Water-Source Heat Pump: What It Is, and Why a Geothermal Heat Pump Is One

If you have been reading about geothermal and then run into the phrase "water-source heat pump" on a spec sheet, in a building's equipment schedule, or in a manufacturer's catalog, the obvious question is whether it is the same thing, a related thing, or something else entirely.

A water-source heat pump is the broader category, and the geothermal heat pump nearly everyone means — the kind that circulates water or antifreeze through the ground — is one member of it. Direct geoexchange is the one exception, and it is worth knowing why. An air-source heat pump trades heat with the outdoor air. A water-source heat pump trades heat with water running through a pipe. What is on the far end of that pipe, a ground loop, a well, or a boiler and cooling tower in an office building, is a separate question, and it is the question that decides whether a given installation counts as "geothermal."

What a water-source heat pump is

The plainest definition available comes from AAON, a commercial HVAC manufacturer that builds these units: "Water-source heat pumps operate the same way as an air-source heat pump, with the difference being that piped water is used to heat or remove heat instead of the outside air."

A heat pump does not make heat the way a furnace does. It moves heat from one place to another using a refrigeration circuit, the same loop of compressor, refrigerant, and heat exchangers that runs your refrigerator. In heating mode it pulls heat from somewhere and delivers it to the building. In cooling mode it reverses and pushes heat from the building out to that same somewhere.

For an air-source heat pump, the "somewhere" is the outdoor air, and the outdoor unit with the big fan is the coil that trades heat with it. For a water-source heat pump, the "somewhere" is water flowing through a pipe. The refrigeration circuit is doing the same job. Only the fluid on the far side of the heat exchanger has changed.

You will see the abbreviation WSHP everywhere in the commercial trade, and also "water loop heat pump," which is the same equipment named after the loop it hangs on rather than after the source. And because a WSHP does not need outdoor air, it does not need to sit outdoors at all. That single fact drives most of what makes this category different in practice.

The family tree: air-source, water-source, ground-source

Most people meet "geothermal heat pump" and "ground-source heat pump" first, then meet "water-source heat pump" later and assume it is a third, separate category. It is not. For the water-circulating equipment that makes up almost all of the market, ground-source sits inside water-source.

The hierarchy has two levels. At the top, the split is air-source versus water-source. Under water-source, the split is what is on the other end of the pipe. Into the earth through boreholes or a trench: a ground-source heat pump, which is what most people mean by geothermal. To a well that draws groundwater: an open-loop geothermal system. To a boiler and a cooling tower in the building's mechanical room: a boiler/tower loop, which is not geothermal at all. The heat pump at the end of the pipe can be the same model in all three cases.

Two manufacturers say this directly. AAON describes the geothermal configuration as one where "ground bores can be used in lieu of the boiler and cooling tower": the ground stands in for the equipment that would otherwise condition the loop. Daikin Applied, describing its commercial SmartSource line, gives the clearest single sentence on the subject: "Commercial water source heat pumps can be applied to geothermal closed-circuit or open-well loops, or on a traditional boiler/tower loop system."

The overlap is not just a commercial-catalog convention. WaterFurnace, whose residential business is geothermal, calls its own residential units water source heat pumps in its 5 Series specification catalog. And AHRI, the Air-Conditioning, Heating, and Refrigeration Institute, runs one certification program for this equipment, with a name that pairs the two terms outright: "Geothermal – Water-Source Heat Pumps (WSHP)."

Two definitions make the nesting explicit, with one scope caveat worth stating up front. The federal definition at 10 CFR 431.92 is written for a particular slice of the market — "commercial package air-conditioning and heating equipment" whose main components include a "refrigerant-to-air heat exchanger" — so it covers commercial water-to-air equipment rather than every machine discussed here. Within that scope it captures the idea exactly: such a heat pump "uses a circulating water loop as the heat source for heating and as the heat sink for cooling." ENERGY STAR defines the narrower term: "A geothermal heat pump uses the thermal energy of the ground or groundwater to provide residential space conditioning and/or domestic water heating." A ground-source heat pump is simply a water-source heat pump whose circulating loop runs through the earth.

There is one exception, and it is where the category boundary actually falls. ENERGY STAR defines Direct Geoexchange, or DGX, as "A geothermal heat pump model in which the refrigerant is circulated in pipes buried in the ground or submerged in water that exchanges heat with the ground, rather than using a secondary heat transfer fluid, such as water or antifreeze solution in a separate loop." In a DGX system the refrigerant itself, rather than a water loop, goes into the ground. That makes it geothermal but not water-source, and it has its own test standard, AHRI 870, and its own ENERGY STAR line. So the test for any piece of geothermal equipment is simple. If water or antifreeze circulates in a separate loop between the unit and the ground, it is a water-source heat pump. If refrigerant goes into the ground, it is DGX.

The second axis: water-to-air vs water-to-water

A second distinction runs across the first, and it is the one most often confused with "water-source."

"Water-source" describes the side the heat pump exchanges heat with — drawing from the loop in heating, rejecting into it in cooling. "Water-to-air" and "water-to-water" describe the side it delivers heat to. A water-to-air unit delivers conditioned air, through ductwork or straight into the room it sits in. A water-to-water unit delivers hot or chilled water, which then feeds something else: a radiant floor, a fan coil, a hydronic system of some kind. Both of them are water-source heat pumps.

The international test standard that governs the equipment splits it the same way. AHRI lists two related standards for its WSHP program: "ISO 13256-1 (1998): Water-source heat pumps — Testing and rating for performance — Part 1: Water-to-air and brine-to-air heat pumps" and "ISO 13256-2 (1998): Water-source heat pumps — Testing and rating for performance — Part 2: Water-to-water and brine-to-water heat pumps." Same standard, same family name, two parts, split precisely along the delivery-side axis. (The "brine" in those titles refers to an antifreeze-and-water mixture, which is what circulates in many closed ground loops instead of plain water.)

Our guide to water-to-water geothermal heat pumps covers the delivery side in depth. One spec-sheet point matters for a homeowner comparing quotes: WaterFurnace's 5 Series catalog states that "Typically, water source heat pumps are limited to producing temperatures around 130°F," a ceiling on the output side that matters for anyone pairing a heat pump with a system designed around a much hotter boiler.

The boiler/tower loop

This is the part a homeowner has almost certainly never seen, and it explains why water-source heat pumps suit multi-zone commercial buildings so well.

Picture a mid-sized office building. Instead of one big central air handler serving the whole floor, there are many small heat pumps, each serving one zone: a corner suite, a conference room, a run of offices along the north wall. Every one of those units is a water-source heat pump. They all connect to a single loop of piped water that runs throughout the building.

Trane's application guide describes the architecture in two sentences: "All the heat pumps are connected to a common water loop (Figure 2). Also connected to this loop are a 'heat rejecter' (such as a cooling tower, fluid cooler, or ground heat exchanger), a 'heat adder' (such as a hot-water boiler or ground heat exchanger), and water-circulating pumps." On a boiler/tower system the heat rejecter is the cooling tower and the heat adder is the boiler. Everything else on the loop is a heat pump.

That loop needs to stay within a workable temperature band, and AAON gives the band plainly: "60 °F and 90 °F." Two pieces of equipment hold it there. At the bottom of the band is a boiler, which is simply a heater for the loop water. As the loop temperature falls toward 60 °F, the boiler fires and adds heat. At the top of the band is a cooling tower, a piece of rooftop equipment that rejects heat from the loop water to the outdoor air, usually by evaporating some of it. As the loop temperature climbs toward 90 °F, the cooling tower runs and pulls heat out. Trane's guide gives the same two numbers independently: the heat rejecter maintains "a leaving-water temperature of approximately 90ºF (32ºC)" in cooling, and the heat adder maintains "approximately 60ºF (16ºC)" in heating. Two manufacturers, same band.

That is the entire control logic of the loop, and it is why the arrangement is named "boiler/tower." The boiler guards the floor, the tower guards the ceiling, and in between the water is a shared medium that every heat pump in the building trades heat with.

Notice what that implies. In the middle of the band, neither the boiler nor the tower does anything. A heat pump in cooling mode is dumping heat into the loop and one in heating mode is pulling heat out, so when some zones are cooling while others are heating, the units are moving heat between each other through the water, and the boiler and tower only make up the imbalance left over.

That drift is not an accident of loose control; it is required. ASHRAE Standard 90.1, a building energy standard, calls for "a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition" — with an exception where "a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity," in which case "dead bands of less than 20°F shall be allowed." Absent that controller, the tower's start point and the boiler's start point sit at least 20 °F apart, and the loop is deliberately allowed to float across the gap between them, and while it floats, the only things adding and removing heat are the heat pumps themselves, so heat one zone rejects is available to a zone that needs it. AAON's 60 °F to 90 °F band is 30 °F wide, comfortably wider than the minimum. The trade literature attaches savings percentages to this effect; they depend on the particular building. What the standard guarantees is the mechanism.

Now connect it back to geothermal. In AAON's geothermal configuration, "ground bores can be used in lieu of the boiler and cooling tower." The loop stays. The heat pumps stay. What changes is that the earth, rather than a boiler and a tower, keeps the loop water in a usable range. That is the exact sense in which a commercial geothermal system is a water-source heat pump system with a different loop on the end; our overview of commercial geothermal systems shows what that looks like at building scale.

Inside the building: units, zones, and where they go

Because a water-source heat pump exchanges heat with piped water rather than outdoor air, it does not need an outdoor location. AAON puts it that using piped water "allows some heat pump configurations the option to be located inside the building." Daikin Applied says its SmartSource units "are available in a variety of sizes and configurations and can be placed in virtually any location within a building."

The units are small. Daikin lists its SmartSource small-capacity single-compressor line at "1/2 to 6 Tons," a ton being the standard unit of cooling capacity in the HVAC trade. Multiply units that size by the number of zones in a building and you have the distributed pattern that defines this category: many small, self-contained heat pumps rather than one central plant doing all the work.

The reason engineers like that pattern is control. Daikin puts it this way: "Each water source heat pump system responds only to the heating or cooling load of the individual zone it serves. This permits an excellent comfort level for occupants, better control of energy use for building owners, and lower seasonal operating costs." A corner office with afternoon sun can be in cooling while an interior room down the hall is in heating, and neither is fighting a building-wide setpoint.

Daikin's list of common applications, "Hospitality Retail Healthcare Education Office," is a list of buildings made of many rooms with different occupants, exposures, and schedules, where one thermostat per floor was never going to work.

Same machine, three water sources

Daikin's sentence gives three loop options for the same machine.

Geothermal closed-circuit loop. A sealed loop of pipe carrying water or a water-and-antifreeze brine runs through boreholes or trenches in the earth and back to the building. In AAON's framing, the ground bores stand "in lieu of the boiler and cooling tower." The heat pump itself is unchanged; ClimateMaster does note that water-side options differ by application, specifying that its copper heat exchanger "with cast iron pump [is] designed for closed loop systems." This is what most people mean by a ground-source or geothermal heat pump.

Open-well loop. Daikin names this as one of the three applications; in an open-loop arrangement the heat transfer fluid is, in ENERGY STAR's words, "part of a larger environment," most commonly ground water, rather than being sealed in a closed piping system. What is different is that the source-side water is real groundwater with whatever chemistry the aquifer has. Requirements for water quality and discharge vary by site and jurisdiction, and they are a design question in their own right. Our guide to open-loop geothermal systems covers the configuration.

Boiler/tower loop. The commercial arrangement above: a closed loop inside the building, held between 60 °F and 90 °F by a boiler at the bottom of the band and a cooling tower at the top, per AAON. There is no ground contact anywhere in it — a water-source heat pump system that is not geothermal.

Trane's own list makes the point. The heat rejecter is "a cooling tower, fluid cooler, or ground heat exchanger"; the heat adder is "a hot-water boiler or ground heat exchanger." A manufacturer's own application guide lists the ground as an interchangeable option in both slots. The geothermal loop is simply what you can put in the boiler's and the tower's place, and the heat pumps on the loop do not change.

When you read that a building has "water-source heat pumps," you have learned about the units. You have not yet learned about the loop, and the loop is where the geothermal question lives.

How they are certified and rated

AHRI's certification program is where the ratings come from. Its scope, in AHRI's words: "This program applies to 50 and 60 Hz Production Models of WSHPs, as defined in Section 1.2, rated below 39,500 W [135,000 Btu/h] in cooling and 58,600 W [200,000 Btu/h] in heating at ISO Standard Rating Conditions." That covers the small-to-mid-sized packaged units that make up the distributed pattern above, not large central chillers, and everything is rated under ISO 13256, Part 1 for water-to-air and Part 2 for water-to-water. The ratings AHRI certifies and verifies by test are cooling capacity and EER, heating capacity and COP, plus part-load equivalents "if applicable."

The one thing to understand about those ratings, for the purposes of this category, is that ISO 13256-1 does not use a single water temperature. It fixes a different entering-water temperature for each of the three applications, and the water-loop, ground-water, and ground-loop conditions are all different from one another. The full set of conditions, and the ENERGY STAR thresholds that sit on top of them, are covered in our guide to geothermal efficiency ratings. What belongs here is what those conditions do to one machine.

ClimateMaster's Tranquility TRT Series catalog rates a single model, the TRT036, in all three applications under ISO 13256-1:

Source: ClimateMaster LC1050, ISO 13256-1 full-load ratings.

One model, three sets of numbers. (The catalog offers selectable options within the model — standard or extended range, copper or cupronickel coax — so this is one model line rather than one configured part number.) On ground water the unit posts an EER of 22.1; on a water loop the identical unit posts 15.0. In heating, the boiler-warmed loop gives it a COP of 5.1 and 43,000 Btu/h of output, while the ground-loop condition drops it to 3.7 and 27,000 Btu/h.

A water-source heat pump is one machine that can be fed by three different loops, and its certified performance is a property of the machine and the loop together. Before comparing a number on a WSHP spec sheet to anything, find the application it was rated under.

One boundary note: ENERGY STAR maps its categories to test standards as "AHRI 870 for DGX systems. ISO 13256-1 for water-to-air models (open and closed loop systems). ISO 13256-2 water-to-water models (open and closed loop systems)." DGX stands apart under its own standard; every water-source type, open or closed, rates under ISO 13256. ENERGY STAR also states that "Commercial (i.e., 3-phase) units are not eligible for qualification under the ENERGY STAR specification at this time," so any three-phase unit sits outside that program — though voltage is a per-model choice, and the same catalogs list single-phase and three-phase versions side by side.

Frequently asked questions

Is a water-source heat pump the same as a geothermal heat pump?

Almost always, yes: a geothermal heat pump that circulates water or antifreeze is a water-source heat pump. AHRI's program is named "Geothermal – Water-Source Heat Pumps," and ENERGY STAR defines a geothermal heat pump as one that "uses the thermal energy of the ground or groundwater." The quick test for your own equipment: if water or antifreeze circulates in a separate loop between the unit and the ground, it is a water-source heat pump. If the refrigerant itself is piped into the ground, it is Direct Geoexchange (DGX), a separate category tested under AHRI 870. The relationship does not run the other way: units on a boiler/tower loop have no ground contact at all.

Is a water-source heat pump the same as a water-to-water heat pump?

No, and this is the most common mix-up. "Water-source" is about where the heat comes from (a water loop). "Water-to-water" is about what the unit delivers (hot or chilled water rather than air). A water-to-water unit is a water-source heat pump, but so is a water-to-air unit. The ISO 13256 standard splits the two delivery types into Part 1 (water-to-air) and Part 2 (water-to-water), both under the "water-source heat pumps" title.

What temperature does the loop run at?

On a boiler/tower system, AAON gives the operating range as 60 °F to 90 °F, with the boiler firing as the loop approaches 60 °F and the cooling tower running as it approaches 90 °F; Trane's application guide gives the same two figures. For rating purposes, ISO 13256-1 fixes a separate entering-water temperature for each application; the values are in our efficiency ratings guide. Actual ground-loop and open-well temperatures in service depend on location, ground conditions, and loop design.

Do water-source heat pumps go in houses?

Residential geothermal units are water-source heat pumps; WaterFurnace uses that term for its own 5 Series units. The boiler/tower loop is a commercial pattern: Daikin's applications for its commercial line are hospitality, retail, healthcare, education, and office.

How efficient is a water-source heat pump?

It depends on the model and, just as much, on the water it is fed. ClimateMaster's TRT036 is rated at EER 15.0 on a water loop, 22.1 on ground water, and 16.4 on a ground loop, all under ISO 13256-1, and it is the same machine in all three cases. Check the certified rating for the exact model and note the application it was rated under.