Geothermal Loop Questions: Slinky, Vertical and Pond Loops

A slinky loop is a horizontal ground loop where the pipe is laid in overlapping flat coils, like a stretched-out Slinky toy, along the bottom of a wide trench. The overlap fits a lot more pipe into each foot of trench than straight runs do. In one published example, 36-inch coils at a 15-inch pitch put about 9.3 feet of pipe in every foot of ditch, against 6 feet for a six-pipe straight trench.

The pipe is high-density polyethylene (HDPE), a plastic, joined by heat fusion. It is not copper. Water, usually mixed with antifreeze, circulates through it and trades heat with the soil. I still find plenty of slinky loops on older Oklahoma installs when a geothermal unit gets replaced.

No. Slinky loops, like every closed water loop, use plastic pipe, almost always HDPE. ClimateMaster’s loop manual says polyethylene is the only acceptable material for pipe buried in the ground.

Copper in the ground belongs to a different design called direct exchange, or DX, where refrigerant runs through buried copper tubing. The Department of Energy’s Building America guide says DX systems are rarely installed today and should stay out of soils that corrode copper.

So if someone tells you your slinky field is copper, either the system type is wrong or you actually have a DX system, and those are serviced very differently. I sort that out on a site visit before anyone touches anything.

The design comes first, and it comes from your house, not a chart. I start with a load calculation, because the loop is sized to that load.

Then I look at the site:

• the soil (a county soil survey describes the layers down to about 6 feet)
• rock, high water tables, fill dirt and steep grades
• trees
• how equipment gets in

Loop length and layout come out of IGSHPA-based design software, which is what ClimateMaster tells installers to use. I walk through those inputs in more detail on my loop design software page, and site conditions on my geothermal site analysis page. Utilities get flagged before the design is final and before a trench is cut. Oklahoma law requires an excavator to notify OKIE811 at least 48 hours ahead, not counting the day of the call, weekends or legal holidays.

All of that goes on paper with a free new-system estimate. Call 405-375-4822.

Trenches get cut with a smooth bottom, free of rocks, because a sharp rock under pressurized pipe is a future leak. One manufacturer’s typical 4-ton layout uses four slinky trenches, each 3 feet wide and 6 feet deep, with at least 4 feet of earth between them and a 6-foot-wide header trench tying them together.

The HDPE coils are stretched out on the ground, dropped into the trench, and shifted into place with a positioning tool so nobody works down in the trench more than necessary. The coil ends are heat-fused to header pipe that carries supply and return to the house. Barbed fittings with hose clamps are not allowed on buried polyethylene.

OSHA requires cave-in protection for workers in trenches 5 feet deep or more unless the cut is in stable rock.

Every fused joint cools completely, then the loop gets a hydrostatic pressure test, water, not air, before the header trench is closed. The Plastics Pipe Institute’s model specification calls for testing per ASTM F2164, and ClimateMaster recommends a hydrostatic test on all buried piping before final backfill. Flow gets verified too, by comparing the pressure drop across the heat exchanger to the unit’s capacity data.

Backfill starts with about a foot of clean, rock-free soil around the pipe, because good contact with the earth is what moves heat.

Next the loop is flushed and purged with a flush cart at 2 feet per second or faster to push out air and dirt. Antifreeze goes in only after the air is out and the loop has been checked for leaks, and its strength is verified with a hydrometer. Last, I pressurize the loop, start the unit, and record flow, pressure and temperatures.

There is no honest one-size number, and anyone who gives you one without a load calculation is guessing. Trench length depends on your heating and cooling load, the soil and its moisture, the pipe size, the coil pitch, and the coldest water temperature the design allows.

Published guidance gives a sense of scale:

• One design article uses one 3/4-inch loop of 600 to 800 feet of pipe per ton in slinky trenches.
• One manufacturer’s typical 4-ton layout uses four trenches about 135 feet long with 4 feet between them, plus a header trench.
• Natural Resources Canada says straight horizontal loops generally take 400 to 600 feet of pipe per ton.

Those are patterns, not a promise for your yard. I size yours from your house and your dirt. If cost is part of that conversation, I break down what drives it on my geothermal cost page.

Deeper than most people picture, and it depends on the design:

• The Department of Energy’s Building America guide says horizontal loops need trenches at least 4 feet deep.
• Natural Resources Canada puts horizontal pipe at 3 to 6 feet, depending on how many pipes share the trench.
• University of Illinois Extension describes trenches 6 to 10 feet deep.
• In one manufacturer’s example, slinky trenches are 6 feet deep and 3 feet wide.

Shallow ground still follows the seasons. Virginia Tech data show soil at 5 feet reaches its warmest in late August, while ground below 30 feet stays nearly constant. That is why a horizontal loop design has to plan for the colder water it will see late in a long winter.

Two pipes in a trench is actually a real layout. DOE describes horizontal loops with two pipes, one at 6 feet and one at 4 feet, or side by side at 5 feet in a trench about 2 feet wide.

What makes it work is everything around those pipes:

• Length. The loop needs enough pipe touching soil to move your home’s heat. That length comes from a load calculation and design software, and it runs to hundreds of feet per ton.
• Flow. The pumps have to move about 2.5 to 3 gallons per minute per ton through it.
• Joints and testing. Every buried joint gets heat-fused, and the loop gets pressure tested and flushed of air.
• Freeze protection. The fluid needs protection where the design calls for it.

Skip any of that and you get a loop that leaks, air-locks, or runs too cold.

An undersized loop puts your system into territory the manufacturer already accounts for with sensors, not guesswork. WaterFurnace, for example, trips a freeze fault when the loop’s temperature sensor stays below 15 or 30 degrees for 30 straight seconds, depending on whether the loop carries antifreeze, and it also faults when loop pressure drops below 3 psi for more than 3 minutes.

Designers set a minimum entering water temperature specifically to keep a properly sized loop out of that zone. That is why I size loop length from a real load calculation and IGSHPA-based design software, not a guess. If you are wondering whether your own field is on the small side, I walk through the warning signs on my is my loop field too small page. Adding loop later means more trenching or drilling, so getting it right at design time matters.

Both go in trenches. A straight horizontal loop lays pipe in straight runs. DOE’s Building America guide says the common layouts are two pipes stacked at 4 and 6 feet, or side by side at 5 feet in a trench about 2 feet wide.

A slinky coils the same HDPE pipe so more pipe fits in each foot of trench. That shortens the trench but widens it: 3 feet wide in one manufacturer’s example.

Which one fits is mostly a question of available land and installation cost. DOE calls horizontal loops generally the most cost-effective choice on new construction where there is enough land, which is also the situation I cover for builders on my ENERGY STAR builder page. Shallow loops of either kind feel the seasons more than a vertical bore does, so both get sized to the load, not the lot.

For a while, yes, and anyone who says otherwise hasn’t run a trencher.

• Horizontal and slinky loops open long trenches. One manufacturer’s 4-ton slinky layout is four trenches 3 feet wide and 6 feet deep, plus a header trench.
• Vertical loops disturb far less area, because the bores go down instead of out. The rig and its work area still need access.
• Pond loops can spare most of the yard.

Done right, backfill starts with clean, rock-free soil around the pipe, which protects the pipe and its contact with the earth. Once the ground settles, nothing of the loop shows above ground except where the supply and return pipes reach the house. I cover what the actual work looks like day to day on what geothermal drilling looks like in your yard and will geothermal tear up my yard.

I won’t give you a number of days until I’ve walked the site. The published sources I trust don’t give one, and the real schedule depends on the loop type, the soil and rock, rain, and access.

What I can tell you is the order of the work:

1. Utility locates
2. Trenching or drilling
3. Fusing the joints
4. A pressure test before anything is covered
5. Backfill
6. Flushing and purging
7. Antifreeze
8. Startup

The pressure test and the flush aren’t steps to rush, because they’re what keeps a loop from leaking or air-locking later. You’ll get a written scope with the free new-system estimate.

I’d talk you out of it, for legal and practical reasons.

• License. Oklahoma’s Construction Industries Board has a Ground Source Piping Contractor license covering all heat pump loop piping buried in the ground or submerged in a well, lake or pond. It requires proof of certification from an approved organization.
• Owner exemption. The exemption in the Mechanical Licensing Act, as written, covers minor repairs like cleaning, adjusting and replacing thermostats. It says nothing about installing a loop, so check with the CIB before assuming you’re exempt.
• Drilling. Drilling a vertical bore takes an Oklahoma Water Resources Board license. On my own projects that part goes to my licensed driller.
• Warranty. ClimateMaster’s residential warranty excludes units not installed by a qualified, trained HVAC contractor, and performance problems caused by improper design or installation.
• Skill. Fusion joints also call for certified technique.

The safe part is records and watching, not tools.

• Keep the loop map, the commissioning sheet and the antifreeze record together.
• Mark the loop area so nobody drives heavy equipment across it or sets a post through a header.
• If you dig, remember that Oklahoma’s 811 law still applies in an easement or right-of-way.
• Tell me about any loop or flow alarm on the thermostat, and any water you can see around the equipment.

What you should not do: open the loop, add or mix antifreeze, loosen flow center fittings, or cut into buried pipe. The alcohols used in some loops are toxic, and flammable when concentrated.

A geothermal diagnostic is $348 flat. It is a full performance audit, including loop pressure, refrigerant charge and controls, with a written report before I leave.

Yes, for the parts I could verify.

• Loop piping. The Construction Industries Board’s Ground Source Piping Contractor license covers installing, altering, servicing or repairing heat pump loop piping buried in the ground or submerged in a well, lake or pond. It requires proof of certification from an approved organization.
• Vertical bores. These are heat exchange wells. The Oklahoma Water Resources Board requires a license to drill or plug them, with a passing exam and two years of qualifying experience. That license belongs to my driller, not to me.
• Retired fields. State rules require a vertical field to be plugged to Clause 10.9 of the IGSHPA C448.3 standard.

I’m IGSHPA accredited and hold a Master HVAC license. Ask every bidder for their license numbers, and their driller’s, before anyone digs.

Yes. Oklahoma’s Underground Facilities Damage Prevention Act requires excavators to give notice at least 48 hours before digging, not counting the day of the call, Saturdays, Sundays or legal holidays.

The law does exempt an individual digging on their own property who isn’t in the excavating business, except inside an easement or right-of-way. A crew putting in a loop for hire is covered.

One loop manufacturer’s installation guide also says to have all utilities flagged before the loop is designed, not just before the digging starts. Knowing where the lines are changes where the loop can go.

When land is short or you don’t want the yard opened up. Vertical loops go in bores about 4 to 6 inches across, drilled roughly 100 to 500 feet deep depending on soil and system size. DOE’s guide describes them about 20 feet apart. University of Illinois Extension notes they’re used where space is limited and have better thermal performance.

In Oklahoma a heat exchange bore is a regulated well:

• Drilling and plugging take an Oklahoma Water Resources Board license, held by my driller.
• The bore has to be grouted with approved material.
• The loop pipe has to meet the IGSHPA C448 standard.

The pipe in each bore is purged, flushed and pressure tested before the borehole is backfilled. How many bores and how deep comes out of the design, not a rule of thumb. I go through that design process on geothermal loop design and vertical borefield design.

Often, if the pond is big and deep enough.

• Depth. DOE’s guide says pond coils sit at least 8 feet under the surface.
• Size. ClimateMaster’s manual calls for a pond of at least half an acre, 8 to 10 feet deep, for an average home, with roughly one 300-foot coil of polyethylene pipe per ton.
• Load limits. Design guidance based on ASHRAE recommends at least 10 feet of depth, and no more than 20 tons per acre in cooling or 10 tons per acre in heating.
• Anchoring. The coils need weights because the pipe and antifreeze float, and they sit up off the bottom on blocks.
• No direct pond water. Pulling pond water straight through the heat pump is discouraged because of fouling.

I want to see your pond late in the summer before I design around it.

That’s an open loop, and it works when the water supply and water quality hold up. Well water runs through the heat pump and then gets discharged.

• Flow. WaterFurnace recommends 1.5 to 2 gallons per minute per ton of cooling, the entire time the compressor runs, so the well has to sustain that.
• Discharge. It goes wherever local code allows, such as a recharge well, drain field, storm sewer or pond. Most codes forbid the sanitary sewer.
• Water quality. WaterFurnace limits include iron oxide under 1 ppm and suspended solids under 10 ppm.

University of Illinois Extension notes open loops depend heavily on local conditions and rules. Wells in Oklahoma are drilled under an Oklahoma Water Resources Board license held by my driller.

For vertical bores, Oklahoma rules set real distances. A heat exchange well has to meet the water well location standards, which include:

• 10 feet from a closed sewer line
• 25 feet from aerobic sprinkler spray
• 300 feet from a feedlot lagoon
• 50 to 100 feet from other pollution sources, depending on slope

Surface water also can’t pond within 10 feet of the bore.

Close to the house, one manufacturer’s guide calls for closed-cell insulation on antifreeze piping that runs within 5 feet of a wall, structure or water pipe.

I found no published setback for trees or driveways over horizontal loops, so I lay those out on site and keep headers where they can be reached if they ever need work.

In Oklahoma it gets plugged to a standard, not just capped and forgotten. State rules require vertical closed-loop heat exchange wells to be plugged according to Clause 10.9 of the IGSHPA C448.3 standard. Plugging is licensed work under the Oklahoma Water Resources Board, done by a licensed driller. The driller is responsible while their equipment is still on site; after that it falls to the landowner.

Before retiring an old field, though, it’s worth testing. If it still holds pressure and flows the way a new unit needs, reusing it may be the better move, which is the situation I cover on my geothermal loop upgrade page.

Either way, abandoning a bore is not a homeowner project.

Almost always high-density polyethylene. The Plastics Pipe Institute calls HDPE the most common ground loop pipe and says it is recognized in virtually all codes. Its 2024 guidance recommends PE4710 material certified to NSF/ANSI 358-1. PEX and PE-RT are also used.

SDR is the pipe’s outside diameter divided by its wall thickness, so a lower SDR means a thicker wall. PPI lists PE4710 at SDR 11 as rated 200 psi at 73 degrees and 125 psi at 140 degrees.

Slinky loops usually use 3/4-inch pipe, because bigger sizes are hard to coil and handle in a trench. The loop pipe coming to your house should be plastic, not copper.

By heat fusion. The pipe and fitting are heated to melting temperature and pressed together so they become one piece of plastic. The Plastics Pipe Institute lists three methods: butt fusion, socket fusion and electrofusion.

• Industry standards and codes require buried connections to be heat-fused or made with approved mechanical fittings.
• PPI’s model specification says barbed fittings with hose clamps must not connect to buried polyethylene.
• Mechanical connections have to stay accessible.
• The same specification requires fusion technicians to hold current certification for the fusion type and pipe size.

Getting it right matters, because PPI notes leaks and failures are more likely at joints than in the pipe itself. I go through the whole process on HDPE pipe, fusion, testing and flushing.

It depends on how cold the loop water gets and on local rules.

• When it’s needed. ClimateMaster says antifreeze is needed wherever entering loop water can drop below 40 degrees or piping runs through areas that freeze. Plenty of southern installs need it for exposed piping. I go into more depth on do geothermal loops freeze in an Oklahoma winter.
• How much. Freeze protection is set 15 degrees below the coldest expected entering water.
• Which fluids. The Plastics Pipe Institute lists propylene glycol, ethanol and methanol, each with approved inhibitors.
• Oklahoma bores. For vertical heat exchange wells, Oklahoma rules approve potable water, food-grade or USP-grade propylene glycol, and solutions whose leaks would dissipate.

Every fluid is a tradeoff. Methanol is toxic, and propylene glycol is hard to pump when it’s cold. Choosing and charging antifreeze is the installer’s job, never a homeowner’s.

A flush cart is a big pump with a reservoir and filter, used to fill a new loop and drive out the air and dirt. ClimateMaster and WaterFurnace both call for at least a 1.5 horsepower pump.

The Plastics Pipe Institute’s model specification calls for flushing at a minimum of 2 feet per second. ClimateMaster says 2 feet per second in all piping, in both directions, with a filter on the return line.

Air and debris have to come out before the system runs. Once they’re out, the loop is pressure checked for leaks, then antifreeze is mixed in through the cart and its strength is verified with a hydrometer. It’s installer equipment, not a homeowner tool.

A properly installed loop typically outlasts the heat pump by decades.

• DOE’s Building America guide estimates 25 years for the indoor components and 50-plus years for the ground loop.
• Natural Resources Canada says properly installed pipe lasts 25 to 75 years.
• The Plastics Pipe Institute says life expectancy is typically well in excess of 50 years when pipe is specified and installed correctly.

Warranty is a separate question. The heat pump maker’s warranty covers its own equipment. ClimateMaster’s residential warranty excludes components it didn’t supply, and performance problems from improper design, sizing or installation. Ask your installer for warranty paperwork in writing before the loop goes in the ground.

Most loop trouble isn’t a hole in the buried pipe. The Plastics Pipe Institute notes leaks and failures are more likely at joints than in the pipe itself, so connections are where I look first.

Loop pressure on a pressurized system also moves with the seasons. WaterFurnace sets it at 40 to 50 psi in summer and 50 to 75 psi in winter, so a single low reading on a warm day doesn’t prove a leak. A reading that keeps falling does deserve a call, and so do low flow or freeze faults.

Chasing a buried leak means isolating sections and pressure testing each one, which is licensed work. A geothermal diagnostic is $348 flat and includes a loop pressure check, with a written report and quote before I leave. Call 405-375-4822.

Often, and it’s worth checking before you replace it. Loop pipe is expected to far outlast the heat pump: DOE estimates 50-plus years for the loop against 25 years for the indoor components, and the Plastics Pipe Institute says typically well over 50 years.

Age alone doesn’t prove a loop is good, though. Before I tie a new unit to an old loop, I want:

• the loop holding pressure
• the loop flushed clean of air and debris
• measured flow matching what the new unit needs (about 2.5 to 3 gallons per minute per ton on WaterFurnace equipment)
• antifreeze tested for strength

A loop that tests bad gets repaired, added to, or replaced. I walk through this exact call on can an existing geothermal loop be reused.

Deep ground does. Shallow ground doesn’t.

The Department of Energy says temperatures about 30 feet down stay relatively constant year-round, and Virginia Tech data show that ground below 30 feet roughly matches the water in 30- to 50-foot wells.

Closer to the surface the seasons still reach down, just late. Soil at 5 feet hits its warmest in late August, and soil at 12 feet in late October. That is why a horizontal or slinky loop sees colder water through a long winter than a vertical bore does, and why its design has to allow for that coldest water.

The buried loop is plastic pipe and fluid, so there’s nothing out in the yard running. The pumps that move the fluid sit inside, in a flow center next to the heat pump, with connections for flushing and filling the loop.

What matters for a quiet, steady system is getting the air out when it’s built. ClimateMaster says all air and debris must be removed from the loop before operation. That means a flush cart pushing fluid at 2 feet per second or more, in both directions, with a filter on the return.

If your system has started making new sounds, tell me what you hear and when. That is part of what I check on a geothermal diagnostic.

On a pressurized loop, some change is normal. WaterFurnace says static loop pressure fluctuates with the seasons and runs higher in winter than during the cooling season, with typical settings of 40 to 50 psi in summer and 50 to 75 psi in winter.

A pressure that keeps dropping over weeks is different, especially with low flow or freeze faults. WaterFurnace controls flag a fault when loop pressure stays below 3 psi for more than 3 minutes.

Don’t add water or open fittings to bring the number back up. Write down the reading and the date, and call 405-375-4822. Loop pressure is one of the first things I check on a $348 geothermal diagnostic.