Geothermal Loop Design: Bore Depth, Headers, Flow, Head, and Antifreeze in Oklahoma
Direct answer: A geothermal loop is not sized by tons alone. A defensible Oklahoma design starts with the building load, equipment data, ground conditions, allowable entering water temperature, loop type, bore or trench layout, header sizing, flow, pump head, and antifreeze assumptions. Change one assumption and the design can change.
Start with load, not tonnage
DOE describes a geothermal heat pump as a system that exchanges heat with the ground through buried or submerged piping. That ground loop has to match the building, not just the nameplate of the old unit. ACCA Manual J is the residential load-calculation anchor before equipment and loop decisions are made.
Loads can change after additions, window changes, insulation work, duct changes, or a different thermostat strategy. A loop that was acceptable for an older house configuration may not fit the next system without testing and design review.
Bore depth is an output, not a guess
Vertical bore depth depends on heating and cooling loads, local geology, bore spacing, grout, pipe size, flow, entering-water-temperature limits, runtime profile, and equipment data. Horizontal and pond loops have different assumptions. IGSHPA points current design and installation work to C448:2025, and PPI notes that qualified designers use project-specific modeling to avoid overheating or overcooling the ground over time.
A rule-of-thumb bore depth can be useful for an early conversation, but it is not a design. The design must state what assumptions were used.
Flow and Delta T are linked
For water, the common field relationship is:
Btu/h = 500 x GPM x Delta T
GPM = Btu/h / (500 x Delta T)Antifreeze changes the fluid factor. The correct factor comes from the fluid type, concentration, and temperature range. This is why the loop design, pump selection, and COP calculation have to use the same fluid assumptions.
Worked example – water-only flow estimate
Example only: If a water-side calculation needs to move 36,000 Btu/h with an 8 degrees F temperature change using water, the flow estimate is:
36,000 / (500 x 8) = 9 GPMThis is not a design target. It leaves out antifreeze correction, unit pressure drop, pipe pressure loss, pump curve, Reynolds number effects, and the manufacturer’s operating limits.
Head loss is only half the equation. Sizing the flow center against that head is what turns the number into a pump selection.
Header sizing and head loss
The pump must overcome total dynamic head, not just move a nominal GPM. Head includes straight pipe, fittings, headers, valves, flow center components, the heat pump heat exchanger, antifreeze viscosity, and elevation or static effects where applicable.
Total head = pipe loss + fitting loss + header loss + unit pressure drop + valve and accessory lossUndersized headers can create low flow and high pump energy even when the buried loop has enough heat-exchange area. Oversizing without design can add cost and fluid volume without solving a load or bore-spacing problem.
Antifreeze assumptions in Oklahoma
Oklahoma closed loops still need a freeze-protection decision. ClimateMaster literature for the referenced TE family requires antifreeze when minimum entering loop temperatures can drop below its stated threshold for that equipment or when piping is routed through freezing areas. That is model-specific guidance, not a universal limit for every unit.
The design record should identify fluid type, target concentration, total loop volume, compatibility, and how the mixed concentration will be verified. It should not guess from jug count.
| Design input | What it controls | What to document |
|---|---|---|
| Manual J load | Heating and cooling demand the loop must support. | Load calculation date, assumptions, and building changes. |
| Equipment data | Required flow, operating range, capacity, and pressure drop. | Model, stage, airflow, EWT range, and tables used. |
| Bore or trench layout | Heat exchange area and long-term ground temperature behavior. | Depth, spacing, grout or backfill assumptions, and field map. |
| Header sizing | Flow balance and pressure loss. | Pipe size, circuit lengths, balancing method, and fittings. |
| Antifreeze | Freeze protection, heat capacity, viscosity, and pump head. | Fluid type, concentration, volume, and test method. |
Field-check list
- Confirm the current building load before reusing an old tonnage assumption.
- Identify loop type: vertical, horizontal, pond, open, or hybrid.
- Record bore/trench map, circuit lengths, pipe sizes, header sizes, and manifold access.
- Check equipment data for required flow, pressure drop, and operating conditions.
- Calculate total head with antifreeze properties included.
- Choose pumps from the pump curve at the design flow and head.
- State allowable entering water temperature assumptions for heating and cooling.
- Verify actual flow, EWT, LWT, pressure drop, and watts during startup.
Safety and professional boundary
Homeowners can collect old loop maps, model numbers, utility history, remodel notes, and comfort symptoms. Do not drill, trench, expose headers, open the loop, change antifreeze, or resize pumps from an online chart. Loop design belongs with qualified geothermal designers and installers using the equipment manual, current standards, and local code.
When to call Hartzell’s
Call Hartzell’s before replacing a geothermal unit on an unknown loop, after a remodel changes the load, when pumps have been changed repeatedly, when water-temperature faults return, or when the old loop map and antifreeze record are missing.
Related geothermal pages
Primary sources used
These sources set the technical context. The page does not replace the model-specific manual, local code, or field measurements for a real job.
- U.S. Department of Energy geothermal heat pump overview – GHP ground loop and heat-pump system overview.
- IGSHPA standards page – Current C448:2025 standard status and design/installation context.
- IGSHPA manuals page – Ground-source design and installation manual topics.
- ACCA Manual J residential load calculation – Load calculation boundary before loop design or equipment selection.
- PPI TN-55 geothermal plastic piping materials – Pressure changes from thermal expansion/contraction, typical static pressures, and design professional boundary.
- PPI BCD design calculator – Head loss, thermal expansion/contraction, static water column pressure, and antifreeze fluid calculation context.
- AHRI water-source heat pump program – Manufacturer performance-data context and related ISO 13256 standards.
- ClimateMaster Tranquility 30 Digital TE installation manual – P/T port flow verification, flushing/purging, antifreeze volume check, and water-side heat-transfer formulas for the referenced model family.
FAQ
Is there one bore-depth rule for Oklahoma geothermal?
No. Bore depth depends on building load, equipment data, ground conditions, bore spacing, grout, loop layout, flow, and allowable entering water temperature.
Can bigger loop pipe fix a weak geothermal design?
Not by itself. Header and pipe sizing can reduce head loss, but loop capacity still depends on heat transfer with the ground and the actual building load.
Should Oklahoma loops use antifreeze?
Closed-loop freeze protection is a design decision based on expected minimum loop temperature, routing, equipment instructions, and local practice. The type and concentration must be verified for the job.