Vertical Loop Sizing concern?

Discussion in 'Vertical and Horizontal Loops' started by hammer, Jan 20, 2011.

  1. hammer

    hammer New Member

    Hey Guys,
    I'm in the process of getting a geothermal HP installed. The quote I accepted was for a 4ton unit with a 4ton loop installed per IGSPHA standards by an accredited well driller. He started the first well today and I inquired how deep the wells would be. He told me he was drilling 2, 280ft wells 18ft apart. I expressed concern to my installer that this vertical loop may not be sufficient for a 4 ton load. My installer is telling me that since he's doing a double loop in each well that he expects to get 20-25% more conductivity per well. Does this sound accurate?

    I thought I trusted him and his experience but everything I can find points to the need for deeper wells per ton and maybe a 15% gain from running a double loop. Also, the well driller has quoted me a different price per foot than what the installer is now saying. At this point I'm concerned that the loop isn't being properly sized to support the load on my 4ton unit. Not sure what kind of data is needed to verify the loop but avg ground temps for my area is 62 degrees and thermal conductivity runs between 2.8-3.1 K w/meters degrees C.
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Methods are less important than results. Does the installer have a predicted entering water temperature lower limit...upper limit?

    A good loop stays within 20-25 degrees of deep ground temp. 62* ground is about as good as it gets for both heating and cooling. If your loop provides entering temps from 40-85 you will be in high cotton as to system efficiency assuming the balance of the system is designed and installed properly.
  3. hammer

    hammer New Member

    So after some more digging I converted the thermal conductivity to BTU/ft hr F and found a design guide at this link.

    Based on my mean ground temps I should have 170 feet per ton. Taking into account the thermal conductivity of the ground 1.5 plus a 10% gain for dual loops K = 1.65 and the thermal conductivity of the grout .85. This gives me...

    (Table 1) × CF(k -ground) × CF(k-annulus). A ground K of 1.65 equals .87 in the table and an annulus of .85 is equal to 1 for that table (this is the grout thermal conductivity). So the calculation reads 170 * .87 * 1 = 147.9 feet per ton.

    So after discounting the top 10-20 feet my loop seems to be short about 60-80 feet or 11-14% short of the recommended minimum.

    I'm sure the system will work as designed but I don't believe it will work as efficiently as it should. That was the whole point in replacing my existing HVAC with a Geothermal unit, increased efficiency (aka cost savings). In the grand scheme of things the extra 60 to 80 feet is a minimal expense that I believe was already part of my quote.

    At this point I'm left feeling like my installer is skimping a bit at my expense. I will be telling him that, albeit a bit more diplomatically.
  4. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Ask them to show the design calculations.

    It should be a report something like what I have attached.

    I am skeptical of the 20-25% improvement by placing two loops in the some bore hole.

    Attached Files:

  5. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    If they can't show a design report, then they might be guessing or using "rules of thumbs"
  6. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    I have always been skeptical of the double loop performance claims. In the end it is all about the local drillers experiance and knowledge of local install methods that work. I would be curious as to wether he has done the double loop before and where is the data to support the claim of 20% increase. I would also be very leary of being the "guinnea" pigfor a system at increased price per foot. Keep us posted as we all are going to be interested in the outcome and the performance.
  7. hammer

    hammer New Member

    He does double loops for all his geothermal installs and has been doing this for years. His experience and use of accredited installers was the reason I chose him for the install. However, he came back to me saying that he didn't do a loop design but will do one for me over the weekend. I wasn't expecting that but I feel it supports my calculations that show the loop is undersized.

    I asked for any research or links to research showing those types of performance gains from the double loop and haven't gotten them. Everything I've been able to find on my own suggests a 10% thermal conductivity gain, although, I did see a study that showed more with the use of spacers.
  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Don't panic.
    While we all encourage folks to avoid rules of thumb, we all know what is typical in our AOs as well.
    In my AO 150'/ton of well is typical (all the usual disclaimers apply). My design software shows ~10% reduction in bore required for two utube/well system.
    So 4 X 150 = 600 - 60 (10%) = 540. You are getting 560' of wells.
    It probably jives here.

    You should explain your concerns to your design/installer. It is his responsibility to keep you comfortable with the design.

    "Also, the well driller has quoted me a different price per foot than what the installer is now saying"

    You also point out another good reason people may want to higher a contractor with an all inclusive price.

  9. hammer

    hammer New Member

    I don't think I've panicked. I think of it as prudent concern. Ultimately this is my system and my responsibility. If it doesn't perform as efficiently as it should I will be the one responsible for paying the bills and/or trying to get it fixed. I doubt my installer is going to be in a rush to fix it because it's not running at optimal efficiency.

    Based on what I was looking at, with a 62 degree avg deep ground temp my wells should have started at 170feet. Are you saying your design software says 150feet of loop is sufficient? Also, I'm thinking the top 10-20 feet of any well should be discounted. Does the design software take that into account?
  10. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I don't know what you were looking at to arrive at 170'. I'm saying we have lots of aquifers and wet soil (in MI) so yes, 150' is ok here. Sometimes a little long, sometimes I'd like a hair more.

    Loop software bases calculations on average depth so ours would be 75' average depth. Therefore it averages the temps of the top 10-20' against the bottom 10-20'. Arbitrary addition or subtraction of loop value makes all your calculations suspect.

    Again, have you addressed this prudent concern with designer/installer?
  11. hammer

    hammer New Member

    I think I stated earlier that the installer didn't do a loop design. He told me he came up with the 2 x 280 ft loop for a 4 ton unit based on experience but that since I had requested a design he would do it over the weekend and provide it next week.

    At this point it's too late to make any changes as the loops have been grouted in. The installer is going to install temp monitoring ports on the loops for me. He is confident based on his experience that the loop is sufficient. I'm confident the loop will work and be fine but my concern is that it's not going to give me the optimal efficiency I am paying for, since I believe it's a bit undersized. I'll just have to see how it performs....

    In regards to the 150 feet you're referring to your regional conditions. The calculations I found use a base number for the feet/per ton based on local deep ground temps. My area deep ground temps average around 62 degrees. Based on that a base well size would be 170 feet per ton. Then you take into account the thermal conductivity of the ground and the grout. Based on my soil thermal conductivity k=1.5 ,taken from a geological drill in the 70's from very close to my lat/long in a granite belt, then adding 10% for the double loop, k=1.65 and with a bore thermal conductivity at .85 for the thermex grout the calculation looks like...

    170 * .87 * 1 = 147.9 feet

    All of this is based on charts found in that design pdf I referenced earlier at:
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I see how you arrived at you number, but a short time ago you were discarding 20 feet of pipe etc.
    See what your installer offers and if it's well thought out, don't be afraid to accept local pro experience.
    Climatemaster as I said does allow a 10+% reduction where two u-tubes are in the ground. Loop temps 30 and above are fine 28* in extreme weather not a deal breaker here.

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