loop trench size

Discussion in 'Vertical and Horizontal Loops' started by tom c, Nov 9, 2018.

  1. tom c

    tom c New Member

    I am having a WF Envision system 4 ton installed they were going to directional bore 4 3oo ft loops but due to rock they will now going to dig 2 2ft wide trenches 300ft long and install the 4 loops in those trenches. It sounds to me that's a lot of pipe in a small area. I just want to make sure this is a viable option. I am loacated in NE Missouri. Thanks for any incite on this.
  2. kk4ej

    kk4ej Member

    I am a novice and learning with doing my own system, so please take my answer based on that.
    In using the software from loop link, the loops in the 2 ft wide trench work fine, the key is how far they separate the ditches and of course the depth. In reading, it seems to me the best Hor loop is a single pipe and a large loop. The second best seems to be the out and back loop in a single ditch. Then the slinkies and other tricks come into play with using less space. If you have the space, space between trenches needs to be several feet.
    From a practical install aspect I learned doing my own ditches, 10 or 12 ft between trenches allow you room to backfill the ditch easier. I did 6 to 7 ft deep ditches, 2 ft wide, and backfill was easier having solid ground to work from as I back filled. I used my backhoe to do all of mine.
    Also playing with the software, going a touch longer on loops don't hurt. The earth you have, depth and pipe spacing is the key.
    Hope I helped some. 4 x 300 ft loops, I assume the pipe length is 600 ft?
  3. wing

    wing Member

    You will need 12 gpm through your loop field for a four ton unit and your installer has proposed 600 feet of loop per ton. I assume this will be .75 inch hdpe pipe.

    This all looks OK as long as the trenches are separated by 10 feet or more, the trenches are at least five feet deep and the ground is fairly moist.

    I would personally look at adding an extra loop, i.e five loops of 600 feet each , just to insure you are not undersized. The financial penalties are one extra coil of .75 hdpe pipe for $300 and an additional 4 hours equipment / labor time to install the extra loop (based on my latest install). Your reynolds number will drop a little bit from 3200 to 2500, but still is within the turbulent flow regime to optimize heat transfer.

    You only want to do this once, therefore deliberately oversizing the loop is not a bad idea.
  4. arkie6

    arkie6 Active Member Forum Leader

    What is the depth of the trenches and what is the soil type and moisture content? 2 trenches 300' long is marginal for 4 tons in my opinion unless everything is optimum, i.e. deep wide trenches, good soil thermal conductivity, and good soil moisture content during peak loads (late winter and late summer). If only going with 2 trenches for 4 loops, I would recommend you use a track hoe with 36" bucket rather than a back hoe with 24" bucket, dig a minimum of 6' deep (deeper would be better), and then separate the loops to the far sides of the trench.

    One way to install the loop pipe safely (i.e. not in the trench that is subject to collapse), would be to layout your 300' out-and-back loops on the ground and use nylon zip ties to tie the out-and-back loop pipes spaced 12" apart to the end of a 5' length of 3/4" 200 psi (thin wall) PVC pipe every 10' of loop length. This PVC pipe would be used to drop the loop pipes into the trench from above and hold them separated and to the sides of the trench during back fill. 4 loops in two 300' trenches would require 120 pieces of 5' pvc pipe, or 600' total length. Lowes has this pipe for $5.25 per 20' stick, so the total material cost for these loop support pipes would be ~$160 and maybe less from a wholesale plumbing supplier. Add another $10 for nylon zip ties.

    If the soil moisture content in your area is questionable (you should be able to determine this when digging the trenches), you could enhance the soil moisture content around the loop pipes during dry periods by connecting the tops of the 5' PVC support pipes with "T"s and more 3/4" PVC pipe running the length of trench with all connected together at the end nearest the house with a connection for a garden hose. This way, if you find your geothermal loop temperatures are getting too cold in deep winter or too hot in late summer, you can flood the soil around the pipes with water to enhance heat transfer.

    Inexpensive 3/4" 200 psi SDR 21 PVC pipe @ Lowes: https://www.lowes.com/pd/Charlotte-Pipe-3-4-in-x-20-ft-200-SDR-21-PVC-Pipe/3359602
  5. wing

    wing Member

    yes, i just caught that there are only two trenches 300 feet long so you'll have two loops per trench in a four square pattern, which makes for a 30 percent performance degradation vs. two pipe per trench as per Siegenthaler.

    really that wont do, at best it will be marginal and risk non performance of all the downstream heat pump and distribution elements.

    maybe the contractor is trying to minimize the amount of trenching 'due to rock ' ?

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