Installing a horizontal loop under a septic field?

Discussion in 'General Discussions' started by fjkeith, Apr 6, 2009.

  1. fjkeith

    fjkeith New Member

    I am in the early planning stages of building a house in a rural area (well water, no county sewer service). Since I have to dig a bunch of 36" wide trenches for my septic absorption field I was thinking...

    Why not install the horizontal loop UNDER the septic field? At the risk of grossing some out, when we are using the house, we will be "wetting" the field regularly, which should improve the thermal conductivity of the installation.

    First stop, I checked with the county health dept. They gave a preliminary OK as long as I backfilled over the loop with an approved fill material (in this case a red quartz sand available from a nearby quarry in North GA).

    From the bottom of the trench to the top I'm looking at:
    - Thin layer of sand for leveling
    - Slinky or web of straight pipes
    - 12" of additional sand
    - ?" of gravel (for the septic field)
    - either septic chamber or perf pipe (not sure about septic field design yet)
    - additional gravel
    - backfill dirt

    Anyone ever done this before? Conceptual fatal flaws? The heat produced in the summer should improve the bacterial action of the septic field. In the winter, the cold loop will retard it somewhat, but the county engineer was not concerned about this. Either way when the house is in use the field will be wetted regularly.

    FYI, I am an electrical engineer with modest mechanical training, so hit me with stuff as technical as you want! :)

  2. fjkeith

    fjkeith New Member

    And, are any of the HDPE types known to be sensitive to sewer graywater?
  3. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    IGSHPA , strongly recommends that you do not put a geothermal field under a septic system.
  4. Bergy

    Bergy Member Industry Professional Forum Leader

    Most codes

    will NOT ALLOW loops even close to the septic. You need to check with ANY government agents that might have a say...

  5. fjkeith

    fjkeith New Member

    Thanks for the feedback. I just ordered the six manual set from the IGSHPA this afternoon, so have not had a chance to review.

    Is the concern one of cross-contamination, or something else?
  6. fjkeith

    fjkeith New Member

    Bergy, is it health dept code issue or some codes that relate to geo loops?
  7. fjkeith

    fjkeith New Member

    I found this at

    3.10.1. Horizontal Loops

    The loop of pipe should not be installed beneath any part of a septic system. Disturbance of the soil underneath a septic bed can lead to inadequate treatment of sewage. Also, heat from the pipes can increase biological growth in the septic tanks, which could lead to costly septic system repairs. Repair of one system would require the excavation of the other.

    My health dept thinks the heat is a GOOD thing, and how it will get into the TANK is beyond me...the heat will be out in the absorbtion field trenches...
  8. fjkeith

    fjkeith New Member

    I found this at

    3.10.1. Horizontal Loops

    The loop of pipe should not be installed beneath any part of a septic system. Disturbance of the soil underneath a septic bed can lead to inadequate treatment of sewage. Also, heat from the pipes can increase biological growth in the septic tanks, which could lead to costly septic system repairs. Repair of one system would require the excavation of the other.

    The backfill requirement that the county laid on me should address the "disturbance of soil" concern, and I have no idea how the heat from the absorption field will get into the septic TANK. Through the pipes from the distribution box? Still seems like a reasonable design to me :)
  9. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Been through this

    We've tossed this one around locally for some time. Included in the discussion have been the Interior Health Agency and P.Eng.'s who design septic fields. As far as geo goes, it isn't really an issue. The vertical separation of systems doesn't really lead to synergies. One is ideally looking for wet conditions the other is looking for dry conditions. One requires heat to operate. The other removes heat.

    But the key is saving a bit of money and space in your excavating. It's the "engineered" backfill requirement that usually makes this a no-go. From the depth of your geo to the depth of your infiltration bed it all needs to be properly engineered (at least around here). This means no backfilling with native soils and having to bring in a fairly large volume of fill. Often this fill is of poorer quality for heat transfer than the native material.

    So, while it would be nice, it just doesn't seem to have any benefits to placing the systems close to one another.
  10. fjkeith

    fjkeith New Member


    I agree, the issue is one of saving money and reducing the total "trenched" area of ones land. Your comment about the heat transfer properties of the engineered vs. native soil fills is interesting, though. Is there a reasonable way for me to determine these values for 1. my native soil and 2. the red quartz sand that the local health dept want used as a fill?

    Ultimately the true bottom of the septic trench is at the same elevation as the location of my geo loop. I figured they spec'd a sand fill for optimum compaction, some bacterial action, and diffusion. But I'm really guessing here.

    In the concept designs that you have tossed around, how much vertical separation did you have between the loop and the septic materials? What sorts of engineered backfill materials did you consider?

    thanks, Joe
  11. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader



    There are standard tables for the thermoconductivities of various soils. Your red "quartz" soil might actually be pretty good. Most septic field material is designed to be porous with a good permeability. This means air, which is not a good heat transfer media. We have our ground loops at 6'+ and the septic usually around 16" so you're looking at a 4' separation. That is a lot of material to have to bring in (and to take away).
  12. fjkeith

    fjkeith New Member

    Loop depth

    Yikes! you're right about bringing in a lot of material. 4' is a lot of fill.

    For the SE US (NE Georgia), is there a recommended depth for the geo loops? I was thinking that my loops would be at about 4' down. If they need to be 6' down then I might end up in the same boat, deciding to separate my two trench sets. Do the IGSHPA manuals have regional recommendations for geo loop depth?

    Any suggestions for links where I can find soil thermal conductivity tables?

    thanks for your input!
  13. sateliteman

    sateliteman Member

    how deep you gonna dig?

    Believe me, I,m not an expert but like your idea. Since the health dept already gave you a preliminary approval, it must be ok to do it. I think I would bury the slinky extra deep so the heat from it wouldn,t interfere with the septic field. Supose you had the ditch about 8 or 9 feet deep. Put in a little sand to level for the slinky. Nex if your dirt didn,t have any rock you could put a little of original dirt without rocks next to the slinky or put a thin layer of sand on it. Then put some of your dirt back on top of that to the level where you wanted to start the leech bed. Flood it with lots of water to insure that the pipe has no air pockets and your fill will compact better. It would be harder to dig deeper but you could get dual use for the trenchs and not have to dig more trenches. I would think that the deep trenchs wouldn,t interfere with the septic field all that much. Just make sure you don,t get into the trench at that depth. I think the osha regulation doesn,t allow getting into a ditch deeper than 5 feet without the walls of the ditch bieing blocked. I have no experience with this but this is my idea of the way I would do it. :rolleyes: vic
  14. fjkeith

    fjkeith New Member

    Fill material

    I like the deep trench idea, except... THe health dept said the fill material has to be a particular local sand, NOT the original soil. I suspect that adequate compaction and later settling (which will distort the septic trenches) are the concerns. I could certainly ask though. :)
  15. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Not sure if plumbing code has anything, but mechanical code itself would not prohibit installation of loops near drain field. It does however require you to follow manufacturer's installation instructions, and that is where you are most likely to find prohibition against this high risk/low reward plan. A minimum of 10' from the drainfield is required by every brand I've worked with and therefore code. It is remarkable how little there is about GSHP's in the IRC/IMC.
    It would be a shame to find that you had frozen all or part of the field, or if you were to damage your ground loops during a drain field repair.
    The symbiosis of the design engineer vs the field engineer is for us field tech's to tell you that the potential for problems (in either function or code compliance) out weighs the benefit, hopefully the design engineers will take our word for it.
    Good Luck,
  16. zach

    zach Member Forum Leader

    conventional septic or sand system

    Does your native soil perk to HD specs? Or, are you being told by the local HD you need to bring in specific approved fill to get the perk you need?

    Bottom line question: can you install a conventional system or do you need a raised bed type set-up?

  17. fjkeith

    fjkeith New Member

    I'm perc'in

    The soil where I'm planning to put the absorption field has passed the Level 3 soil test for percolation, so no raised bed needed. I'm debating between conventional pipe-n-gravel vs. chamber-type systems. I may combine chamber and gravel or chamber and sand.
  18. zach

    zach Member Forum Leader


    I am not an expert, just a skilled homeowner. But, last summer I installed/renovated the septic system on our property. I installed a new tank and set a new field enlarging the system from two bedroom design to four bedroom design.

    The system as it sat was a hybrid raised bed(gravity fed, no pump) and the local HD required me to bring in another foot of sand as they did not like the depth of the sand fill in the existing field. I now have about 4.5 feet of sand fill in the field.

    Your idea is certainly interesting but I would be concerned about disturbing the native soil. You definitely do not want to have septic issues. If you can do it, those chambers are the way to go. Smaller field and an easier install.

    Also, having to truck in fill can get expensive quick too. Here in upstate NY I paid about 12 bucks a yard for septic sand delivered. I had close to 300 yards brought in. It was the largest expense of the job.

    Just my .02 and good luck

  19. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    You had me worried I thought you where goind to re-dig the septic for a loop field.

  20. zach

    zach Member Forum Leader


    No way, no more digging there. The wife would string me up after looking at that mess most of last summer!

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