Bad Water and Geothermal in Oklahoma: Scale, Corrosion, and the Mineral Myth

I am Dave Hartzell. 47 years in HVAC, 15+ years running Hartzell’s Heat & Air out of Kingfisher, and geothermal is what I am known for. The call goes like this: “My geo quit cooling, and the guy said the loop is full of minerals.”

Sometimes that is true. On a closed loop it usually is not, and it is expensive to be wrong about.

Two systems, and only one of them drinks

An open loop pulls raw water out of the ground, runs it through the heat pump once, and sends it somewhere else. It drinks: whatever is dissolved in your well goes through the machine every minute the compressor runs. A closed loop is sealed polyethylene pipe, fused in the ground and filled one time with water and antifreeze, circulating that same fluid forever. That difference decides almost every water question. More on closed loop vs open loop geothermal.

What scale does inside a coaxial heat exchanger

The water side of most geothermal heat pumps is a coaxial heat exchanger: one tube inside another, water in one passage and refrigerant in the other, a thin metal wall between. All the heat the system moves crosses that wall. Scale plates out on the water side, and scale is an insulator. It does not have to be thick to hurt you, because you did not buy the tube, you bought the heat that moves through it.

• Capacity falls first, quietly. It runs longer for the same result, and people call it a hot summer.
• Pressures move. In cooling it cannot dump heat into the water, so head pressure climbs. In heating it cannot pull heat out, so the low side drops.
• Late on, the passage narrows. Pressure drop rises, flow falls, and lower flow fouls faster.
• It locks out. The safety trips, and by then the compressor has fought high head for a season.

Brass, bronze and cast iron: where the metal actually is

People picture corrosion out in the yard. On a closed loop there is nothing out there to corrode: the buried pipe is fused polyethylene, no threads and no metal joints underground. The metal is all where you can reach it, at the brass adapters, hose kits, valves, flow center and the exchanger.

• Dezincification. Brass is copper and zinc. Aggressive water pulls the zinc back out and leaves a porous copper skeleton. The fitting still looks like a fitting and has the strength of a sponge.
• Galvanic corrosion. Couple brass or bronze to cast iron with conductive water between them and you have built a battery. The less noble metal gives itself up. Chemistry, not a defect.
• Cast iron tuberculation. Iron rusts inward in lumps that choke the passage and shed rust into strainers, pumps and the coax.
• Hydrogen sulfide. Rotten egg water goes after copper specifically. That is why cupronickel exchangers exist, and why the water test belongs before the equipment order.

One fill against a fresh load every minute

Skip the arithmetic and look at where the water comes from, because that is the whole difference. An open loop is a river. Every minute the compressor runs, water that system has never seen before enters the exchanger, trades heat across the wall, and leaves for good. Whatever the well carries, hardness, iron, sulfur, dissolved oxygen, shows up fresh and keeps showing up, all day and all season. The exchanger never gets a break from it.

A closed loop is a bucket. It gets filled one time, on install day, with water and antifreeze, and then it is sealed shut. Nothing is added after that. The same fluid runs past the same metal wall over and over for the life of the system, carrying exactly what went in and nothing else.

Why the closed loop mineral story is usually wrong

A closed loop has no resupply. Whatever came in with the fill water is all that system will ever see, and it deposits once and stops. Corrosion is the same story: rusting iron needs dissolved oxygen, a sealed loop holds a fixed amount, the metal consumes it in the first weeks, and the reaction runs out of fuel. An open loop is the opposite, and every gallon brings a fresh load of oxygen and mineral, all season.

Usually, though, not always. Closed loops do foul: filled from a dirty well and never flushed, install debris in the pipe, biological growth in a system that sat, a packed strainer. Every one is a single event or a maintenance fault, not an ongoing supply. If a flush fixes it and the same symptom is back next season, it was never minerals.

What it usually turns out to be instead

1. Air in the loop. Never fully purged, or a leak let air back in. An air pocket kills flow and reads almost exactly like a fouled exchanger.
2. A weak circulator, or a valve left part way closed after the last service call.
3. Air side, not water side. A loaded filter, a dirty coil, closed registers, crushed flex. Starve the indoor airflow and head pressure climbs.
4. A pinched or crushed pipe. Backfill settles or somebody trenches a water line, and flow drops on one circuit.
5. A loop short for the load. Water temperature drifts every season, which is design, not chemistry. Losing fluid is its own problem: why a geothermal loop keeps leaking.

How I tell them apart

Scale and low flow both look like bad heat transfer. They separate on measurement. I take entering and leaving water temperature across the coax, pressure drop across the water side, actual flow, refrigerant pressures with superheat and subcooling, and the air side numbers. Bad transfer with normal flow and normal pressure drop points at the wall, because a film insulates before it restricts. Low flow with high pressure drop points at a restriction, and that is air, a pump, a valve or a crushed pipe long before it is heavy scale. When a flush is the answer I use a cart and micron filter bags, then read what came out. More on geothermal system problems in Oklahoma.

What this costs

A geothermal diagnostic is $348 flat, any hour. You pay it up front, there is no refund and no credit on a repair, and you get a written report before I leave. Replacement work is quoted separately before approval. A residential service call is $99 dispatch plus $111 diagnostic, $210 total. The tech gives a written repair estimate before repair work starts. There is always a dispatch fee and always a diagnostic fee. A new system estimate is free.

Said plainly: the federal residential credit under Section 25D has expired. Tax questions go to your tax professional. OMPA pays up to $800 per cooling ton in Kingfisher, Okeene and Watonga, a cap and not a guarantee, requiring a desuperheater.

Where I work

Geothermal is statewide for me. I go anywhere in Oklahoma for a geothermal customer: Kingfisher, Okarche, Hennessey, Watonga, Piedmont, Edmond, Enid, Oklahoma City, Lawton, Broken Bow and Durant. Daily service calls stay inside Kingfisher, Canadian, Oklahoma, Garfield, Major, Logan and Blaine counties. Geothermal does not. I am IGSHPA accredited and Master HVAC licensed. 4.8 stars, 300+ Google reviews.

Questions people ask me about geothermal and water

My tech said my geothermal loop is full of minerals. Is that even a real thing?

On a closed loop it is usually wrong. The loop is sealed and was filled once, so whatever came in with the fill water deposits one time and stops. Air, a weak circulator, a closed valve or a dirty coil cause the same symptoms.

Does hard water hurt a geothermal heat pump in Oklahoma?

On an open loop yes, because raw well water runs through the exchanger continuously and scale insulates the transfer wall. On a sealed closed loop that same chemistry is a one time exposure.

Will bad well water eat the brass fittings on my geothermal system?

It can. Aggressive water leaches zinc out of brass and leaves porous copper that fails under pressure, and brass bolted to cast iron in conductive water sets up a galvanic cell. On a closed loop that exposure is inside, not underground.

Does bad water void my geothermal warranty?

I will not answer that from a web page. Coverage and acceptable water chemistry live in the documents for your specific unit, so bring me the paperwork and I will read it against your lab report.

How much does it cost to get a geothermal system diagnosed in Kingfisher Oklahoma?

$348 flat. Replacement work is quoted separately before approval.

Told your loop is full of minerals? Get a second opinion before you spend the money. Call 405-375-4822. Hartzell’s Heat & Air, 602 S Main St, Kingfisher OK 73750. You can also book online.