Geothermal heat pump field guide: loop, flow, and commissioning

Ground-source heat pump, and the loop that does the work

A geothermal heat pump, also called a ground-source or water-source heat pump, is a refrigerant machine that exchanges heat with the ground or with well water instead of with outdoor air. A few feet down, the earth holds close to the local average annual temperature all year, commonly somewhere around 45 to 70°F depending on where you are. The unit runs its refrigerant cycle against that stable temperature, so it never has to fight a 5°F morning or a 100°F afternoon the way an air-source unit does.

That one difference drives everything good about the machine. The compressor works across a smaller temperature lift, so it moves more heat per watt, and the equipment lasts longer because nothing sits outside in the weather. The box is rarely what fails on a geo job. The loop and the water are.

The work splits into two trades that have to agree on the same load. The loop side is drilling or trenching, pipe fusion, flushing, and antifreeze. The unit side is refrigerant, air or water distribution, and controls. A geo job goes wrong at the seam between them: the loop sized for one tonnage and the equipment for another, or a loop that nobody purged before the unit was started. Get the loop wrong and no amount of work on the unit fixes it.

Why does a geothermal heat pump beat air-source?

The advantage is the stable source temperature, and it shows up as a higher coefficient of performance. Where an air-source heat pump loses capacity and efficiency as the outdoor air drops toward 0°F, a ground loop is still feeding the unit water in the 30s or 40s, so the compressor keeps a manageable lift. Field and manufacturer COP figures for ground-source units commonly land in the range of about 3.5 to 5 in heating, meaning 3.5 to 5 units of heat delivered per unit of electricity in. Verify the rated COP and EER for the specific model against its AHRI listing, because the number moves with entering water temperature and flow.

Cooling tells the same story in reverse. In summer the loop is cooler than the outdoor air the air-source unit would reject heat to, so the geo unit rejects heat into cooler water and runs at a higher EER. That is why a geo system can carry both heating and cooling from one machine and a reversing valve.

Life expectancy is the other selling point. The indoor unit is protected from weather and commonly lasts on the order of 20 to 25 years, and a properly fused HDPE ground loop is generally expected to last 50 years or more. The cost that buys all of this is up front, in the drilling or trenching, which is exactly where the design has to be right because you do not get to redo a bore field cheaply.

Closed loop or open loop?

There are two families. A closed loop circulates a sealed water-and-antifreeze mix through buried HDPE pipe, picking up or rejecting heat through the pipe wall and never touching the ground water. An open loop pumps actual ground water out of a well, runs it through the heat pump's heat exchanger, and then discharges it, either to a second well, a pond, or a surface drain. Closed is the more common choice and the more forgiving one.

Closed loop is the default because it is sealed, predictable, and works almost anywhere you can drill or trench. The fluid stays clean, the freeze protection is in your control, and there is no well chemistry to fight. The cost is the loop install itself.

Open loop can be more efficient when the site has the water for it, because ground water arrives at a stable temperature and water is a strong heat carrier, so you move the load with less flow. The catch is that you have to have the well capacity, the water has to be clean enough not to scale or corrode the heat exchanger, and you have to have a legal place to put the discharge water. Where the water is good and plentiful, open loop is attractive. Where it is iron-heavy or scarce, closed loop is the safer call, and the permit alone can decide it.

Closed-loop types: vertical, horizontal, and pond

Closed loops come in three layouts, and the site usually picks for you. Vertical goes deep in boreholes when surface land is short. Horizontal spreads out in shallow trenches when land is cheap and plentiful. Pond or lake drops the loop into a body of water when one is close enough to use.

Vertical is the urban and suburban answer. Boreholes commonly run on the order of 200 to 400 ft deep, take little surface area, and reach soil that holds a steady temperature regardless of the season at grade. They cost more per foot because you are paying a drill rig, but on a tight lot they are often the only option.

Horizontal is cheaper to install where you have the room, with trenches commonly dug about 6 to 10 ft deep over a wide area. The tradeoff is that the shallow pipe sees seasonal swings in soil temperature and moisture, so a horizontal field tends to run a little less efficiently than a vertical one at the same load and needs far more land. Pond and lake loops are the cheapest when a suitable body of water is available, with the coils sunk and anchored below a depth that stays cold-stable year round, commonly cited as at least 8 to 10 ft of water. Confirm the minimum pond depth and the loop length for your climate, because a shallow pond that freezes or overheats defeats the point.

How deep are geothermal wells and how long is the loop?

Loop length is sized per ton of load and it depends almost entirely on how well the ground moves heat. A common planning range for vertical bore is on the order of 150 to 300 ft of bore per ton, but that span is wide for a reason: saturated clay can move heat far better than dry sand, so the same load needs a shorter loop in wet ground and a longer one in dry. Climate shifts it too, with northern heating-dominated jobs and southern cooling-dominated jobs landing at different ends. Treat any per-ton figure as a starting point and let the soil and the design software set the final length.

The variable underneath all of it is formation thermal conductivity, how fast the rock and soil around the bore conduct heat away from the pipe. Undersize the loop and the loop temperature drifts: every winter the loop pulls heat out faster than the ground replaces it, the entering water temperature creeps colder year over year, and the unit's capacity and COP fall off until the antifreeze is the only thing keeping the loop from freezing. That drift is the classic failure of a guessed loop, and it does not show up until the second or third season.

On larger and commercial bore fields, the design is backed by a formation thermal conductivity test, also called a thermal response test (TRT). A test bore is drilled, heated fluid is circulated for a couple of days while the response is logged, and the measured conductivity feeds the loop-field model. On a big field that test pays for itself many times over, because guessing the conductivity wrong across dozens of bores is a six-figure mistake. On a single residential bore the test is often skipped in favor of conservative regional values, which is a defensible call as long as the values really are conservative.

The vertical bore field: U-bend, grout, and the header

Each vertical borehole carries a single loop of HDPE pipe with a U-bend fitting fused to the bottom, so fluid goes down one leg, around the bend, and back up the other. The U-bend is a factory-fused fitting, not a field joint, for a reason that runs through this whole guide: you cannot get a wrench back down a 300 ft hole to fix a leak. The pipe and its bend go in as one tested assembly.

Once the loop is in the hole, the bore is sealed from the bottom up with thermally enhanced grout, commonly a bentonite-based grout blended for higher conductivity than plain drilling mud. The grout does two jobs at once. It seals the bore so the loop cannot become a vertical channel that lets surface water or one aquifer contaminate another, which is what the well codes care about, and it puts a conductive material in tight contact between the pipe and the formation so heat actually crosses. A bore backfilled with a poor grout or with cuttings is a bore that drills heat resistance right where you need conductivity.

Multiple bores tie together at a header, often in a buried vault or pit, where the individual loops are manifolded and fused into the supply and return runs back to the building. Bore spacing matters because boreholes too close together thermally crowd each other and the field behaves like a smaller, hotter or colder mass than the count of bores suggests. Common residential spacing runs on the order of 15 to 20 ft between bores. Confirm the spacing, grout product, and conductivity against the loop design and the IGSHPA and local well requirements.

The horizontal loop and the slinky

A horizontal loop trades depth for land. Instead of deep bores, you dig trenches commonly about 6 to 10 ft down and lay HDPE pipe in them, either as straight runs or as overlapped coils, the layout the trade calls a slinky. The slinky packs more pipe into a shorter trench by overlapping the loops, which cuts excavation length at the cost of more pipe and a little thermal crowding within the coil.

The depth is a balance. Go too shallow and the pipe sits in soil that swings with the seasons and can approach freezing in a hard winter, which costs capacity exactly when you need it. Go deeper and you pay for excavation and hit the water table or rock. The sweet spot keeps the pipe below the frost line and in soil that holds moisture, because wet soil conducts heat and dry soil insulates.

The honest limit of horizontal is land. A horizontal field for a whole-house load wants a sizable yard, and on heating-dominated sites the trench pipe footage per ton climbs. Where the land exists and the budget is tight, horizontal is the cheapest loop to put in. Where it does not, you are drilling vertical whether you wanted to or not.

HDPE pipe and heat fusion

The buried loop is high-density polyethylene, and every joint in the ground is heat-fused, not mechanical. Fusion melts the pipe ends or the pipe and a fitting together and lets them cool into one continuous piece of plastic, so the joint is as strong as the pipe and has no gasket, no clamp, and nothing to back out or corrode. IGSHPA-style practice approves HDPE and PEXa for closed loops specifically because they can be joined this way.

The rule that follows is blunt: no mechanical joint goes in the ground. A compression fitting or a barbed clamp buried under a yard or down a bore is a leak waiting to happen, and when it lets go you have lost the loop fluid, the antifreeze, and the heat transfer, with no practical way to reach the joint. Reputable geo programs flatly prohibit buried mechanical fittings for this reason. Every connection that gets covered with dirt or grout is a fusion weld made by a trained operator.

After the loop is fused and before it is buried or grouted in, it gets pressure tested and held, so a bad weld shows up while you can still dig it out. A loop that fails its pressure hold gets found and fixed at the trench, not after the landscaping is back and the unit is running. Skip or rush that test and you own the excavation when the loop loses pressure two winters later.

Why do you purge a geothermal loop?

You purge to drive every bubble of air and every bit of debris out of the loop before the unit runs, because air in a closed loop wrecks both flow and heat transfer. An air pocket parked at a high point throttles the flow through that circuit, an air-bound pump cavitates and loses prime, and a loop full of micro-bubbles carries heat poorly because gas is a terrible conductor. A geo system that was never properly purged runs low on capacity and nobody can find the refrigerant problem, because there is no refrigerant problem.

The fix is high-velocity flushing with a purge cart, a pump and tank rig built to push fluid through the loop fast enough to sweep the air out ahead of it rather than letting bubbles cling to the pipe wall. The practical target is a flushing velocity of at least 2 ft per second through every circuit, which works out to roughly 4 gpm for 3/4 in pipe, about 10 gpm for 1-1/4 in pipe, and around 20 gpm for 2 in pipe. The cart runs through a filter on the return so the debris it sweeps up gets caught instead of recirculated, and you flush each circuit until the return runs clear and air-free.

Purge before you add antifreeze and before you trust any flow reading. A loop that looks like it is flowing can be partly air-bound, and the delta-T across the unit will lie to you until the air is gone. This is the step the schedule pressures crews to shortcut, and it is the one that creates the most callbacks. Flush it right the first time and the rest of the commissioning goes clean.

Antifreeze and freeze protection

A closed loop runs colder than the freezing point of plain water on a heating-dominated winter day, so the loop fluid is a water-and-antifreeze mix sized to stay liquid well below the lowest expected entering water temperature. Get the concentration wrong on the low side and the loop slushes or freezes in the heat exchanger, which can split the heat exchanger and kill the unit. The freeze point is set below the design minimum loop temperature with margin, not right at it.

The common antifreezes are methanol, ethanol, and propylene glycol, and each is a tradeoff. Methanol and ethanol carry heat well and stay thin when cold, but both are flammable above roughly 25 percent concentration and methanol is toxic, so handling and local rules matter. Propylene glycol is nontoxic and non-corrosive, which is why some jurisdictions and projects require it, but it carries heat the worst and turns thick and hard to pump when it is cold, which can cost you flow at exactly the wrong time. The choice is part performance, part toxicity rules, part what the AHJ allows.

Match the antifreeze and its concentration to the lowest loop temperature the design predicts and to whatever the local code and the manufacturer permit. Record the fluid type and the measured freeze point after you charge the loop, because the next technician needs to know what is in there before they top it off or work on the heat exchanger.

Flow, the flow center, and delta-T

Flow is what the heat pump actually feels from the loop, and like any water-side balancing job it is set in gallons per minute, not in pump pressure. A closed-loop geo unit commonly wants on the order of 3 gpm per ton of capacity, which lands the water-side temperature difference across the unit near 10°F at design. Open-loop systems can run leaner, on the order of 1.5 gpm per ton, because raw ground water carries more heat per gallon than an antifreeze mix. Treat these as starting points and set flow to the manufacturer's table for the model.

The flow center is the pump package that moves the loop fluid, usually one or two circulators in a sealed module, sometimes a pressurized flow center and sometimes a non-pressurized tank type that doubles as the fill point. It has to deliver the design gpm against the loop's pressure loss, which on a long bore field is real. Size the circulator to the loop, not just the unit, because a long loop with an undersized pump never makes flow and the unit starves no matter how clean the purge was.

Delta-T is your check that flow and load agree, the same logic that drives water-side balancing on any hydronic system: too little flow and the delta-T opens up wide, too much flow and it collapses toward nothing while the pump burns energy for no gain. Measure the entering and leaving water temperatures at the unit under real load and compare the delta-T to the manufacturer's design spread. If it is off, fix the flow before you ever touch the refrigerant. A flow problem read as a charge problem sends a technician chasing a fault that is not there.

The heat pump unit and the desuperheater

The unit is a refrigerant heat pump with a water-to-refrigerant heat exchanger on the loop side. Water-to-air units send the building heat into a duct system through a refrigerant-to-air coil, the way a conventional heat pump does. Water-to-water units make hot or chilled water instead, for radiant floors, fan coils, or a buffer tank. A reversing valve flips the refrigerant flow so the same machine heats in winter and cools in summer by swapping which heat exchanger is the evaporator and which is the condenser.

Inside, it is still a refrigerant circuit, and it is set and judged the same way as any other. The charge is proven on superheat and subcooling against the metering device and the conditions, exactly as a split heat pump is, with the loop water standing in for the outdoor air. Charge it in cooling and verify it in heating where the manufacturer directs, and read the subcooling and superheat the same way you would on any other system rather than trusting the factory weigh-in alone.

Many geo units include a desuperheater, a small auxiliary heat exchanger that grabs heat off the hot refrigerant gas leaving the compressor and dumps it into the domestic hot water tank. When the unit is running for space heating or cooling anyway, the desuperheater makes hot water nearly for free as a byproduct. It is plumbed to the water heater with its own small pump, and it is one of the quiet efficiency wins that gets skipped or piped backward on a rushed install, so confirm it actually moves heat into the tank and not the other way.

Open-loop specifics: water quality and reinjection

Open loop lives or dies on water quality, because that ground water runs straight through the unit's heat exchanger. Hard water scales the heat exchanger and chokes its heat transfer over a season. Iron and certain bacteria foul it and clog the discharge side. Aggressive, low-pH water can corrode and, in the worst case, eat the heat exchanger out of the machine. Before you commit to open loop, you test the water for hardness, iron, pH, and the rest, and you size or select the heat exchanger to survive what the well actually delivers.

Well capacity is the next gate. The well has to deliver the design flow continuously while the unit runs, not just for a quick draw, so the production well and pump are sized to the heat pump load on top of any domestic demand. Undersize the well and the unit short-cycles or loses capacity whenever the water table drops.

Then there is the water you used. Open loop is pump-and-dump or pump-and-reinject, and the discharge almost always needs a permit. Many jurisdictions require the water be returned to the same aquifer through a reinjection well rather than wasted to the surface, and reinjection has its own failure mode: iron that oxidizes when the water hits air will plug a reinjection well over time, so the discharge is often kept sealed from air all the way back down. The well, water, and discharge rules vary by state, county, and city, and the AHJ and the water authority control what is allowed. Sort the permit before the design, not after.

What entering water temperature should the loop hold?

Entering water temperature (EWT) is the temperature of the loop fluid arriving at the unit, and it is the single number that tells you whether the loop is sized and running right. A practical closed-loop EWT lands somewhere in the 30s to low 70s°F across the year for most U.S. sites: coldest in late winter when months of heating have pulled the ground down, warmest in late summer when months of cooling have pushed it up. The unit is rated at specific EWT points, so the EWT you measure is what sets the real capacity and COP, not the nameplate.

The two seasons pull opposite directions. In heating the loop gives up heat and the EWT drifts down, and the colder it gets the harder the compressor works and the lower the COP. In cooling the loop absorbs heat and the EWT drifts up, which costs cooling efficiency. A well-sized loop keeps both extremes inside the unit's rated window with margin; a loop that is too short lets the heating EWT sag year over year, which is the loop-drift failure showing up as a number you can read.

Flow has to climb as the EWT falls to keep the leaving water above freezing. Manufacturers commonly call for more gpm per ton at low EWT for exactly this reason, with minimum loop temperatures and minimum flows spelled out in the unit data. When the loop runs cold, confirm both the EWT and the flow against the manufacturer's low-temperature limits, because that is where a marginal loop freezes a heat exchanger.

Commissioning the system

Commissioning a geo system is a sequence, and skipping a step hides the failure instead of finding it. Start at the loop: confirm it was pressure tested and held, confirm it was flushed to clear, air-free return, and confirm the antifreeze type and freeze point are recorded and right for the climate. A unit started on an air-bound or under-protected loop will read wrong on everything downstream.

Then prove the water side. Set and measure the flow in gpm against the manufacturer's spec, and read the entering and leaving water temperatures under real load to confirm the delta-T matches design. With flow and delta-T good, turn to the refrigerant and read superheat and subcooling against the unit's targets and the measured EWT, the same way the refrigerant charging side of this work is verified. Run the unit in both heating and cooling and confirm capacity and the reversing-valve changeover in each.

Finish with the extras and the controls. Confirm the desuperheater is moving heat into the domestic tank and not robbing it, confirm any auxiliary or backup heat stages and the lockout that keeps them from running when they should not, and walk the thermostat and loop-pump control through their staging. Then write all of it down. A geo unit that was started but never documented is one no one can troubleshoot later, because there is no baseline to compare a future reading against.

Distribution: air handler or radiant

How the geo unit delivers its heat is its own job, and it follows the rules of whichever side it feeds. A water-to-air unit pushes air through ductwork, so the duct system has to be sized, sealed, and balanced like any forced-air system, and the airflow has to be set before you judge the unit, because low airflow over the coil reads as a charge or capacity fault that is really a duct problem.

A water-to-water unit makes hot or chilled water for radiant floors, panel radiators, or fan coils, and now the delivery is a hydronic system with all that implies: the loops have to be balanced to flow, the air has to be purged from the distribution side too, and the delta-T across each emitter has to be set, the same water-side balancing discipline covered in the hydronic guide. Geo pairs especially well with radiant because radiant runs at a low water temperature, and a heat pump is most efficient when it does not have to make very hot water.

The mismatch to watch is supply water temperature. A geo unit makes warm water comfortably but struggles to make the high water temperatures that old cast-iron baseboard was designed around. If you are putting geo on a building with high-temperature emitters, either the emitters get upsized to run cooler or the unit fights an uphill lift all winter. Match the distribution to what the heat pump likes to make.

Electrical, backup heat, and controls

The electrical side is a dedicated circuit sized to the unit and the loop pump, plus whatever backup heat the design includes. Most geo systems carry auxiliary electric resistance heat, often as strip heat in the air handler, for the coldest stretches and for emergency heat if the compressor is down. That backup is a real load, so the service and the circuit have to carry the compressor and the strips, and the controls have to lock the expensive strip heat out until the heat pump genuinely cannot keep up. Strip heat that runs whenever the thermostat calls is a power bill nobody understands.

A soft start or a similar inrush-limiting device is common on geo compressors, both to ease the starting current on the service and to keep the lights from flickering on every cycle. On a generator-backed or weak-service site, the soft start can be the difference between a unit that starts and one that drops the breaker.

Controls tie it together. The thermostat stages the compressor, the backup heat, and on multi-speed units the capacity, and it should bring on auxiliary heat only on a real deficit, not on a normal recovery from setback. The loop pump control runs the flow center, sometimes continuously and sometimes only on a call, and on variable-speed flow centers it modulates to hold flow as the load changes. Confirm the staging logic and the aux-heat lockout at commissioning, because a miswired stage shows up as a comfort complaint and a utility bill long before anyone suspects the controls.

Efficiency, COP, and the tax credit

Geo efficiency is rated two ways: COP for heating and EER for cooling, both measured at standard entering water temperatures under the water-source rating standards. Higher is better on both, and geo units post numbers an air-source unit cannot match in cold weather because the loop never gets as cold as the winter air. The rated figures assume the unit is set up right, so a real install only earns its rated COP if the flow, the EWT, and the charge are all where they should be.

The incentive picture changed sharply in 2025. The federal 30 percent residential credit under Section 25D, long cited as running through 2032, was terminated early by the 2025 reconciliation law (the One Big Beautiful Bill Act): it no longer applies to expenditures for installs completed after December 31, 2025, though unused credit from a pre-2026 install can still carry forward. Commercial and some larger systems may still reach the separate Section 48E investment credit on its own phase-out schedule, and many states and utilities keep their own rebates. Incentives change with legislation and with the equipment list, so confirm the current federal and state credits, the qualifying efficiency, and the documentation the IRS wants for the install year before you promise a number to a customer.

Beyond the federal credit, many states, utilities, and local programs add rebates, and commercial projects fall under a different set of rules. The credit is real money on a job whose up-front cost is the main objection, so it is worth getting right, but it is a tax and policy question, not an HVAC one. Send the customer to a tax professional for the claim and keep the install paperwork clean enough to support it.

Maintenance the owner inherits

A geo system is low-maintenance, not no-maintenance, and the owner inherits a short list that keeps it running at its rated efficiency. The loop fluid and pressure get checked: a closed loop should hold its pressure for years, and a loop that needs topping off is a loop with a leak somewhere, which is a problem to find, not a habit to feed. The antifreeze concentration is verified on the same visit so the freeze protection has not been diluted by a previous top-off.

Flow and filtration are the rest of it. The loop-side flow gets confirmed against the original commissioning numbers, any loop strainer or filter gets cleaned, and on the air side the filter is the one thing the owner has to change on their own schedule, because a clogged air filter starves the coil and reads as a sick unit. On open loop, the maintenance is heavier: the heat exchanger gets inspected for scale and fouling, and the well and reinjection side get watched for the clogging that water chemistry causes.

The desuperheater is the part that gets forgotten. Its small pump and connections want an occasional look so it keeps moving heat into the hot water tank. None of this is hard, but it has to be tied to the original commissioning record, because maintenance with no baseline is just guessing at whether anything has drifted.

Geo and large commercial loop fields

At commercial scale the same physics drives much larger loop fields, and the design rigor goes up with the stakes. A campus or a large building runs a bore field of dozens or hundreds of vertical bores tied to a central loop, often serving distributed water-source heat pumps on each floor or a central plant. These are the jobs where the formation thermal conductivity test is not optional, because guessing the conductivity across a field that large is a budget-breaking error in either direction.

Large fields also have to balance heating and cooling over the year so the ground stays in equilibrium. A building that rejects far more heat to the ground in summer than it pulls out in winter will slowly heat its own bore field until performance falls off, and the reverse happens on a heating-dominated load. Designers manage this with field sizing, with hybrid setups that add a cooling tower or boiler to shave the imbalanced extreme, and with monitoring that watches the loop temperature trend over seasons rather than days.

Data centers are a growing case, because they reject heat year round and that steady, cooling-dominated load matches well to a ground or water loop with the right balancing strategy. The thermal guidelines and the loop design that suit a data center are their own specialty, but the field fundamentals are the ones in this guide: size the loop to the real conductivity, fuse every joint, purge the air, and prove the flow and the temperatures at startup.

What to document

A geo system that was commissioned but not documented is one the next technician cannot troubleshoot, because there is no baseline to compare against. The record is what tells you, two winters out, whether the loop is drifting or the charge has moved or nothing has changed at all.

Capture the loop type and total length, the antifreeze type and measured freeze point, the loop pressure-test result, the set flow in gpm and gpm per ton, the entering and leaving water temperatures with the delta-T, the subcooling and superheat in both modes, the COP or capacity check, the desuperheater confirmation, and the control staging and aux-heat lockout settings. Write down which were design targets and which were measured, so a reviewer can tell what the system was supposed to do from what it actually did.

Common mistakes

• Sizing the loop by a guessed per-ton number instead of the soil conductivity, so the EWT drifts colder every winter.
• Starting the unit on a loop that was never purged to an air-free return, then chasing a phantom refrigerant fault.
• Burying a mechanical fitting in the loop instead of a heat-fused joint, which becomes an unreachable leak.
• Setting the wrong flow or reading the delta-T wrong, then adjusting the charge to fix a water-side problem.
• Missing or under-concentrating the antifreeze, so the loop freezes and splits the heat exchanger.
• Putting open loop on iron-heavy or aggressive water with no plan for scaling, fouling, or the discharge permit.
• Letting auxiliary strip heat run on every call because the lockout was never set at commissioning.
• Piping or wiring the desuperheater backward so it cools the hot water tank instead of heating it.

Field checklist

Standards and references

The ground-loop side leans on IGSHPA, the International Ground Source Heat Pump Association, for loop design, HDPE pipe fusion and jointing, grouting, and flushing and antifreeze practice. The CSA/ANSI/IGSHPA C448 series covers ground-source loop design and installation across these areas and is a common reference for the loop trade. Use it for the loop, the fusion, and the grout, and confirm the edition and any local adoption.

The heat pump itself is rated under the water-source heat pump standards: ISO/AHRI/ASHRAE 13256-1 for water-to-air and brine-to-air units and 13256-2 for water-to-water and brine-to-water units, with units listed through AHRI certification. ENERGY STAR criteria set the efficiency thresholds that historically tied into the federal tax credit and still gate many state and utility rebates. The manufacturer's installation and operation manual governs the flow, the charge, the low-temperature limits, and the antifreeze the unit will tolerate, and it wins over any rule of thumb in this guide.

The well, the water, and the discharge fall under the local AHJ and the water authority, and those rules vary by state, county, and city. Open-loop reinjection or discharge almost always needs a permit, and the antifreeze a closed loop may use can be restricted locally. Confirm the permit, the allowed fluids, and the well requirements with the jurisdiction, and let the project specification override any common figure when it is stricter. The standard numbers and their editions shift over time, so verify them against the documents in force on the job before citing them.

Units, terms, and conversions

Geothermal work mixes the loop trade's terms with the refrigerant trade's, so the same system can read differently across a loop design, a unit data sheet, and a controls drawing.

Capacity is in tons, where one ton is 12,000 Btu per hour. Loop flow is in gallons per minute (gpm), often quoted per ton of capacity. Temperatures are entering and leaving water temperature (EWT and LWT) in °F, with their difference the delta-T. Efficiency is COP for heating and EER for cooling. Loop length and bore depth are in feet, and pipe is HDPE sized by nominal diameter. The same loop fluid may be called the brine, the loop fluid, or the antifreeze mix depending on who is writing.

EWT / LWT

Entering and leaving water temperature at the heat pump, in °F; their difference is the water-side delta-T

Closed loop

A sealed, buried HDPE loop of water and antifreeze that exchanges heat through the pipe wall

Open loop

A pump-and-discharge system that runs actual ground water through the unit and reinjects or discharges it

Heat fusion

Melting HDPE pipe and fittings into one continuous joint, the only joint allowed in the ground

Flow center

The circulator pump package that moves loop fluid at the design gpm against the loop pressure loss

Desuperheater

An auxiliary heat exchanger that captures compressor discharge heat to warm domestic hot water

COP / EER

Coefficient of performance for heating and energy efficiency ratio for cooling, both measured at rated EWT

TRT

Thermal response test, an in-situ measurement of formation thermal conductivity for sizing large loop fields

Related tools

Calculators and readiness checks for this work

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