Field case · Geothermal repair · Mustang, OK · September 2026

Geothermal set on 73, stuck on 75: a Mustang repair from the job file

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This one came in the second week of September 2026. The homeowners were new to the house and new to geothermal. The thermostat was set on 73 and the house would only get down to 75. Upstairs ran a lot hotter than downstairs, and the electric bills were high and climbing. The system itself was only from 2023, so this was not an old unit wearing out.

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What the homeowners told me

• Set to 73, only reaching 75.
• Upstairs significantly hotter than downstairs, and they wanted the ductwork looked at.
• High electric bills that kept getting higher.
• A 2023 geothermal system they had inherited with the house.

What I did first

It went on the schedule as a geothermal diagnostic, which is $348 flat, weekday or after hours. That covers loop pressure, refrigerant charge, the desuperheater, the controls and the heat exchanger, with a written report and quote before I leave. On a geothermal call I test the loop side before anybody touches the refrigerant, because charging refrigerant for a loop problem is the most expensive mistake in this trade.

What I found

The problem was on the loop side, at the pump station. One of the two loop pumps had failed. The second pump was still running, but it showed signs of neglect, and a system that is only a few years old should not look like that. When one pump in a two-pump station fails, the other is carrying a load it was not sized to carry alone, and flow through the loop drops. Low loop flow means the unit cannot reject heat into the ground the way it was designed to.

The repair

• Replaced the whole pump station with a new two-pump flow center, both pumps Grundfos UP26-116.
• Changed the second pump along with the failed one instead of waiting for it to quit too.
• Refilled the loop with an antifreeze and water mixture and confirmed the water level was good.

After the repair the system was cooling properly. The repair was quoted in writing before any work started.

What is still open, and what I will not claim

The plenums need sealing. I wrote that down and left it with the homeowners. Leaky plenums are part of why one floor can run hotter than another, and the new pumps do not fix that.

I also want to be plain about what the record does not show. There is no before and after temperature reading written in this job, so I am not going to give you one. And I am not going to tell you their electric bill came down, because I do not have that number. What I can tell you is that a geothermal unit with a failed loop pump is working against itself, and that was fixed.

What this job says about geothermal in general

When I counted 727 completed geothermal jobs in my records, loop pump and flow center work came up 48 times and compressors came up 13 times. Not one of them replaced a loop field. The buried pipe lasts. The pump that moves the fluid is a moving part, so it wears, and it wears faster when nobody has looked at it. If you bought a house with geothermal, have the loop checked before the first summer, not after the bills start climbing.

Questions I get about jobs like this

How do I know if my geothermal loop pump is failing?

The house stops reaching setpoint, the unit runs long, and you may get a flow or water temperature alarm. A failing pump is one of the things I rule in or out on the $348 geothermal diagnostic by checking loop pressure and flow before anything else.

What does a geothermal flow center replacement cost?

It depends on the flow center and the loop, so I quote it in writing after the $348 geothermal diagnostic, before any work starts. Replacing a single loop pump usually costs less than a full two-pump flow center.

I just bought a house with geothermal. What should I do first?

Have the loop pressure, flow and pumps checked, and ask for the readings in writing. I do that on every geothermal diagnostic. It is also worth asking the seller for any service records on the system.

Do you service geothermal in Mustang and Canadian County?

Yes. Canadian County is one of the seven counties I run every day, and for geothermal I also travel across Oklahoma and into most states that border it.