A twist: Horizontal Loop in River Alluvium

Discussion in 'Vertical and Horizontal Loops' started by RiverMan, Mar 30, 2021.

  1. RiverMan

    RiverMan New Member

    I am new here. In search of some experience in a non-typical situation. I am a future homeowner looking at installing a Geothermal system for the home. My geothermal contractors are unsure about what I have and are desiring to over engineer the ground loop and spend too much of my money.

    The property has a small river running through it. In many parts of the country you would call it a creek. There is a small pond on the property. It is dug into what once was dredge tailings from dredge gold mining in the 1800s. A hydrology engineer told me the pond water is there as part of the river alluvium. Basically, the part of the river that is below ground, but nominally at the same level as the river itself. My observations are, and the engineer confirmed, that the water in my pond is part of the river system. It actually visually flows in the same direction as the river. There has been no observation of the river ever being dry in the channel. It is constantly moving in the pond. The pond was never frozen this past winter, even with extended periods of cold near 0 degrees F. I took temperature measurements during the winter and the pond surface temp was 39 degrees to 42 degrees throughout the winter. If you want to get a pictorial, look up on Google Earth: 123 Davenport Loop, Breckenridge, CO. You'll see the river running through the north(east to west) and the pond toward the west end of the property.

    What I am thinking I would do is use the currently excavated pond as a hybrid river loop. Since the water is continuously being exchanged by the moving water in the river alluvium, I have a similar environment to a river loop. I would place the loop as a slinky in the pond, then cover with a material with porosity to not inhibit the flow, and then fill in pond. (it is not very attractive).

    I would gain the advantage of not needing as much length as the Geo guys are thinking as well as having a pre-excavated pit. They proposed 500 feet per ton as if this was a standard horizontal ground loop. (I need 14 tons of capacity for heat only, no cooling load). My reasoning is my loop will pull the cold out of the loop quickly because of the moving water in the alluvium.

    My questions specifically are:

    1) Am I crazy here that the river alluvium is truly similar to a river loop and I will have as good of heat exchange as I desire? If I am crazy, please tell me why.
    2) If I were to bed the loop in a material, what should it be? Sand? Coarse sand? Pea gravel? How do a we balance the benefits of the bedding material for optimizing water flow but minimizing any potential damage to the loop?
    3) As anyone else have a working model that has a similar ecosystem? What are your experiences?

    I look forward to hearing from you all.
  2. gsmith22

    gsmith22 Active Member Forum Leader

    any reason to not do an open loop - pulling water from stream, running through system, and putting it back? eliminate the ground/river/pond loop completely

    Edit: just occurred to me that freezing might be an issue with 40 degree water and the heat pumps dropping the outgoing water temp below freezing. would definately need the ground loop with antifreeze if the stream temp is around 40 in winter
  3. SShaw

    SShaw Active Member Forum Leader

    I would think the loop would not perform similar to a river loop, unless the velocity of water movement is equivalent to the river, and 100% of the pipe's surface is exposed to the water as it would be in a river.
  4. wing

    wing Member

    Wow, 14 tons heating load. What are you planning on building ?

    Given the millions your project is going to cost, I would recommend to take a conservative approach to the loop field and plan on 7000 feet of loop. No sense scrimping to save a few thousand on this aspect of the job.
  5. RiverMan

    RiverMan New Member

    Thanks for the replies so far. 7000 feet slinky? Overlapping? I’m not worried about pipe cost. More so excavation.

    I will say when I got the numbers from the HERS consultant on the load size, I was surprised. I don’t think I’m too out of line of cases I’ve studied on this forum. Most similar sized cases were showing 6-7 tons in the Midwest or Texas. I’m at 4900 square feet living space plus a heated two car garage. But we are at 9300 feet altitude. Rarely do we need cooling at this altitude. I am planning on the ability of running cool water through the radiant floors to take the edge off occasionally. My local jurisdiction has the strictest, greenest building codes in the state.

    I’m am watching costs everywhere because lumber is currently 3X, steel 2x, pre COVID prices.
  6. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    I am late to the party, but here now.
    I would think that a loop in your configuration could be sized using a pond model, and do well. Using a river model with the UK amount of turnover in your pond would be risky. So if you can use a pond model and make it work with ft of pipe vs. pond size, any turn over you get in the pond via the tailings being part of the river system will help the performance of the loop, not hurt it.
    Hope this helps!
  7. SShaw

    SShaw Active Member Forum Leader

    Are you saying you think the proposed idea of burying the loop under some sort of fill material will work the same as a loop in clear water? Seems like physics would dictate it would perform like a ground loop once it's buried.

    Looking at the Google images the pond looks to be about 60 feet in diameter, or 0.1 acre in area. Depth is unknown. Do you think the proposed idea of burying the pipe in the area of this pond (whether filled-in or open water) would work? I believe ASHRAE recommends a maximum of 10 tons per acre, as assumes something like 10' minimum depth of water.

    A heat exchanger submerged in the river (if large enough) or vertical bores seem like better solutions. but someone with all the details needs to do some design work.
  8. gsmith22

    gsmith22 Active Member Forum Leader

    you guys are better than me, I can't even see the pond on the aerial. And at best, that looks like a "creek" and nothing like a river. SShaw is right, ponds/rivers assume complete water contact with the loop not dirt/stones/soil. Engineers are right to oversize that loop because it is more likely to work like a horizontal ground loop in saturated soil than a river or pond loop.
  9. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    I am not so sure about being better... but geo is not complicated, people are. I have not and will not google earth this guys project. He asked a question and I answered. If you submerge pipe in water that never freezes in winter, and it has a recharge of source water, the only question is what is the load it will handle. In my thinking this is or would be more of a geo battery, rather than a field or a loop.

    The late great! Mark Custis created a geo battery in a buried tank that was filled with coils of copper pipe that was fed by a slow feeding natural spring. All I am saying is that you can get the data on recharge by doing a simple die trace and this discussion is mute.

    If the OP can save some coin in the long run, than he or the installer should get the data upfront and then decide.
  10. gsmith22

    gsmith22 Active Member Forum Leader

    Eric, I think the issue is that it really isn't a loop buried in water (pond or river). At best its a loop buried in stones with water moving through the stones. And 50% (or thereabouts) water isn't 100% water as there will be loop to loose stone contact ala a boring with incomplete grout.

    OP, maybe design a pond loop and then design a ground loop in saturated earth and take the average of the two loop lengths?

    14tons is a huge load and is more akin to commercial work. And it isn't uncommon in commercial work to test ground heat exchange to verify design parameters.
  11. Eric Kurtz

    Eric Kurtz Member

    I like the idea of testing the actual flow rate of water thru the ground. If the water in the loop field is getting changed over every day (as an example), that is alot of potential btus available. Then the question comes down to the transfer capacity of the actual pipe in the ground. What is the minimum feet of pipe needed to transfer 28000 btu per hour at 39* surrounding ground temp. Which would basically be the equivalent of a pond loop. These are very rough ideas, but my gut would say that it is definitely worth sinking some time and dollars studying the feasibility of it.

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