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Ground source heat pump loop ground temperature

Discussion in 'Home Power and Microgeneration' started by Astro, Dec 28, 2004.

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  1. Astro

    Astro Guest

    A few weeks ago, I had a GSHP system installed. Along with the loops, I
    installed temperature sensors so that I could monitor the temperature of
    the ground at the loops. In this way, I figured that I could monitor
    system efficiency during the course of the seasons. This is a four ton,
    direct expansion system with four, 100ft vertical loops. The ground here
    is solid granite for hundreds of feet down and around. Little to no water

    The measurements I'm making are as follows -

    Run the system normally for some period, i.e. a couple of days of
    operation. While the system is running, I'm monitoring the vent, room, and
    outside temperatures throughout the house. I'm also monitoring the
    temperature of the refrigerant lines to/from the compressor unit and have
    measured the current draw at the breaker panel.

    Next, I turn the system off for a period, typically the better portion of
    a day. During this period, I monitor the loop temperature sensors to see
    ground temperature recovery.

    Finally, I have a monitor buried at roughly the same depth (these are all
    vertical loops) so as to provide a ground control temperature.

    The results I'm getting are illuminating (and disappointing!)

    Control ground temperature started at 51F on 12/8/04. It is now 49.9F on
    Total system run-time during this period has been 150 hours.

    The ground around the loops is freezing within <24 hours of system
    operation. Along with the loop cooling goes the vent temperatures. When
    the system is fresh (i.e. loop temperature recovered to the mid 40's), the
    best vent temperature is around 93F. (My ductwork is tight and well
    insulated). Maximum vent temperature is within 2F of the temperature
    measured right at the air handler.

    Ground loop recovery is getting progressively worse as the usage
    increases. I assume this is because the ground around the loops is getting
    colder. Currently, it takes about 6 hours for the loops to "unfreeze" from
    about 30F to 32+F. Then, the loops rise at 1.1F/hour until they reach 38F.
    Then they slowly rise a couple degrees above that. Measurements just a
    week ago showed loops rising up to a about 42F before leveling off to
    about 45F. My suspicion is that within a couple more weeks of use the
    loops won't even rise above freezing.

    My computed BTU output for the system runs from 32000BTUs/hr to
    42000BTUs/hr. This system is rated at 4 tons, and I was told that it
    typically puts out closer to 5 tons at the start of the season and it
    drops to about 40000BTU by the end of the season as the ground cools.

    The system was "professionally installed". The installer came highly
    recommended by the manufacturer.

    On the initial install, the loops were backfilled incorrectly resulting in
    the tubing making minimal contact with the surrounding ground. These
    measurments are from after I corrected this problem.

    The system was supposed to have six loops, 100ft but the drill rig broke
    during installation. However, the manufacturer states that four, 100ft
    loops should be sufficient.

    The loops were supposed to be installed at least 10ft apart but they are
    6ft apart.

    After doing many hours of research, I feel that there are several issues

    First, the thermal diffusivity of the ground is such that much more ground
    loop is needed to balance the thermal load being placed on it. I have
    typically read that about 250ft/ton is required for conventional gshp
    systems and under good conditions, this can drop to 200ft/ton for a direct
    expansion sysetm. so from the getgo, I'm at least 50% of the desired loop

    Second, the loops are too close together. At 6ft apart, the BTU capacity
    of the surrounding ground is seriously compromised. Most of what I've read
    suggests that 225 sq. ft/loop is required for long term ground temperature
    stability. This is running at 16% of this.
    The manufacturer stated that even 8ft loop separation is within spec.

    Ok, now besides telling me that I've been ripped off, I'm interested in
    other peoples experiences with GSHPs. Has anybody besides me taken the
    time to actually quantify the performance of their GSHP system? I've read
    everything on the standard GSHP sites and none of what I've seen actually
    shows what expected ground loop temperature is in the immediate vicinity
    of the loops. I've read about the overall ground temperature of the loop
    field, but haven't encountered any literature discussing what I'm

    I've also seen measurements of water loop systems, where the water
    temperature to/from the ground loop is measured. This would be a good
    indication of the loop field temperature. Anybody have numbers from their

    I am now investigating remediation for the system. I am considering two
    1) Install 6 additional vertical loops, spaced 15ft apart from one
    another. this would give the required 225 sq. ft/loop and would increase
    the system to 250ft/ton.

    2) go to a horizontal loop field since drilling here is so damned hard and

    Any suggestions on an effective, long term remediation would be greatly
    appreciated. Spare me the snide remarks about installing a gas burner and
    throwing away the GSHP. I actually have a working baseboard hot water
    system that this system is supposed to replace. Now I'm alternating
    between the systems to save oil.
  2. daestrom

    daestrom Guest

    Sorry to hear of your troubles, I have been looking forward to installing a
    GSHP here in update NY. But your experience is giving me second thoughts.
    In my area, there is a lot less granite, and a lot more ground water, so
    maybe your experience is unique to your situation? Anyway, I'm planning a
    horizontal loop system.

    I'm curious, how are you measuring the ground loop temperatures when the
    system is off? Are these temperatures near the surface or deep within the
    'wells'? With 100 ft deep wells, I would have expected the bottom to be
    much warmer than the surface.

  3. Astro

    Astro Guest

    (snipped out original long posting)
    My guess is that in an area with plenty of water movement, the problem
    will be greatly reduced as water in motion is great at carrying heat. My
    bores are pure granite. I live on a "mountain" which is a local diabase
    intrusion. I have two wells on the property. The backup well which the
    previous owner drilled in 1990 is 950 ft. deep and has a recovery of

    As for the measurements - I embedded temperature sensors in epoxy then
    lowered these 50ft into the boreholes along with the loops.

    I believe that you have to go considerably deeper to start seeing warming.
    I have read that the geothermal gradient is on the order of 75F/mile. When
    the loops were first installed, the baseline measurements showed a
    temperature of 51F at all loops.

    After all the research I've done now, I would suggest that anybody
    considering a geothermal system pay for a thermal diffusivity analysis to
    see what the thermal gradient will be around the loops during actual use.
    For example, I would like to see the thermal gradient based on a 100%
    runtime of a 50k BTU load. Can the heat from the surrounding strata come
    in as fast as it's being pumped out? How fast would it come in? Where's
    the break point - i.e., if you run the system X% of the time, it will be
    pumping out the same amount of heat as can diffuse back in. I think that
    is the magic number for any loop configuration. Without this analysis, I
    wouldn't trust any GSHP installation.

    As for Tim's comments:
    That will certainly help some. However the balance here is definitely
    weighted towards heating degree days, so my guess is that, if I'm lucky,
    I'll get really good Air Conditioning performance early in the season,
    then as the ground warms up, it will warm back to somewhat above the
    baseline temperature. The problem I see is that the current loop field is
    so small that in a matter of a couple weeks of use, the heating system
    will draw out all the useful heat. But time will tell. Thanks for the
    Another good idea. Actually, since they botched the backfill on
    installation, I did that myself using hot water (we have a direct hot
    water hose connection), so the bores got filled with 120+ water. In
    practice, the heat in the water was sucked out PDQ by the ground thermal
    Funny you should mention that one. I was running an analysis on that
    option as well and may yet go that route. The problem, and please correct
    me if I'm wrong, my references show about 25,000BTU/day per panel is
    pretty good. So if I put 8 panels on my roof, then I'd be pumping in, at
    best, about 200,000 BTUs/day into the system. Now, when the system is
    running to spec, it should be pumping out about 50,000 BTUs/hr. Some of
    this will come directly from the compressor power, and the rest from the
    ground. So I was figuring on some 35,000BTU/hr being pumped out. During my
    test runs, I found the system to be running between 16+ hours/day at 15F.
    So I'd still be pumping out 16*35000 = 560,000BTU/day which is
    360,000BTU/day more than I'd get from the solar panels.

    I still think solar could be a good addition as that would really decrease
    the thermal load on the wells.

    One other option - if I install solar, then would it not be more efficient
    to just add radiant heating wherever I can to my house? Then I'd be
    pumping the heat right to where it's being used. as such, all those BTUs
    would go into the house and reduce the load on the ground thermal mass.

    Lots of thoughts. Please keep em coming.

  4. Astro

    Astro Guest

    Thanks for the practical background info.
    We'll see about corrosion. I hope not. The groundwater pH here isn't to
    acidic and they installed a galvanic protection system, so at least that
    portion (hopefully) won't be an issue, but time will tell.

    The DX systems don't grout because of the high temperature swings.
    Research I've seen indicates that grouting is a mistake with DX as the
    cement tends to shrink away from the tubing and crack. In this
    installation, they use sand, which can shift and fill and maintain a good
    thermal contact. So goes the theory. I had asked for a good Bentonite
    grouting and this is what they told me, as did an independant reference at
    one of the colleges that specializes in GSHP.

    I checked the possibility of open loop first, since that's typically the
    most efficient system available. Unfortunately, I've got extremely little
    ground water here. The two wells on my property together can only supply a
    couple gallons a minute.

    Thanks again for your input.
  5. Steve Spence

    Steve Spence Guest

    get away from the air handler, and use your baseboard. the needed temps
    are less, and your system will run more efficiently. I have no
    experience with vertical loop systems, only horizontal slinky coils.

    Steve Spence
    Dir., Green Trust
  6. When I talked to one DX system manufacturer, I was told that the wells would
    be drilled at a 45 degree angle, so that the well heads would be in a 15
    foot diameter circle, but that the majority of the wells would be far enough
    apart for adequate thermal separation.
  7. Interesting idea. Is that a fairly regular test or analysis? Eg, for
    well drilling they can test flow/recovery to know if it is sufficient.
    I wonder if they could do something similar for a GSHP...

    A neighbor has a GSHP and really likes it. However his is open loop,
    and the outflow maintains a nice pond (1/2acre?).
    The fact that the ground sucked the heat out of the water so quickly is
    a good sign for summer heat storage. After all, that heat did not just
    disappear, it was dispersed via conduction.

    One interesting point with solar though, is that lower operating
    temperature is more efficient/effective at harvesting and transporting
    that solar energy. For radiant heating, you probably want water at
    least 100F at the panel output. However to dump heat into the ground
    you could probably get significant benefit at only 80F or even colder
    output temperatures.

    In the summer you could easily get temperatures high enough to provide
    all your domestic hot water needs. That may even be a better design
    starting point -- add solar water heating sufficient to provide all
    domestic needs, and if it works out well, add capacity for space
    heating. Lower initial investment for a trial.

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