Diagonal Loop length per ton

Discussion in 'Geothermal Loops' started by bawalker, Jan 7, 2013.

  1. bawalker

    bawalker New Member

    I am curious whether there is an established general length for a bored diagonal loop. I am aware that 150 feet per ton is typically used for vertical wells but should diagonal wells be longer?

  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    It really depends on the depth, soil conductivity, pipe size, ...among other things. DEPENDING ON THE DEPTH, it follows the rules of horizontal loopfields.
    usually around 600' /ton as a rule of thumb, which is pipe in the ground, meaning 300'/ton borehole.
  3. bawalker

    bawalker New Member

    Thanks for the information. Are there guidelines for the depth? At what depth, assuming soil connectivity and pipe size are not issues, would the horizontal rules not be applicable? And once horizontal rules did not apply, would the vertical rules apply?
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    It depends on the spacing, the load , the....and I could go on and on and on. But here is an example:

    Lets say for a certain load, lets say 22,800 btuH heating load here in Buffalo NY, a certain diffusivity and conductivity etc etc, in 6ft deep, 2ft wide trenches you need about 687ft of straight pipe not to drop you loop below 30F. Probably due to the pipes being closer together you need about 560ft of horizontal boring (1120ft of pipe) for this to not drop below 30F (almost double). Now 499ft at 7ft depth, 457ft at 8ft, 428 at 9ft, 408 at 10ft. So 335ft at 20ft. So it is not linear, and it is not a big difference anymore between 9 and 10ft of depth, but a huge difference between 5 and 6, or 6 and 7ft. Then the conductivity changes when you get deeper and hit more water.
  5. bawalker

    bawalker New Member

    Thanks, I think I can extrapolate from the example. Getting to 20 feet as quickly as possible is the key, which takes obviously only 20 feet for a vertical loop. The difference then is the effective length of the pipe getting to that 20 foot depth in a diagonal boring.
  6. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    That should not be an issue if you are pumping enough GPM to keep the heat pump happy.

  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Well, it is an issue for overall performance, if you don't have the correct design of the heat exchanger. The most common reason systems fail, or at least run inefficient, is loop design.

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