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Building Science

Open-Loop vs. Closed-Loop Ground Source Heat Pumps

Some basics on the two types of GSHP systems and cautionary advice against a side-by-side comparison

A ground source heat pump draws heat from or dumps heat into the ground, groundwater, or surface water. Source: U.S. Dept. of Energy Building America Solutions Center

I mainly talk about air source heat pumps here. But ground-source heat pumps are an important type of heat pump, too. So today, let’s look at the issue of open-loop vs. closed-loop design when installing this type of heat pump.

What is a ground source heat pump?

First, I use the term ground source heat pump, but they’re also called ground-coupled or geothermal heat pumps. They do exactly the same thing as air source heat pumps: They move heat between indoors and outdoors. The equipment is really just a heat exchanger with a lot of interesting physics that keeps your house cool in summer and warm in winter.

The only difference is that in a ground source heat pump, the outdoor exchange happens with the ground, groundwater, or surface water. The diagram below shows four different ways that outdoor heat exchange can occur.

The main advantage of ground source heat pumps is that it’s easier to draw heat from the ground in winter and dump it into the ground in summer. Why, you ask? A couple of reasons. First, the ground temperature doesn’t vary like the air temperature does. Second, the ground or water that you’re drawing heat from or dumping it into can have a higher heat capacity than air. (See the comment by RoyC below for a better understanding of that second issue. It’s the one that begins with “I have to disagree with Allison…”)

Open-loop vs. closed-loop

Anytime you’re in the position of choosing a heating or cooling system, you have to make decisions. One that comes up with ground source heat pumps is deciding between the open-loop vs. closed-loop types for the outdoor heat source or sink. The diagram above shows four closed-loop systems.

In that type, the fluid that exchanges heat with the ground is a mixture of antifreeze and water. As the name suggests, it’s contained in a closed system that circulates the fluid through the pipes. That same fluid going through the pipes over and over.

Open-loop ground source heat pump
Source: U.S. Dept. of Energy Building America Solutions Center

One type of open-loop system is sometimes referred to as a pump-and-dump system. That pretty much tells you how it operates. You pump water from the ground or a pond, run it through the heat exchanger, and dump it onto the surface or into a storm sewer. That type is widely illegal (see next section).

But there are three other types of open-loop systems that handle the water responsibly after the heat exchange. The diagram above shows one of them, the doublet. It has water being pumped out of one well and then sent back into a different well. The other two open-loop types are the standing column well and the dynamic closed loop. The latter is really an open-loop/closed-loop hybrid. See Jay Egg’s article on this topic for more detail.

Pretty simple concept, right?

Which type is better?

As with most things, which type is better depends on whom you ask. But there is a pretty good agreement among a lot of people in the industry that one type is better than the other. Before I tell you which it is, though, let’s list the important issues.

  • Water quality
  • Efficiency
  • Groundwater contamination
  • Aquifer depletion
  • Legality

Let’s start at the end because the last three are connected. In some places, you’re not allowed to install an open-loop system (or certain types of open-loop systems anyway). In dry areas out West, water is scarce and highly regulated. Aquifer depletion and groundwater contamination can make that problem worse.

An open-loop system can pump A LOT of water from the ground. In a large, inefficient house, that could be many tens of thousands of gallons per year. Groundwater contamination happens when the used water just gets dumped onto the ground. There it can carry surface contaminants down into the ground.

That leaves the issues of efficiency and water quality from the list above.

Efficiency

On the surface, open-loop systems seem to be more efficient than closed-loop systems. One reason for that is that water transfers heat more readily than the closed-loop antifreeze mixture because it’s more conductive. Also, if you have a large supply of groundwater, you’ll have a more constant temperature over the course of the heating or cooling season. The area around a closed-loop system can heat up in summer and cool down in winter, making it harder to cool or heat the house.

But there’s more to it than that. I spoke with John-Paul Kiesel of Water Furnace, one of the biggest names in ground source heat pumps. He told me that when you look up the efficiency of the two types of systems, you’re not getting the full picture. It’s like comparing apples to oranges. That’s because a significant source of energy use is absent from the open-loop efficiency.

The closed-loop system efficiency includes the pump energy whereas the open-loop efficiency does not. And pump energy can be much higher in an open-loop system.

Ground source heat pumps in a net zero energy apartment building
Ground source heat pumps in a net-zero energy apartment building

Let’s say you have matching systems that are of the vertical well type. The open-loop system works against gravity the whole time as it pulls water from the ground. The closed-loop system, however, is pulling the antifreeze mixture up one pipe while the fluid is going down the other side. The fluid going down helps push the other side up, resulting in less pump energy use.

Kiesel said about the efficiency question, “If you have a properly designed closed loop, it shouldn’t make any difference.”

Water quality

The quality of the water in an open-loop system is what can make a huge difference. You’re putting groundwater or pond water in contact with the heat exchanger. That can lead to pitting, corrosion, scale, or other problems. The acidity and mineral content of the water as well as the amount of silt and even iron bacteria can shorten the life of the equipment.

Kiesel told me that the expected lifetime of a closed-loop system is 25 to 30 years. With poor water quality, an open-loop ground source heat pump may last only 10 to 15 years. He also told me he doesn’t have a preference for open-loop or closed-loop. As with just about everything in the world of building science, both types can work well when designed, installed, and commissioned properly.

Comments from industry pros

I posted a question on LinkedIn about this topic to see what others had to say. I especially wanted to hear from those with direct experience with either or both types. Here are some of the comments I got there:

Bradford White (engineer): “I’m definitely in the closed-loop camp as a default. Control your water, know your water.”

Ty Branaman (HVAC instructor): “Closed loops require purging but less contamination and less energy usage in the pump since what is pushed is pulled in the loop.”

Rob Brown (former contractor, now working for a manufacturer): “Open loops are FAR more susceptible to failure.”

John-Paul Kiesel (manufacturer’s rep): “Open loop works great as long as the water quality is in range and flow rates are adequate.”

Dan Nall (architect): “I’ve had bad experiences with the 2 open-loop systems with which I’ve been involved. An open-loop system is sort of like having unprotected s3x with a relative stranger.”

Ross Trethewey (engineer): “We have designed 250+ geothermal heat pumps systems in the Northeast over the last 15 years and every single one is a closed-loop system.”

The sum and substance

There’s your quick overview of open-loop vs. closed-loop ground source heat pumps. One thing I didn’t mention is cost or ease of installation. For both of those, the word on the street is that the open-loop system wins. It’s just easier and cheaper to install a pump-and-dump system. But the drawbacks, especially the water quality issue, can eat into any savings you get upfront if the equipment lasts only half as long. As always, do your homework, get a full design upfront, find a good contractor to install it, and then get it commissioned.

Afterword

My description of open-loop ground source heat pumps above just scratches the surface. To understand the intricacies of it better, see this article by Jay Egg: When Does Aquifer Thermal Energy Transfer Work Best?

________________________________________________________________________

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to our newsletter and follow him on LinkedIn. Images courtesy of author, except where noted.

21 Comments

  1. Expert Member
    Michael Maines | | #1

    Great overview, Allison. Your LinkedIn commenters don't exactly provide a ringing endorsement for open-loop systems! Would it be possible to share the comment by RoyC that you mentioned?

  2. Expert Member
    MALCOLM TAYLOR | | #2

    "The main advantage of ground source heat pumps is that it’s easier to draw heat from the ground in winter and dump it into the ground in summer (because) the ground temperature doesn’t vary like the air temperature does."

    An article this fall seemed to say that this advantage may disappear as the system is used, and the ground temperature may vary.
    https://www.greenbuildingadvisor.com/article/geothermal-heat-pumps-reputation-vs-reality

  3. Expert Member
    ARMANDO COBO | | #3

    I would like to see an US economic analysis of single residential geothermal H&C system installation vs. other single residential H&C system installation for an average american house, without any financial interests, bias or professional/personal relationships.

    Are there any differences depending on climate zones? Are there any differences depending on soil types? Which, if any, is more cost effective?

    Does anyone knows if any of the national labs have worked on any of these studies?

    1. Expert Member
      DCcontrarian | | #4

      Whether it works seems to be highly dependent on local geology. If you look at the business, there seem to be highly localized installers who know their local area (and presumably live someplace where the geology makes it feasible.)

      This was the premise of Dandelion Energy -- they would get extensive local knowledge and try to saturate that area. They were initially developed by Google and then split off. I found their approach interesting but it seems like they've been quiet for a while.

      About a year ago there was an NPR show about them, I think it was "Planet Money." One of the interesting tidbits was that they initially planned to partner with existing HVAC contractors to do their installations. They eventually realized that it was impossible to find contractors who would install the equipment to their specifications and had to pivot and bring all the installations in-house.

    2. paul_wiedefeld | | #5

      I think that’s partly the issue - there is no average American house and for a technology that’s niche, it’s going to fail for the average home. The successful factors in my opinion aren’t geological and are actually easier to identify: cold climate, expensive electricity and expensive other fuels, high heat loss, and low interest rates. Basically, that leaves drafty New England homes when interest rates are low, and nowhere when they are not.

      1. Expert Member
        DCcontrarian | | #6

        Which may be why Dandelion has been focused on New York and Connecticut.

        New England also has poor access to natural gas, a lot of places have put moratoria on new gas hookups because the network can't handle it.

        I'm from New England and still spend about a quarter of my time there. Until about ten years ago there was a guy in my town who specialized in geothermal. But he closed up shop before I ever got to talk to him.

        1. paul_wiedefeld | | #7

          Yup! But when capital does not flow as easily, I can’t see much of a place for geothermal.

      2. Expert Member
        DCcontrarian | | #8

        The University of the District of Columbia recently did a big geothermal installation, they dug a bunch of wells under their football field. My sense is you would experience significant economies of scale with a larger installation than a typical house.

        1. paul_wiedefeld | | #9

          Maybe! A big institution would pay cheap electric rates and have strong air source economies of scale too. I’m not holding my breath for geothermal to make an impact.

        2. iainb | | #12

          There’s an experiment in MA right now installing three pilot geothermal shared networks. They put in infrastructure networked to a bunch of buildings and residential homes. They started construction, but no one is hooked up yet. The two major gas utilities are funding it. What they hope is that the economics works and they can go from supplying gas to supplying thermal exchange.

          It’s been done successfully on some college campuses and a two locations I know of in the UK. None of them, however, supported the profit interests of a utility. So we’ll see what happens.

          https://heet.org/2023/08/28/snapshots-of-the-networked-geothermal-progress-in-ma/

          1. Expert Member
            DCcontrarian | | #14

            I'd think if you were going to go to all that trouble you'd be better off using a lake or river as your sink/source. See my post #13 below.

            In Framingham there's the Hultman Aqueduct, Lowell is right on the Merrimack River.

  4. user-723121 | | #10

    Has Drake Solar Landing been mentioned?

    Doug

    1. Expert Member
      MALCOLM TAYLOR | | #11

      Doug,

      https://www.dlsc.ca/about.htm

  5. Expert Member
    DCcontrarian | | #13

    Came across this today:
    https://www.siemens-energy.com/global/en/home/stories/mvv-mannheim.html#tblciGiCK-q9ahfgUP6UPIzyMasjYPnOHyql0Nokiq4uh3CjlyiD6yGMor6ulmvn7jrOwAQ

    Mannheim, Germany.

    "The new river heat pump from MVV supplies climate-friendly heat from the Rhine water to around 3,500 households - One of the largest heat pumps of its kind in Europe."

    They claim 20 MW of heating.

    1. Danan_S | | #15

      Pretty neat, because climate change is warming rivers, which de-oxygenates them, threatening species that live in them.

      https://theconversation.com/as-climate-change-warms-rivers-they-are-running-out-of-breath-and-so-could-the-plants-and-animals-they-harbor-213297

      Not sure if the effect of the river heat pump is enough to offset that, but maybe with enough of them.

      1. lance_p | | #18

        The effect would be river cooling in the winter and warming in the summer as the heat pump both takes heat from, and releases heat to, the river. If rivers are that sensitive to temperature change, I would think altering temperature with heat pumps could be a disaster for the local ecosystems.

        Referring to the article you linked to, I can think of many reasons our water systems might be changing, some of which may be human activity related, but none of which are climate change. I'm skeptical of studies whose conclusions are based solely on modeling, and report "this is definitely caused by climate change, and we need more funding for further studies."

        1. Expert Member
          DCcontrarian | | #19

          Most of the US and Europe is heating dominant climate. Which means you'll be taking more heat out of the river in the winter than you put back in the summer.

    2. wastl | | #16

      DC,
      I was earlier this year in a hotel in Bavaria which was heated that way. They claimed that it was only possible because the hotel was down river of a coal plant which prevented the river to freeze in winter. Mannheim can freeze over once in a life time so this is a risk unless some second fuel is available

      edit: we see here more "cold district heating systems" Several building share a group of geothermal wells and every house has a water-water HP.

      1. Expert Member
        DCcontrarian | | #17

        It's probably not so much that the river freezes over, but that the water has very little available heat when it's that cold. In a lake the deep water won't ever get below 39F, in a well it will stay closer to 50F. A river mixes the water pretty well so if the surface is frozen all of the water is going to be close to freezing.

        1. wastl | | #21

          I guess the limit is that the discharge temp. back into the river must never be below freezing - that will kill the evaporator.
          That becomes difficult in winter time.

    3. iainb | | #20

      The network effect really can work with any heat source/sink. Connecting sources on a network lets you peak shave and use fewer boreholes/loops/lake loop than you would need if you did every house individually.

      The network also provides an opportunity for shared thermal storage and supplementing what comes out of the ground. Which could be important in cooling dominated climates.

      I’m hoping the economics work out, because it could provide a path forward for natural gas distributors. If they have a path forward other than ceasing to exist they might actually help!

      (And please no H in my gas lines. It’s a bad idea.)

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