Vertical loop depth requirement for GSHP

Discussion in 'Vertical and Horizontal Loops' started by freestyle35, May 24, 2011.

  1. freestyle35

    freestyle35 New Member

    We just had a 5-ton Waterfurnace Envision series furnace installed by Thermal Associates (TA) from Glens Falls, NY. The install was completed last week (5/18/2011). We live in upstate, NY where the temperatures can get very cold. I'm concerned about the well depth. TA told us that they only need 500'. We had 2 300' wells drilled. The well is through slate beyond about 10'. I could not find data on the thermal conductivity of slate but I'm guessing it's a pretty good conductor since it's dense. Everything I've read online suggests a minimum of 150' per ton.

    Should we plan on drilling another well.

    thanks for any advice/insight
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Loops is what it is all about

    So, "IF".

    Granite is a good heat conductor. Boots on the ground know more than Internet consultations.

    Where did you get 150'?

    How is the system working so far?

    In other words you did not give us enough information to render an opinion about a system that you are not complaining about.

    More input.

  3. freestyle35

    freestyle35 New Member

    It's slate actually and I agree with your "boots on the ground" comment

    Google "vertical loop depth per ton for geothermal"- lots of hits

    The system was just commissioned last week - brand new. We aren't in our heating or cooling season. The installers did a great job - very neat and precise work.

    The system was sized based on heat loss calculations for our home. I assume a 5-ton system will be sufficient but was just curious about "rules of thumb" or "experiences" with bore hole depth requirements in slate - just an academic curiosity at this point. I'm not ready to go out and mess up our yard again with another well.

    Thanks for the response
  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Sometimes 150'/ton is too much and sometimes not enough.
    Wait and see.
  5. scnewconstruction

    scnewconstruction New Member

    AMI you bring up a point I was wondering about. Other than the cost associated with too much ground loop is there a determent to the system effectiveness with too much ground loop?
  6. Jamesck

    Jamesck Member

    Mine has been in for (4) months now. The software called for (4) holes 175 foot deep for a 4-ton system. When the driller started it went really well and they ended up going 200 feet deep. We have mostly red clay and shale in this part of Wv. They said that I should not need any strip heat, which is what I wanted. We installed a 2-stage Carrier split 50YDS049 with a FV4006 variable speed air handler. At first I did know if it was producing enough hot water. Once hot weather got here it seems to be doing fine. Our electric bills are averaging $50.00/month so far. I am very pleased with it so far.:D
  7. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    For any mechanical system to work efficiently the peices of the puzzle need to be matched to each other. To little loop and the load will exceed the capacity of the loops. To many loops and your wallet lost more funds than neccassary for the system to operate effeciantly.

  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    It also depends on the size of pipe you have installed. 1.25" pipe has about 10% higher heat transfer rate than 0.75" pipe.

    Temperature and Energy logging by: Web Energy Logger

    Above is the link to a system which has a 6 Ton load, 133' of 1.25" pipe/ton. As you can see at the graph called "Annual Entering Loop Water" it never dipped below 31.5 F EWT. Generally, the consensus is that one has a loop which is a perfect compromise between higher installment cost (longer loop) and higher operating costs (shorter loop) when the lowest EWT at the peak of the heating season is around 30 F. So one could argue that this particular loop is still oversized.
    Most people or contractors do not monitor their systems, and therefore the general 150'/ton thumb rule has a lot of safety margins. I also purposely have sometimes undersized vertical loops, dipping the peak season EWT down to 28F, increasing operating costs by $150/year, but saving $6000 of upfront installment costs. I sometimes do the oposite, especially for horizontal loops.
    It becomes a science of itself. If they did a very nice job and are reputable, chances are that they knew what they were doing and how far to dip into those safety margins, saving you a lot of money upfront.
  9. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Alright, I should have mentioned that we are getting into rock pretty quickly, and my driller is using a 6 inch drill as standard. We started to use 1.25" for flow reasons in lots where we had to go deeper, but then we were realizing that we were getting about 10% more heat transfer out of it when we monitored it. Surface area of the pipe is much bigger, but offset to some degree by thicker wall. Both IGSPA and climatemaster loop software agree on the 10% higher performance. If you know how the geology performs, you can move away from the rule of thumb (even the software only lets you pick 4 different ground conductivities). At the end of the day, that is why you are doing a conductivity test for larger applications. You want to reduce upfront costs without sacrificing performance.
  11. AlaskanNoob

    AlaskanNoob New Member

    Has anybody tried installing 5 to 10 or so more shallow 50 ft deep boreholes per ton that are connected together with a manifold such that you can oversize the loop and if it turns out it's not efficient, one or more boreholes can be shut off from the system to make it more efficient?
  12. SR

    SR New Member

    My 3 ton system was designed originally for three 150-deep boreholes, but during the actual boring, the holes kept collapsing due to sandy conditions. The best they could do was 6 boreholes of 75' average each. The installer said that this arrangement would reduce the pumping losses. It has been great system for over 10 years in Minnesota.
    Deuce likes this.
  13. gsmith22

    gsmith22 Active Member Forum Leader

    it would generally be considered time inefficient from a construction standpoint to have to set up to drill a lot of shallow holes instead of drilling fewer deeper holes so you will almost always get better pricing for the fewer wells that go deeper. This also assumes your shallow holes go deep enough that seasonal variabtion of ground temps don't affect them. With respect to reducing head loss/pumping energy use, that is entirely dependent on how the various wells are connected together. 3 - 150ft holes connected in parallel would have an equivalent head loss to 6 - 75 holes if the 6 holes were arranged with 3 groups of 2 holes connected in series and the 3 groups are themselves connected in parallel. 3 - 150 ft holes connected in parallel would have substantially less head loss than 6 - 75 ft holes connected in series.

Share This Page