Geothermal Ground Loop Components: What's in the Ground and Why

A geothermal quote can describe the ground loop in one line ("vertical closed loop, 2 bores") and then spend three pages on the heat pump. That's backwards. The loop is the part going under your lawn, the part that should outlast the equipment attached to it, and the part nobody can inspect once it's buried.

A closed ground loop is five things: pipe, joints, a water-based fluid, a pump package, and valves. Each has a published specification behind it.

This guide walks through the configuration most residential quotes describe: HDPE pipe with heat-fused joints. It isn't the only permitted build. PPI MS-7 also covers PE-RT and PEX tubing, which have their own approved fitting systems, and a full loop field includes hardware this guide doesn't cover, such as U-bends, grout and tracer wire. If your quote names PE-RT or PEX rather than HDPE, the specifics below about fusion won't map onto it.

First, What the Loop Is — and What Never Enters It

In a conventional closed-loop system, refrigerant never enters the buried pipe.

The refrigerant stays inside the heat pump cabinet, in its own sealed circuit, the same arrangement as your refrigerator. What travels through the buried loop is water, or water mixed with antifreeze. The two circuits meet inside the cabinet at a water-to-refrigerant heat exchanger, where the loop fluid gives up heat to the refrigerant in winter and absorbs it in summer, without the two fluids ever mixing. Manufacturers commonly build that exchanger as a coaxial, tube-within-a-tube assembly, but the construction is a specification of the equipment rather than a rule of the technology. Your unit's literature will name it. IGSHPA describes it plainly: "For water-based, water or antifreeze solution is circulated through plastic pipes buried beneath the earth's surface."

Engineers call the buried loop the ground heat exchanger. What governs its design is fluid flow, pressure, freeze protection and air removal.

A quote may instead specify a direct-exchange (DX) system. These are what the DOE's research roadmap calls "a niche form of closed-loop system that circulate refrigerant from the heat pump directly through buried metal tubing instead of using a secondary glycol/water loop." Metal tubing, refrigerant underground, no water loop at all. Everything in the rest of this article describes the conventional water-based system, the configuration our how geothermal heating works guide walks through end to end.

Closed-loop pipe may run down drilled boreholes or across horizontal trenches; either way the circuit is sealed. The open-loop alternative pumps groundwater directly and involves a different set of components and questions. See open loop vs. closed loop for that comparison.

The Pipe: What PE4710 on Your Quote Means

The pipe in a conventional loop is high-density polyethylene (HDPE), and there's a model specification that says which HDPE qualifies. It's PPI MS-7, published by the Plastics Pipe Institute and hosted by IGSHPA. MS-7 describes itself as a model specification and a guide. A project or an authority having jurisdiction can adopt all of it, part of it, or none of it, so what actually binds your installation is your local code and your contract. Where MS-7 is adopted, HDPE ground-loop pipe must:

• be made from PE4710 material, evaluated under ASTM D3350;
• meet one of the product standards CSA B137.1, ASTM D2737, ASTM D3035, or ASTM F714;
• be listed in PPI TR-4 with a minimum hydrostatic design stress of 800 psi at 73°F;
• be certified to NSF 358-1 and NSF/ANSI/CAN 61.

"PE4710" is the piece to look for on a quote or a pipe print line. If a spec sheet mentions PE3608, read carefully: MS-7 permits PE3608 for PE-RT tubing, a related but distinct product that may be PE3608 or PE4710 and carries a lower minimum design stress of 630 psi at 73°F. PE3608 is not the grade MS-7 accepts for HDPE loop pipe.

SDR: the wall-thickness number

The second number on loop pipe is the SDR, or standard dimension ratio: the pipe's diameter divided by its wall thickness. MS-7 puts it directly: "A numerically-lower Standard Dimension Ratio (SDR) value indicates a thicker wall for a given diameter."

There is no single SDR for every ground loop. MS-7's table sets a maximum SDR and a minimum pressure rating by material and nominal size; across the ¾"–2" HDPE sizes used for loop circuits it lists a maximum of SDR 13.5, with the pressure rating varying by size and by whether the pipe is going in vertically or horizontally. Read the row that matches your pipe rather than the headline number.

MS-7's wall-thickness limits also protect the pipe from installation stress, kinking, abrasion, scratches and gouges. The spec is explicit that pipe must not be dragged over rough ground or obstructions, and a vertical loop is uncoiled and lowered into its borehole once. The wall has to survive that handling intact.

Depth adds its own check. The deeper the borehole, the taller the column of water standing in the pipe, and MS-7 requires verifying the pipe against that static water-column pressure. If your project involves deep bores, this is one of the calculations your designer runs — our well depth guide covers what drives boreholes deeper in the first place.

The Joints: Why HDPE Gets Fused Underground

In an HDPE loop, buried connections are made by heat fusion — melting the pipe and fitting together so they cool into a single piece of polyethylene. A fused joint is a permanent, homogeneous connection; there's no gasket, thread, or clamp in it to loosen. (PE-RT and PEX tubing take manufacturer-approved fitting systems of their own, which is one more reason to know which material your quote specifies.)

MS-7 recognizes four fusion fitting types for HDPE: socket, butt, saddle, and electrofusion. Butt, socket, and saddle fusion must follow ASTM F2620; electrofusion must follow ASTM F1290 plus the fitting manufacturer's instructions. And the person running the fusion equipment matters as much as the equipment: MS-7 requires fusion technicians to be trained and currently certified for the fusion type and the pipe diameter range being installed. Ask who will perform the fusion work, and for that technician's current certification for the method and pipe diameter being used.

MS-7 puts three restrictions on buried joints:

• Barbed fittings with mechanical clamps may not connect directly to polyethylene pipe underground. The push-on, hose-clamp style connection familiar from irrigation work is excluded from burial on loop pipe.
• All mechanical connections must remain accessible. Anything with a joint you could conceivably tighten or take apart has to stay somewhere a human can reach it.
• Vertical-loop fusion joints must follow the cited ASTM procedures, and the number of joints must be minimized. Fewer joints, fewer places for anything to go wrong.

Together those keep serviceable mechanical connections reachable and limit buried HDPE connections to standardized fusion. The standards don't quantify field failure rates for either joint type; they simply require the buried ones to be fused.

The Loop Fluid: Water Plus What, Exactly

The fluid in the loop is water — and, wherever any part of the loop could freeze, water mixed with antifreeze. MS-7 lists these water-mixed antifreeze options:

• Propylene glycol, with approved corrosion inhibitors and environmental stabilizers;
• Ethanol, with approved inhibitors and stabilizers;
• Methanol, with approved inhibitors and stabilizers;
• other fluids permitted by ANSI/CSA/IGSHPA C448 or local codes.

DOE workshop training material states the trade-offs directly: methanol is comparatively inexpensive and high-performing but toxic and flammable; propylene glycol is non-toxic but more expensive and lower in heat-transfer performance. Ethanol, like methanol, is flammable, and MS-7 requires manufacturer safety procedures and local regulations to be followed for flammable fluids.

All antifreeze must comply with local regulations, and the DOE material warns that local law and codes may determine which fluid is approved where you live. Concentration is a separate calculation, based on the required freezing point for your loop design and the fluid manufacturer's specifications; a colder-running loop needs more freeze protection than a milder one. There is no universal "X% glycol" figure. Fluid condition is one of the loop-side items a service visit can check, and our maintenance guide covers where that fits.

The Flow Center: The Pump Package on the Wall

The flow center moves fluid between the heat pump and the ground loop: a packaged pumping-and-valving assembly. It's the small cabinet on the wall or on the side of the heat pump with two loop pipes entering it.

WaterFurnace's installation manual for its GeoLink flow centers documents what's inside: one or more circulating pumps in fixed- and variable-speed versions, an insulated cabinet, and three-way valves. In operation, per the manual, "the pump circulates loop fluid through the heat pump, ground loop, and FC Reservoir."

Fixed- versus variable-speed matters here for the same reason it matters in the heat pump itself: a pump that can throttle draws less power at part load, and pumping energy is one of the inputs behind the rated efficiency figures in our guide to the most efficient geothermal heat pumps.

In a non-pressurized flow center, the reservoir separates air from the circulating fluid and provides a point to check the fluid and add more. Continuous air separation reduces loop noise and helps maintain circulation.

The three-way valves are the other half of the package. They give the flow center its operating, bypass, flushing, and purging configurations — which means a technician can flush or purge the system by turning valves rather than cutting pipe. When your loop was first commissioned, those valves are how the air and debris came out.

Headers, Manifolds, and the Valves That Matter Later

Where a loop field uses a centralized distribution manifold — paired supply and return headers — the individual bore or trench circuits connect to it in parallel, and larger-diameter pipes carry the combined flow between those headers and the heat-pump equipment. That's per PPI TN-55, the Plastics Pipe Institute's technical note on ground-source piping. It isn't the only layout TN-55 describes: series and reverse-return arrangements exist too, and some buried headers are built without valves at all. Ask which one you're getting.

Parallel circuits create a flow problem: fluid takes the path of least resistance, so without correction, some circuits draw more flow than others. Two valve types on the manifold handle this and everything after:

• Balancing valves add resistance where necessary so each parallel circuit receives appropriate flow. A circuit starved of flow moves less heat than its length suggests, no matter how well it was drilled.
• Shutoff valves, typically installed at the manifold, isolate individual circuits for purging, repair, and maintenance — so one circuit can be worked on without draining the field.

Purge valves provide high-flow access for removing construction debris and trapped air from a new loop. The 2016 IGSHPA Design and Installation Standards specify placement that allows: the ground loop and the heat-pump circuits can be flushed separately, purge flow is not forced through the circulator pumps, and the ground loop can be flushed before the heat pump is even installed. (The current C448 standard is a paid document; the 2016 IGSHPA standards are the publicly readable statement of these provisions.)

That last provision lets the loop field be flushed, purged and verified while the mechanical room is still empty.

Where the Serviceable Points Are When Something Goes Wrong

These standards don't publish field failure rates for leaks, pumps or fluid degradation. They do identify the accessible components a technician works from:

• Mechanical connections. MS-7 requires them to stay accessible, so the joints that can be opened — at the flow center, at reachable manifold locations — are where diagnosis starts.
• The flow center. A non-pressurized reservoir shows fluid level and condition, and the valving allows flushing or bypass without excavation.
• Shutoff valves at the manifold. Where circuits are individually valved, a problem can be narrowed circuit by circuit rather than treated as one buried system.

If your system is showing symptoms such as poor heat transfer, pump noise or pressure loss, the diagnostic path runs through those accessible points first. Our geothermal repair guide walks through what technicians actually check and in what order.

Frequently Asked Questions