What it is

Two numbers make or break a geo system's performance: loop flow (GPM) and entering water temperature. Get those right and the refrigeration side usually behaves. Get them wrong — air in the loop, a weak pump, a short or fouled loop — and the unit makes lousy heat or cool while showing refrigerant symptoms that send techs chasing the charge. This article covers verifying/setting flow, how geo charging differs from air-source, and the field problems that actually cause most geo complaints.

The big mindset shift: on geo, prove the water side before you touch the refrigerant side. Most "it doesn't heat right" calls are flow or air, not charge.

How it works

Flow rate. The unit needs a design GPM through the water-to-refrigerant heat exchanger to move heat properly — typically sized around a couple-plus GPM per ton. You verify flow by measuring the pressure drop across the heat exchanger and comparing it to the manufacturer's pressure-drop-vs-GPM chart for that unit (each unit publishes the relationship). Pressure-drop ports on the exchanger let you read it. Low flow = poor heat exchange, weak capacity, and in heating, freeze risk.

Entering water temperature (EWT). This is the loop water temperature coming into the unit. It's the geo equivalent of "outdoor temperature" for an air-source heat pump — and it's far more stable. Performance and the "correct" refrigerant readings depend on EWT, so you always interpret pressures in the context of the current EWT and flow.

Charging geo vs air-source. Geo systems are commonly factory-charged and sealed, and you charge/verify against EWT and flow, not outdoor air temperature. The manufacturer's commissioning data gives expected pressures, superheat, and subcooling at a given EWT and GPM. You can't judge the charge without knowing both. Many geo charge problems are actually misdiagnosed flow/air problems — so step one is always "is flow correct and the loop free of air?" before adjusting refrigerant.

Air in the loop and purging. A closed loop must be purged of air at commissioning (and after any opening) using a purge cart/flow center at high velocity to sweep air out. Trapped air kills flow, causes noise, and can freeze the exchanger in heating. Open loops don't have this issue but have their own flow/quality concerns.

In the field

Verify flow (closed loop):

  1. Measure pressure drop across the heat exchanger at the ports.
  2. Look up the unit's pressure-drop chart to convert that to GPM.
  3. Compare to the design GPM (≈ a couple-plus GPM per ton). Low flow → purge air, check the pump/flow center, clean the strainer, look for restrictions.

Verify entering water temp and ΔT:

  • Read EWT and leaving water temp; compute water ΔT and compare to the unit's target at the rated GPM. Off-target ΔT plus EWT tells you whether the loop and flow are healthy.

Then, and only then, look at refrigerant:

  • Use the manufacturer's commissioning numbers for the measured EWT and GPM. Compare actual superheat/subcool/pressures to those. Now a charge discrepancy means something — before you confirmed flow and EWT, it didn't.

Purge a closed loop if you suspect air: run the purge cart at high flow to carry air bubbles out, watch the sight glass/return for air, and keep going until it runs air-free. Then set/verify static loop pressure and antifreeze.

Check antifreeze freeze protection on closed loops — weak antifreeze plus any low-flow event can freeze the exchanger in heating.

Normal values & targets

  • Design loop flow: roughly ~2.25-3 GPM per ton on many residential units — confirm the unit spec. Verify via pressure drop, not by guessing.
  • Entering water temp (closed loop): mid-30s to ~50°F in heating, warmer (~70-90°F) in cooling, depending on climate/loop/run time. Stable.
  • Water ΔT across the unit: several degrees at rated flow — per the unit's data.
  • Charging reference: match measured superheat/subcool/pressures to the manufacturer's values at the measured EWT and GPM — geo charge is judged against water conditions, not outdoor air.
  • Antifreeze: protected below the coldest expected loop temp with margin.

Representative — always confirm against the unit's pressure-drop and commissioning tables.

Common faults & what they mean

  • Weak heating/cooling, refrigerant looks "off" — verify flow and EWT first. Air in loop, low flow, fouled exchanger, or wrong EWT (short/undersized loop) cause refrigerant-looking symptoms. Don't add/remove charge until the water side is proven.
  • Low flow — air-bound loop (most common), failing flow-center pump, clogged strainer, kinked/restricted line, or partially closed valve. Confirm with pressure drop, then purge/repair.
  • Exchanger freezing in heating — low flow and/or weak antifreeze let the water side drop below freezing. Fix flow and antifreeze; respect any freeze-protection lockout.
  • EWT trending wrong over a season — undersized or short-circuited loop (supply/return too close), so the ground can't recover. Heating EWT keeps falling or cooling EWT keeps climbing. Design/install issue.
  • High head (cooling)/low suction (heating) from the water side — poor heat exchange from flow or fouling, not a refrigerant fault.
  • Real charge issue — only callable once flow and EWT are confirmed; then compare to the EWT/GPM commissioning data.

Tech tips & gotchas

  • Prove flow and EWT before touching the charge. This is the cardinal rule of geo. Adjusting refrigerant on a unit that's really air-bound or low-flow just makes things worse and wastes the visit.
  • Verify flow with pressure drop, not vibes. Each unit has a pressure-drop-to-GPM chart; two port readings give you real GPM. "Feels like it's flowing" isn't a measurement.
  • Air is the #1 closed-loop gremlin. It mimics low charge, makes noise, kills capacity, and can freeze the exchanger. Purge thoroughly with a cart at high velocity; don't trust a half-hearted purge.
  • Charge against water conditions. Geo commissioning data is keyed to EWT and GPM, not outdoor air. Pull the right table and match to the measured EWT and flow.
  • Weak antifreeze is a hidden time bomb in heating. A loop that runs cold with marginal antifreeze freezes the exchanger on the coldest night — often cracking a brazed-plate. Verify the freeze point during service.
  • A loop that won't hold EWT points to design, not the box. If heating EWT keeps sagging or cooling EWT keeps climbing across the season, the loop is undersized or short-circuiting — that's not something you fix with a screwdriver at the unit.

Safety / code notes

  • Refrigerant work (on sealed factory-charged units) follows EPA Section 608 — recover, don't vent.
  • Closed-loop antifreeze and open-loop discharge are regulated — follow the applicable environmental/plumbing code; dispose of loop fluid properly.
  • Loop fluid is under pressure and can be hot or cold — relieve and verify before opening ports or the flow center.
  • Electrical: compressors and loop/well pumps are line-voltage loads — lock out/verify dead; confirm disconnect per NEC §440.
  • Never defeat freeze-protection or pressure safeties — diagnose the root cause (flow/antifreeze) instead.