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Community and Q&A

Split air-source heat pump using ground loop instead?

Lance Peters | Posted in Mechanicals on

I’m scratching my head over this. Several companies offer air source air-to-water heat pumps that are split (indoor compressor unit with outdoor coil/fans). Would it be such a stretch for them to offer an indoor heat exchanger that used a ground loop as a heat source instead of the outdoor fan coil? This would essentially be a super efficient ground source heat pump, far more efficient than most traditional ground source heat pumps that do not offer modulating compressors.

I’m building in CZ6A on a lot with plenty of space for a horizontal ground loop, and because the lot has very shallow bedrock there will be lots of fill brought in. It just seems like a perfect situation to bury some PE pipe and end up with a far more efficient system than an air source unit at our fairly cold temperatures. It would eliminate defrost cycles as well, and with a much higher COP as the ground loop would likely supply water at or above freezing instead of using air at far beyond sub-zero temperatures (our 99% design temp is -27C/-17F).

I’m picturing a brazed plate heat exchanger/pump taking the place of the outdoor coil/fan unit. Anyone have any insight?

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  1. GBA Editor
    Martin Holladay | | #1

    Your question isn't clear (at least to me). Ground-source heat pumps are common. Many of these ground-source heat pumps have buried horizontal ground loops circulating water or a water-antifreeze mix.

    In short, you should talk to a contractor who specializes in ground-source heat pumps.

    The type of system you are talking about is common. You aren't the first person to invent it.

  2. Lance Peters | | #2

    Martin, I'm not concerned that I'm inventing something.

    What I'm wondering is, why do ground source heat pumps cost so much? High-end mini splits have shown what kind of performance is possible and how it can be packaged and sold at retail for about $2000. The reality is, if these units could use a ground loop instead of an air heat exchanger they would be FAR more efficient and have greater output. They would be pulling heat from the ground which would be at or above freezing in most cases, instead of extracting heat from frigid air. Higher COP, quicker ROI, etc.

    Now to go one step farther, if they offered a hydronic option rather than a wall mount fan unit they could heat water and be used with an ordinary air handler, heated floors, room by room radiators, domestic hot water etc.

    Instead we seem to be stuck with a handful of companies that make air to water heat pump systems for 3-5X the cost, or traditional ground source heat pumps that cost far more. Yes I understand the cost of the loop is extra, but in a new construction scenario where significant grading and/or backfilling is to take place the ground loop shouldn't cost a fortune.

    I guess the easiest way to word my question is this:

    Why does a Ground Source Heat Pump cost several times the price of an Air Source Heat Pump? They're both heat pumps, and the ASHPs seem to offer much more advanced technology (fully modulating compressors, valves and fans) where GSHPs typically don't.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Why does a ground-source heat pump cost several times the price of an air-source heat pump?"

    A. Fortunately for you, I wrote an article on that exact topic. Here is the link: Are Affordable Ground-Source Heat Pumps On the Horizon?

  4. Lance Peters | | #4

    Thanks for the link, and a great article BTW! This quote pretty much sums up my feelings:

    “”Kavanaugh still hopes that the cost of GSHP systems will eventually drop. “It is a product that has a 20% smaller compressor than an air-source heat pump,” Kavanaugh told me. “Compared to a split system, it should have half the sheet metal, half the aluminum, and half the copper in it. It should have less than half the controls in it. If the product were widely used, if it were common, it would cost less than a split-system air conditioner/heat pump. But the current situation is that that device costs more than a split system unit, and it should not. ... If you could install the system inside the building, with the pump and ductwork, for the equivalent of an air-source system, the economics would be better. The only premium would be for the ground loop.”

  5. Expert Member
    Dana Dorsett | | #5

    Methanol is commonly used as antifreeze in ground source heat pump systems with shallow ground loops. Some US states have restricted that use, some based on depth, allowing it for shallower systems, but barring it below some depth.

    With shallow ground loops operating at temps well below freezing ground heaving of the soil is fairly common, but if you don't mind a lumpy back yard that needs re-leveling every few years it's not necessarily a no-go.

  6. John Semmelhack | | #6

    Lance - The difference in equipment cost between air source and ground source heat pumps isn't particularly high when they are relatively close to "apple-to-apples" on features. The big difference in price is the installation of the ground loop.

  7. Steve Knapp CZ 3A Georgia | | #7

    I visited a new-home community recently where all the houses had to be on individual ground-source heat pumps. The developer's representative was quite proud that it had negotiated a lower price on the well portion of the install. (Vertical loops installed in wells.) Instead of the HVAC contractor's normal fee of mid-twenty thousand for each well, the homeowner's cost would be mid-teens. (I am not remembering the quoted costs exactly, but these are the sort of numbers he was citing. This was just the loop install cost BTW.)

    I visited two other communities where the developers also were mandating ground-source heat pumps. New homeowners paid a portion of the HVAC cost as part of the purchase price. The balance of the system cost was billed to the owner through a five-year lease.

    This approach is also used for spec builds in my current community, but the lease payments here are typically much higher for some reason.

    In all these communities, the developers make some claims about the superior performance of ground-source systems, (It's free energy etc.) but efficiency doesn't really seem to be the chief reason they decided to go with geothermal. To me, the big driver seems to be eliminating noise pollution. Three of the developments are quite dense and one is next to a scenic fast flowing river.

    In these communities, the developers decided to use ground-source despite the high upfront costs. I am guessing they concluded buyers would pay more (probably a lot more) to avoid hearing fan noise every time they stepped outdoors or cracked a window.

  8. Lance Peters | | #8

    John S., can you direct me to the GSHP manufacturers who are most cost and performance competitive? Everything I'm seeing so far is much more expensive than an equivalent size ASHP, even before the cost of the ground loop is counted for.

  9. Lance Peters | | #9

    Dana, how shallow is "shallow" when talking ground loops? I've read that deeper is generally better. Our lot will have about 5ft of backfill brought in, fairly level in the back yard to a retaining wall, and gently sloping to the road in the front yard. I would like to think this much fill would make for a trivial ground loop installation.

  10. Lance Peters | | #10

    Steve K., good points! I had considered the cosmetic advantage of not having hardware installed outside the house but hadn't thought much about the noise. Funny, because I live in a townhouse subdivision right now and when it's hot out all you can hear is compressors running and housings rattling.

  11. Expert Member
    Dana Dorsett | | #11

    In terms of regulatory restrictions for use of methanol as antifreeze, some states that allow it draw the line at no deeper than 20'.

    Shallow is relative, but in GSHP terms anything that's near or above the 100 year frost line could be considered "shallow". But successful systems can still be designed to work at 3' of depth in most locations.

    The shallower the ground loop, the more pipe it takes to be effective throughout the heating season. By mid winter the average soil temp on a 3' deep loop can be colder than the average temperatures of the outdoor air, and if the loop is too small it can be cold enough to take a huge hit in efficiency & capacity.

    The temperature and thermal conductivity of your bedrock may help you out a bit there, since the 5' of over fill could be treated more like insulation than thermal mass, with most of the heat coming via the bedrock, assuming it isn't exposed above ground very much.

    Have you priced out drilling the ground loop? Some stone isn't too expensive to drill, and the size of the ground loop (and the pumping power needed) can be reduced by quite a bit.

  12. Lance Peters | | #12

    I was thinking the same thing, Dana; that bedrock would be a good conductor and the backfill would more or less just isolate the loop from the environment above. However, I've been able to find next to nothing as far as reliable information on horizontal loop design and soil/rock types, let alone loop on bedrock. Best I've come up with is a general guideline of 500-600 ft/ton depending on the pipe diameter. More is better, but overdoing it will add pumping losses.

    No I haven't gotten a quote for a vertical bore. I figured with over an acre of graded yard (minus septic field) there would be no need to go vertical. Water well drilling in this area reportedly costs around $10k, plus or minus depending on how deep it ends up being. I would think a vertical borehole for a geo loop would be similar with multiple boreholes a possible necessity?

  13. John Semmelhack | | #13

    Lance, regarding minimal cost difference in the equipment itself -

    Exhibit A: 2-ton Climatemaster, 2-stage GSHP for ~$3,500

    Exhibit B: 2-ton Goodman, 2-stage ASHP for ~$3,000

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