'Double piped' well?

Discussion in 'Vertical and Horizontal Loops' started by Viper GTS, Sep 14, 2018.

  1. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    @ gsmith22,
    I would like to know more about your design modeling and data for your project please.
  2. gsmith22

    gsmith22 New Member

    I'm in somerset county, NJ. I bought a house last year that is ~30 years old with original propane furnaces and a/c. They need to be replaced, propane is ridiculously expensive, and I don't have natural gas service. Geo was a natural option and I'm in the process of having it converted. I'm on a hill with rock relatively close to the surface so horizontal piping was out and vertical was my only real option. Only considered closed loop, local geology would prevent open loop. As you know, its expensive to drill vertical wells so I wanted to make sure the heat exchanger side of the work was optimized. I also wanted it to work well the first time, especially given the current 30% fed credit (If I have to spend money, its best to do it now). Being an engineer and having some back ground in heat transfer and fluid dynamics, I took the opportunity to research how systems are designed. So I got IGSHPA's design and installation guide for residential and light commercial and mainly just followed its design process chapter to chapter. Its very well written, easy to understand, and provides a nice roadmap for anyone looking to design a complete geo system. Part of the loop design process is to figure out if there is an imbalance in heating and cooling. They have you do what is called a weather bin analysis that ultimately provides an estimate for the amount of heating and cooling runtime as well as Btus pulled from/rejected into the ground. Runtime and heating/cooling imbalance gets factored into the equation that provides the minimum amount of borehole length. In my case, it caused the overall length of boreholes to be ~20% greater than I would need if heating and cooling were balanced.

    As it turns out, I ultimately asked for my ground loop to be increased over what the mechanical contractor was going to specify. He came up with the loop design with what appeared to me to be a simplified computer algorithm created by Waterfurnace - GeoLink I think its called. It seemed to have alot of generic inputs that weren't necesserily specific to my location/geology/usage and didn't seem to be readily understood by the contractor. One of the things that stuck out to me in the design manual's discussion of loop design is that for unbalanced heating and cooling situations, the min/max water temps of the loop will change over time if unbalanced loading isn't taken into account. Essentially, you can make up for the imbalance by either putting more vertical borehole in the ground and/or separating the vertical wells further apart. I used a combination - added about 50ft to each hole and spaced them at 20ft vs standard 15ft spacing. Going to 25ft spacing really didn't help nearly as much as going from 15 to 20 so the rest of the imbalance I made up with borehole length. Hope this helps.
  3. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Thank you for your response. It was very helpful.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    What you were describing with unbalanced loads having a memory effect in vertical loop fields is only really relevant in larger commercial systems, where the inside bores (in the middle of the loopfield) are more isolated from undisturbed ground, thus heat (or coolness) traveling through the ground reaches the inner loops much more difficult.

    It is not an issue in smaller vertical loopfield with only a few holes.
  5. gsmith22

    gsmith22 New Member

    The physics of heat transfer remain the same regardless of a residential or commercial installation. It is true that a larger loopfield (more often found in a commercial application) is more prone to an unbalanced load memory because of the grid effect of many bores reducing the ability of the interior bores in the grid to be replenished by ground heat/cool. But, unbalanced load is unbalanced load. Did it massively affect my borefield? No, (3 bores in a line) but the effect wasn't zero either as you seem to indicate. The IGSHPA's manual that I listed above goes through the calculations and it is clear that unbalanced load needs to be accounted for. Spacing at 20ft vs 15ft had a very distinct effect for me and I would have ended up with a more longer bore length than I used.

    With regards to the OP's original issue, he has a single bore so no overlapping heat cone effect from adjacent bores. I was merely pointing out that data from one (mild) winter is probably not sufficient to declare victory and to keep an eye on entering water temps especially given his climate (same as mine) with much more heating than cooling.

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