Geothermal for HVAC Technicians: What Air-Source Guys Get Wrong
Geothermal heat pumps aren’t bad. We just need to teach the technicians how to work on them. Heat pumps don’t need to improve. Technicians do.
I have said that for years. This page is me making good on it. Dave Hartzell, 47 years in the trade, IGSHPA accredited.
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One rule for this page: no temperature, no delta, no pressure, no flow rate. There is no universal number here. The data for the model in front of you governs.
1. It is a water to air machine, not a condenser
On air source, half your diagnostic model is the outdoor unit: ambient, coil condition, fan, condenser approach. Here that half is a coaxial heat exchanger with liquid in it. No coil, no fan, no outdoor temperature. The water half is the source side, the duct half the load side.
What replaces the readings you lost is entering water temperature, leaving water temperature and flow. Most guys walk in and never take them. What they mean depends on which loop you are in front of, so know your closed loop from your open loop.
2. Reading the loop: entering and leaving water
Entering water temperature is the most important input on the machine. Capacity, power draw and operating pressures in that unit’s data are indexed to it. Change it and you change what normal means for every other reading.
The pair is what matters. Leaving water alone is just a temperature. The difference across the coax, with flow, is the heat actually moved.
A single reading lies. High entering water in cooling could be an undersized loop field, a fouled exchanger, poor flow, or a machine running hard since noon: same number, four different repairs. Take both at steady state and read them against that unit’s published performance, the same way COP is a value at stated conditions.
3. Flow, and the good compressors condemned over it
Inadequate flow mimics a refrigerant problem. Starve the source side and the machine cannot move heat across the coax. In cooling, head climbs. In heating, suction falls. On gauges that reads like a charge problem, and I have seen good compressors condemned over it. More on common geothermal system problems.
Your flow indicator is pressure drop across the heat exchanger. Read the pressure either side of the coax and convert the drop to flow on the manufacturer’s curve for that exchanger, which is model specific and why there is no number here. Flow gets taken by air, a weak circulator, a closed valve, a plugged strainer, or a heavy antifreeze mix.
Prove flow before you touch the refrigerant circuit.
4. Pressures do not read like the numbers in your head
Every air source tech carries a memorized set: on a day like this, head belongs about there. Those numbers are anchored to outdoor ambient, and here the anchor is gone: the source side is a loop at ground temperature, stable year round. Head in cooling will not look like the July afternoon you are standing in, and suction in heating will not look like a January morning. The machine is not sick. Your reference is.
The fix is not new numbers to memorize, it is that you stop diagnosing from memory. Pull the data for that model, enter it at the entering water temperature and flow you measured, and read what it should be doing. That is what stops a tech charging a healthy sealed system.
5. The flow center, cavitation and air in the loop
The flow center is the pump module: circulators, isolation valves, fill and purge ports. It has no air source equivalent. Air is the enemy. A circulator is built to move liquid, and fed air it cavitates, loses flow and wears out early.
Air drops flow, and low flow reads on gauges as a refrigerant fault. Air gets in three ways: a top off without a proper purge, a fill port left open, or a leak pulling air in on a pump’s suction side. So purge until air stops coming out, then pressurize and seal. Pump and loop pressure belong in every geothermal maintenance visit.
Never put sealer or stop leak in a loop
Dave, plainly: don’t be putting seal or crap in it.
It does not fix a mechanical fitting. A weeping flange gasket, a tired o-ring, a cracked fitting: no additive tightens a joint. Worse, it does not stay at the leak. It circulates through the flow center, the circulators and the coaxial heat exchanger, so you risk the expensive parts to avoid finding a gasket.
6. Pressurize it and seal it properly
Tech to tech: when a loop genuinely loses fluid, the cause is almost always human. It was never pumped up to proper pressure, or never sealed properly. That is workmanship, not a product defect. Fused polyethylene in the ground has no gasket in it, so the joints that fail are the ones a person made.
7. Where the leaks actually are
Work from the flow center outward:
- Flow center and pump flanges. The weep shows underneath, where nobody looks.
- Hose kits and o-rings. Rubber ages, clamps relax, crimped ends are favorites.
- Fittings and adapters. Every threaded and barbed transition, especially where material changes.
- Valves and the fill port. Stem packing, and a port nobody closed after a top off.
- Pressure and temperature ports. Easy to skip, enough of a drip to matter.
- Unit connections. Where the loop lands on the heat pump, coax included.
The one real exception is older loop pipe assembled with push together fittings, not fused joints. Seasonal expansion and contraction works that joint until it cracks. Even that is a fitting failure, not the pipe wall.
Mineral staining is your best tell. But a weep slow enough to evaporate as it forms leaves no stain at all, which is why these go unfound for years. Isolate and pressure test the loop, which separates an above ground problem from a buried one before anybody digs.
8. The desuperheater, briefly
A small refrigerant to water heat exchanger that uses hot discharge gas to preheat domestic water, with its own pump and plumbing. What gets missed: it only makes hot water while the compressor runs, so output follows the season, not the shower schedule. And when its pump fails the complaint arrives as a hot water problem, so nobody checks it. Detail on what a desuperheater is.
9. Go get your IGSHPA accreditation
Get the manufacturer side too. I am a ClimateMaster GeoElite dealer, and the practical value is not the logo, it is factory training on specific equipment, a technical support line that knows the model, and warranty claims you can process yourself instead of arguing through a supply house.
IGSHPA is the International Ground Source Heat Pump Association. Its accreditation covers the ground side: loop design and sizing, pipe fusion, purging, flushing, pressurizing, flow center work, and the water side commissioning no air source curriculum touches. It is a credential, not a license, and not required by law. If you work on this equipment, go get it.
Nobody taught us this
I am not running down other technicians. The guys getting this wrong can pull a nuisance fault off a furnace in ten minutes. They are wrong here for one reason: nobody taught us this. Geothermal was a small slice of the market, and training followed volume. A training gap, not a character flaw.
Homeowner reading over his shoulder: when somebody condemns your geothermal system, the question is not whether he was trying. It is whether he was ever taught this equipment. Ask what the entering water temperature was, and whether flow was verified.
Questions I get about working on geothermal
How do you check water flow on a geothermal heat pump?
Measure pressure drop across the coaxial heat exchanger and convert it to flow on the manufacturer’s curve for that exchanger. Verify flow before you put gauges on the refrigerant circuit, because low flow imitates a charge problem well enough to condemn a good compressor.
Can you put stop leak in a geothermal loop?
No. It cannot repair a mechanical joint like a flange gasket, an o-ring or a cracked fitting, and it does not stay at the leak. It circulates through the flow center, the circulators and the coaxial heat exchanger, the expensive parts.
How much does a geothermal diagnostic cost in Kingfisher Oklahoma?
$348 flat, the same price at any hour, weekday or after hours. That covers a full performance check with a written report and a quote before I leave. No credit and no refund. A quote on a new system is free.
Call 405-375-4822
If your system got condemned, get a second opinion from somebody trained on the water side. My Geothermal Diagnostic is $348 flat at any hour, with a written report and a quote before I leave. A quote on a new system is free. Geothermal repair is statewide Oklahoma.
Technician with a machine that is not adding up? Call anyway.
Dave Hartzell, Hartzell's Heat & Air, 602 S Main St, Kingfisher, Oklahoma. Master HVAC License, IGSHPA Accredited. 47 years in the trade, 15+ years in business.