the loop | Erratic flow, gurgling at pumps, EWT bouncing 5 F or more cycle to cycle | Purge with a flush cart at 50 gpm minimum |
| 2 | Pump weakness or pump failure | Low pressure differential across pump, low flow GPM | Replace circulator (Grundfos UP, Taco 0011, Wilo Stratos) |
| 3 | Antifreeze degradation | Slimy fluid, odor, freeze point drift toward 32 F | Drain, flush, recharge with proper concentration |
| 4 | Undersized loop field (design flaw) | Performance fine in spring/fall, degrades after 30+ days of continuous load | Loop addition (drill another bore or add a horizontal trench) |
| 5 | Loop fluid contamination (debris, biological) | Cloudy fluid, fouled brazed-plate heat exchanger | Power-flush, possibly chemical clean of the BPHX |
| 6 | Crushed or kinked loop tubing | Sudden flow drop after landscaping or excavation work | Locate damage with pressure decay test, repair with thermal-fusion coupling |
| 7 | Refrigerant charge issue (last suspect) | Superheat/subcool out of spec confirmed at proper EWT | Recover, leak-test, recharge by weight per nameplate |
The order matters. Charging a system that has 30 percent air in the loop wastes the technician's time and the customer's money, because the air will continue to slug the BPHX and the refrigerant pressures will keep swinging.
Diagnostic decision tree
Step 1: Read entering water temperature (EWT) and leaving water temperature (LWT)
Most geothermal heat pumps have a service display showing EWT and LWT. If not, port a digital thermometer onto the source-side piping at the unit. Compare to design specification.
| Mode | Typical EWT (closed loop) | Typical LWT |
|---|---|---|
| Heating | 30 F to 50 F (regional) | EWT minus 4 F to 6 F |
| Cooling | 65 F to 85 F (regional) | EWT plus 8 F to 12 F |
If EWT is at design but LWT delta is off, the loop is fine, look at the refrigerant circuit. If EWT is wildly off design (heating EWT below 25 F, cooling EWT over 95 F), the loop is the problem.
Step 2: Check flow rate (GPM)
A geothermal system needs 2.5 to 3 GPM per ton at the loop side. A 4-ton system needs 10 to 12 GPM. Measure flow with a ball-valve flow meter on the source-side hose bibs, or read pump head against the pump curve and the loop pressure drop.
- Flow at or above spec, EWT at design: refrigerant circuit.
- Flow below spec, EWT off: air, pump weakness, or restriction.
Step 3: Purge if air is suspected
Connect a flush cart (typically Geo-Flo, Hydron, or shop-built with a 1 HP pond pump and a 30 gallon tank). Run flush at 50 GPM minimum for 30 minutes in both directions. Watch the return-side sight glass for bubbles. The loop is purged when the sight glass runs clear for 5 consecutive minutes.
If air keeps coming after 30 minutes, the loop has a leak somewhere admitting air. Pressure-test the loop at 50 psig static and watch for decay over 30 minutes.
Step 4: Check antifreeze concentration and freeze point
| Antifreeze type | Typical concentration | Freeze point | Notes |
|---|---|---|---|
| Methanol (20 percent by volume) | 20 percent | 5 F | Lowest pumping power, flammable when concentrated |
| Propylene glycol (25 percent) | 25 percent | 15 F | Non-toxic, higher viscosity (more pump power) |
| Ethanol (denatured, 20 percent) | 20 percent | 5 F | Permitted in some jurisdictions, restricted in others |
| Calcium chloride brine | varies | depends on percent | Industrial only, corrosive to copper |
Test with a refractometer calibrated for the antifreeze type. A propylene glycol refractometer reads wrong for methanol; carry both calibration kits.
Concentration drifts DOWN over time on a closed ground loop, and that is the direction that hurts. A closed loop has no open expansion tank to evaporate from. What it has is a service history: every time somebody tops off a pressure drop with a garden hose instead of premixed fluid, the mix gets weaker and the freeze point walks back toward 32 F. That is the drift in the causes table above. Antifreeze also degrades chemically over 8 to 15 years. A 15-year-old loop with no service should have its fluid sampled and tested for pH (should be 7 to 9 for glycol, slightly lower for methanol).
Step 5: Pump diagnostics
Read pressure across the pump (suction and discharge ports). The pump should produce its rated head at the rated flow. A Grundfos UP 26-99 rated for 30 GPM at 9 feet should show 9 feet of head at 30 GPM (3.9 psi). If it shows 3 feet of head, the impeller is fouled or worn, or the pump is failing.
Replace pumps in matched pairs on dual-pump systems. Replacement pump must match the loop-pressure-drop curve, not just GPM. Oversized pumps cost the customer money in continuous power; undersized pumps starve the loop.
Resolution per cause
Air in the loop
Flush at 50+ GPM for 30 minutes per direction with a flush cart. Verify clear sight glass. Top off antifreeze to restore concentration. Repressurize the loop to operating pressure (typically 15 to 30 psig static). If air returns within a season, hunt for the leak: connect a pressure gauge to a low-side schrader, isolate sections of the loop with the appropriate ball valves, and watch decay.
Pump weakness
Replace the circulator pump. Common replacement units: Grundfos UP 26-99, Taco 0011, Wilo Stratos ECO 16, B and G Series 100. Match flow and head to the loop. After replacement, repurge the loop.
Antifreeze degradation
Drain the loop, flush with clean water, recharge with new antifreeze at proper concentration. Dispose of used antifreeze per local hazardous-waste rules. Methanol is flammable and requires hazmat shipping; propylene glycol is non-hazardous but typically still routed through a waste-water permit.
Undersized loop
Confirm it before anyone quotes anything. A short loop performs correctly in mild weather and only drifts after weeks of continuous load, with EWT climbing through the cooling season or falling through the heating season while flow, antifreeze, and pump all check out. It shows as a trend, so log EWT across a season rather than judging it from one visit.
Verify what was actually installed against the design. Bore count, bore depth, trench length, and loop separation routinely differ from the drawing because the driller hit rock or ran out of yard. Compare installed footage to the required length per ton for the local soil conductivity, and re-run the building load rather than trusting the original if the house has changed since (finished basement, addition, new glazing).
Then choose the remedy honestly:
- Add loop. Another vertical bore or an added horizontal circuit, tied into the existing header with a balancing valve so flow splits correctly. This is the right fix, and it is the expensive one, because it means bringing a rig back into a finished yard.
- Reduce the load. Envelope work or a supplemental heat source covering the peak. On a marginal shortfall this often costs less than drilling and it lowers the customer's bills year round.
- Rebalance the annual load. A field can be adequately sized and still drift when the ground is charged in only one direction. A heating-dominated house slowly cools its field over years; desuperheater use or a shoulder-season rejection strategy pushes back on that.
Say plainly that this is a design or installation shortfall, not a failed part, and that no amount of service work makes it go away. Quantify what it is costing in supplemental heat or lost capacity and price the addition. Repeated symptom-chasing calls on a short loop damage the relationship far more than one direct conversation does.
References
- IGSHPA Closed-Loop/Geothermal Heat Pump Systems Design and Installation Standards, 2017 edition.
- ASHRAE Handbook, HVAC Systems and Equipment, 2020 edition, Ground-Source Heat Pumps chapter.
- ASHRAE Standard 90.1-2022 (geothermal-heat-pump efficiency requirements).
- AHRI Standard 13256-1 (Water-Source Heat Pumps Performance Rating).
- Manufacturer service literature: ClimateMaster, WaterFurnace, Bosch, Trane.
- IAPMO Uniform Mechanical Code 2021, Chapter 12 (closed-loop systems).
- Manuall internal: HVAC Geothermal Systems, HVAC Refrigerant Leak Diagnosis.