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Closed-Loop vs Open-Loop Geothermal Systems

Updated 21 September 20267 min readGeothermal & Heat Pumps

The distinction is simple: a closed loop circulates its own sealed fluid through buried pipe and never touches groundwater, while an open loop pumps groundwater through a heat exchanger and returns it to the ground. Open loops transfer heat better because nothing stands between the water and the exchanger. That same directness is what brings their complications.

Key takeaways

  • Closed loops are sealed circuits of plastic pipe; open loops move actual groundwater through the building's heat exchanger.
  • Heat has to cross pipe wall, grout and soil to reach a closed loop. An open loop removes those layers, which is its whole advantage.
  • An open loop's viability is a question about the aquifer — yield, chemistry and somewhere to put the water afterwards — not about the heat pump.
  • Water chemistry is the recurring open-loop failure mode: scale, iron and particulates foul exchangers that a closed loop never exposes.
  • Both must respect the ground's heat balance. Returning water too close to where it was drawn creates thermal breakthrough, the open-loop version of an undersized closed loop.
On this page
  1. What each one circulates
  2. The heat-transfer advantage, and where it comes from
  3. What the water brings with it
  4. Where the water goes afterwards
  5. Closed-loop variants, and the hybrid in between
  6. Choosing: the site decides

What each one circulates

A closed loop is a sealed circuit. Water with antifreeze circulates endlessly through buried plastic pipe, collecting heat through the pipe wall and carrying it to the heat pump. The same fluid returns to the ground and goes round again. Nothing enters and nothing leaves.

An open loop pumps groundwater from a well, passes it through a heat exchanger where the heat pump takes what it needs, and then returns the cooled water to the ground through a second well or another approved discharge. The water makes a single pass.

Closed loop and open loop side by sideTwo ground-coupled systems in section. On the left a closed loop: sealed plastic pipe buried in the ground carries an antifreeze solution in a continuous circuit down into the ground, back to the heat pump indoors, and out again, with nothing entering or leaving the fluid. On the right an open loop: a submersible pump lifts groundwater from a supply well, passes it through a plate heat exchanger that feeds the heat pump, and returns the cooled water to the aquifer through a separate injection well some distance away.Closed loop — a sealed fluid circuitsoil / rockHeat pumpindoorsantifreeze solution,sealed for decadesgrout fills the borearound the pipeNothing enters or leaves the circuit. The ground isreached only by conduction through the pipe.Open loop — the aquifer's water is the fluidsoil / rockaquiferHeat pumpindoorsplate heatexchangersubmersible pumpsupply wellinjection wellseparation mattersGroundwater passes through the building and isreturned. The site has to permit that.
The same heat pump in both cases. What differs is whether the ground's own water passes through the building, and what has to be true of the site for that to work.

The heat pump itself does not care. Both arrangements deliver a fluid at some temperature to the evaporator, and the vapour-compression cycle proceeds identically. What changes is how efficiently heat gets to that fluid, and what else comes with it.

The heat-transfer advantage, and where it comes from

Follow a joule of heat from the soil into a closed loop and count the barriers it crosses: through the soil to the borehole wall, through the grout that fills the borehole, through the wall of the plastic pipe, and finally into the fluid. Each layer has a thermal resistance, and they add up. Plastic pipe is a poor conductor — a necessary compromise, since it also has to be durable and cheap to bury.

An open loop deletes that stack. The water *is* the heat carrier, arriving at the exchanger already at the aquifer's temperature.

How many barriers the heat has to crossTwo heat paths compared as a chain of thermal resistances with the temperature dropping at each step. The closed loop path crosses undisturbed soil, the grout around the borehole, the pipe wall and finally the circulating antifreeze, arriving at the evaporator several degrees below the ground temperature. The open loop path brings groundwater directly to a plate heat exchanger, so only the exchanger plate stands between the aquifer and the refrigerant and the total temperature drop is much smaller.Closed loop: four barriers between the ground and the refrigerantundisturbed soilgroutpipe wallantifreeze filmevaporatora few degrees lostat every stepground temperatureOpen loop: one barrier, because the ground's water comes to yougroundwater, pumpedexchanger plateevaporatora smaller total dropaquifer temperatureSource temperature at the evaporator is what sets the lift the compressor works across, and lift is whatefficiency is made of. Every barrier removed is lift the compressor never has to pay for.
Each barrier costs a few degrees of source temperature, and source temperature is what sets the lift the compressor has to work across.

The effect shows up as a better source temperature at the evaporator, which is a smaller temperature lift, which is directly a better coefficient of performance. In a cold climate that margin is worth having — the divergence described in ground-source versus air-source in extreme cold is precisely about how valuable a stable, warm-ish source is on the coldest day.

What the water brings with it

Groundwater is not clean water. It is a solution that has been in contact with rock for a long time, and everything it dissolved comes with it into the building.

  • Hardness and scale. Dissolved calcium and magnesium precipitate on heat exchanger surfaces as conditions change, and scale is an insulator, so a fouled exchanger quietly gives back the efficiency the open loop was chosen for.
  • Iron and manganese. Common in groundwater and prone to oxidising into deposits once exposed to any oxygen, often with the help of iron bacteria, which produce a sludge that blocks fine passages.
  • Particulates. Sand and silt abrade pump impellers and exchanger plates and settle where flow slows.
  • Corrosion. Chloride content, dissolved gases and pH decide which exchanger materials survive. This is a materials selection problem, not a maintenance one.
What deposits do to a heat exchanger plateA plate heat exchanger channel shown in three states. Clean, with groundwater on one side and the clean circuit on the other, transferring heat across a small temperature difference. Scaled, with a mineral layer on the water side adding thermal resistance so a larger temperature difference is needed to move the same heat. Fouled, with iron deposits and sediment narrowing the channel, restricting flow and reducing heat transfer substantially.Cleanwell waterclean circuitheat crosses easilyScaledmineral layerclean circuitsame heat now needsa bigger differenceFoulediron and sedimentclean circuitflow narrowed as wellWhy it is easy to missScale is an insulating layer growing exactly where the design needed a bare surface. Nothing breaks; thesystem simply works a little harder each month, so it looks like a slow decline rather than a fault.This whole failure mode is absent from a closed loop, whose fluid is whatever was put in at commissioning.
Scale is an insulating layer in exactly the place the design was trying to keep clear. It shows up first as a slow decline in performance, not as a fault.

The standard defence is to keep the groundwater out of anything delicate: a robust plate exchanger between the well water and a clean secondary circuit, filtration sized for the particulates present, and materials chosen from a water analysis taken before anything is ordered. A closed loop needs none of this, because its fluid is whatever was put in at commissioning.

Where the water goes afterwards

An open loop moves substantial volumes of water. Where it goes is a bigger question than where it came from.

Reinjection into the same aquifer through a second well is the usual answer: the water is returned, a few degrees colder in winter, having given up its heat. That requires the aquifer to accept it as readily as it gave it up, which is not guaranteed — injection wells foul more readily than supply wells.

Thermal breakthrough between injection and supply wellsPlan view of an aquifer with groundwater flowing from left to right. In the upper case the injection well sits well downstream of the supply well, so the plume of cooled returned water drifts away and never reaches the intake. In the lower case the wells are too close together or the flow direction was misjudged, so the cooled plume reaches the supply well and the source temperature the system draws on falls further each season.Injection well downstream and far enough awaygroundwater flowsupplyinjectioncooled plume drifts awayseparationWells too close, or the flow direction misjudgedgroundwater flowinjection, upstreamsupplythe system drinks its own cold returnBreakthrough is the open-loop version of an undersized ground loop: the source cools season by season whileevery component still tests as healthy, which is what makes it hard to diagnose.
Thermal breakthrough is the open-loop equivalent of an undersized closed loop: the system slowly cools the very resource it depends on.

Separation matters, and so does direction. Put the injection well upstream of the supply well and the cooled plume drifts straight back, so the system draws progressively colder water and its performance declines season after season. Diagnosing that is harder than it sounds, because every component tests as healthy.

Abstracting and returning groundwater is a regulated activity in most places, and the rules differ by jurisdiction. The requirements come from the authority responsible for the aquifer, and they are best established before the design is fixed rather than after.

Closed-loop variants, and the hybrid in between

Closed loops trade thermal performance for independence from water, and their variants trade land for depth.

Choosing between the arrangements
Closed loopOpen loopStanding column well
What circulatesSealed antifreeze solutionGroundwater, once throughGroundwater within one deep well
Heat transferThrough pipe wall and groutDirectDirect, through the rock column
Site requirementLand area or drilling accessA productive aquifer and somewhere to return waterSuitable hard-rock geology and depth
Water chemistryIrrelevantDecisiveDecisive
MaintenanceCirculation pump and antifreezeWells, filtration, exchanger cleaningWells and exchanger, with bleed control
Slow failure modeGround cools year on year if undersizedThermal breakthrough, or foulingColumn cools if bleed is insufficient
Regulatory burdenLighter, rarely nilAbstraction and discharge approvalAs open loop

Comparison of design characteristics. What decides a specific site is a well yield test and a water analysis for open loops, or a ground thermal assessment for closed loops — neither of which is a property of the heat pump.

The standing column well sits between the two families. Water is drawn from the bottom of a deep borehole and returned to the top of the same one, so it circulates through the rock rather than across the site. It keeps much of the open loop's direct heat transfer without needing two wells or a large water footprint, and in hot or cold extremes a small "bleed" — discharging a fraction of the flow and drawing fresh groundwater to replace it — pulls the column temperature back toward the aquifer's.

A standing column well, and what the bleed doesSection through a single deep borehole in hard rock. A submersible pump draws water from the bottom of the column, sends it to the heat pump's heat exchanger, and returns it to the top of the same column, so the water circulates down through the bore while exchanging heat with the surrounding rock. A small bleed line discharges a fraction of the returning flow, which draws fresh groundwater in through the rock and pulls the column temperature back toward the aquifer's own temperature when a heating or cooling season runs long.fractured hard rockone boreholesubmersible pump at depthheat exchangerto the heat pumpreturned to thetop of the same columnwater moves betweencolumn and fracturesBleeddischarging a small fractionpulls fresh groundwater in,moderating the columnOne bore does the work of a supply and an injection well, which is why standing columns appear where rock iscompetent and land is tight. The bleed is the control: it is used at the extremes, not continuously.
One borehole doing the work of two wells. The bleed is the control: discharging a small fraction pulls the column back toward the aquifer's own temperature when a season runs long.

Choosing: the site decides

The honest summary is that this is not a choice between two products. It is a question about the ground.

Ask about the water first. Is there an aquifer that will sustain the required flow through a whole heating season? What is in the water? Is there somewhere lawful and practical to return it? If any answer is unsatisfactory, the comparison is over and a closed loop is the design.

If the water answers well, an open loop offers a better source temperature for less drilling, at the price of water handling, filtration and a maintenance regime that must actually be followed.

If in doubt, closed. A sealed loop has fewer ways to disappoint slowly, and slow disappointment is the characteristic failure of ground systems — not sudden breakdown but a quiet decline that every component passes a test for. The same is true of the air-side diagnostics in why a heat pump freezes up: the machine is rarely the thing that is wrong.

Frequently asked questions

Which is more efficient?

An open loop usually delivers a better source temperature to the heat pump, because heat reaches the exchanger without crossing pipe wall, grout and soil. That narrower temperature lift shows up directly as a better coefficient of performance. The advantage only exists where a suitable aquifer does.

Can an open loop run out of water?

Yes, and it is a real design risk. The supply well must sustain the required flow continuously through the heating season, not just on the day it was tested. Seasonal drawdown, nearby abstraction and drought all affect it, which is why the well test matters more than the equipment selection.

What does 'thermal breakthrough' mean?

It is when the water returned to the ground finds its way back to the supply well. In winter that means drawing progressively colder water, and performance drifts down. Separation and an understanding of which way the groundwater moves are what prevent it.

Do open-loop systems need approval?

Almost everywhere, yes. Abstracting groundwater and returning it are regulated activities in most jurisdictions, and the requirements differ by place, so it is a question for the local authority rather than for a heat pump manufacturer. Closed loops face lighter requirements but are rarely unregulated either.

Is a standing column well closed or open?

Open, with a twist: water is drawn from the bottom of a deep borehole and returned to the top of the same one, so it circulates through the rock column. It combines an open loop's direct heat transfer with a much smaller water footprint, and it depends on suitable hard-rock geology.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

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Research, drafting and review

Articles are drafted from primary engineering and physics references with AI-assisted tools, then reviewed and fact-checked line by line by a human editor before publication. We publish explanations, not recommendations: no products, no pricing, no country-specific rules, and no invented author personas.

Last reviewed 21 September 2026. How we research and review