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Musings of an Energy Nerd

A Fresh Look at Ground-Source Heat Pumps

Because of their high cost, these systems are hard to justify for single-family homes

This type of ground-source heat pump system has vertical closed loops. The type of system shown in the illustration includes vertical tubing inserted into boreholes. Once the lengths of U-shaped tubing are inserted in the boreholes, the holes are filled with grout. Other systems include horizontal lengths of tubing installed in trenches that are between 4 and 5 feet deep. [Illustration courtesy of Fine Homebuilding]

Most residential heat pumps in the U.S., including ductless minisplits and ducted minisplits, are air-source heat pumps. During the summer, this type of heat pump acts like an air conditioner: it collects heat from indoor air and transfers the heat to the outdoor air. During the winter, the heat pump reverses direction: it collects heat from the outdoor air and transfers the heat to the indoor air.

A second type of heat pump is the ground-source heat pump (GSHP). During the summer, this type of heat pump transfers heat from indoor air to the soil around your house. In some cases, it may transfer heat from indoor air to the water in a drilled will, or to a nearby lake or pond. During the winter, a GSHP collects heat from the soil around your house (or from the water in a drilled well or nearby pond) and transfers the heat to indoor air.

To effect this heat transfer, a ground-source heat pump system requires either buried tubing—vertical loops of tubing inserted into boreholes, or horizontal lengths of tubing buried in deep trenches—or the circulation of water from a drilled well or pond through tubing that conveys the water to the heat pump.

Some manufacturers of ground-source heat pumps state that their equipment gathers renewable energy, or label their equipment as “geothermal.” To clear up these misconceptions: a ground-source heat pump does not use geothermal energy. It isn’t a type of renewable energy system. Like an air-source heat pump, it’s simply a type of heating and cooling system that uses electricity as its fuel.

Air-source heat pumps are much cheaper and almost as efficient

GBA has dozens of articles on ground-source heat pumps (many of which are listed in the “Related Articles” sidebar). If you are…

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6 Comments

  1. charlie_sullivan | | #1

    This is all fair criticism. As an engineer, I was delighted to participate in the engineering project of designing a system for my house and tweaking it to get excellent performance, all for a cost at the low end of your range (15 years ago). But the experience also underscored how difficult it would be for someone without engineering expertise to achieve a similar result.

    However, there is an advantage that I don't see mentioned here that I think will be increasingly important in the coming decades. That is, even if the total annual electric consumption of a good air-source heat pump installation is not significantly different from that of a good ground-source heat pump installation, the peak electricity consumption on a cold January night is significantly more for an air source heat pump. For the consumer, that doesn't matter, because it's the total energy consumption that you pay for not the peak load. But for grid operators, that difference really matters. With an increasing percentage of variable renewable sources on a grid, including solar (which doesn't produce anything through a long January night), and with increasing electrification of heating, the fact that air-source heat pumps’ COP drops right when you need them most is a concern, and is a motivation to continue trying to improve approaches to cost-effectively installing and commissioning these systems.

    Companies like Dandelion are working on making the installation more systematic, to lower cost and avoid problems, and to do drilling with right-sized equipment and approaches that minimize overhead.

    It’s not a good choice right now for a typical homeowner, but I’m not ready to give up on it as a technology that should be used in the future.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    "Some manufacturers of ground-source heat pumps state that their equipment gathers renewable energy, or label their equipment as “geothermal.” To clear up these misconceptions: a ground-source heat pump does not use geothermal energy. It isn’t a type of renewable energy system. Like an air-source heat pump, it’s simply a type of heating and cooling system that uses electricity as its fuel."

    A basic but important distinction that somehow up to now had eluded me.

    1. charlie_sullivan | | #4

      Alternatively, we could say that air-source heat pump utilize renewable tropospherothermal energy.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        Charlie,

        Maybe you could. I've tried three times to say that word and can't.

  3. iainb | | #3

    Have you seen the pilot geogrid projects? The basic idea is a network of boreholes at the street sharing water in a network between houses, businesses, etc. They're testing to see if it makes economic sense and if current gas providers could essentially switch to thermal.

    https://heet.org/2022/02/28/six-geogrid-demos-approved-for-massachusetts/

    There are multiple locations for the pilots, both MA gas utilities are participating.

  4. superman22x | | #6

    It does seem one of the details of the Inflation Reduction Act includes renewal and updating of section 25C. It changes the rebate on all heat pumps (among other things) to 30% with a cap of $2,000/year. The current geothermal tax rebate is extended until 2033 at 30% as well. This bill has not yet passed at this time, but worth noting.

    EDIT: After a little closer look - the rebate is 50% of the heat pump cost ($8k max) if your household income is less than 150% of the median. Which for my metro Detroit area, ends up being $136k! The 30%/$2000 max is if your income is above the 150% threshold. Rebate is 100% if your income is less than 80% of the area median. I think we will see a LOT more air source heat pumps in the coming years. It will be interesting to see how the technology evolves.

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