Nebraska Geo Thermal Open Loop Design

Discussion in 'General Discussions' started by Jason Flippin, Apr 30, 2018.

  1. Jason Flippin

    Jason Flippin New Member

    A couple questions that I wanted to bounce off the experts. I have a dual HVAC system currently split between upstairs/downstairs. I am looking at the options of replacing the upstairs unit with a Geo-thermal system currently and have been kicking around possible design scenarios based on what I understand. I am currently on a well and it sounds like my acquifer plenty deep and the gpm on my pump could handle the load of a 5 ton geo thermal heat pump (8.5-10 gpm~ the way I understand it for the heat pump). I still need to have the water tested to make sure the water meets the heat pump manufacturer's water quality requirements, but that is the last hurdle that I am aware of on that front.

    A couple questions that I haven't figured out and would like to get some feedback on.
    I am most interested in an open loop because of the consistent water temperature year round and being able to leverage my existing well to provide that consistent water. I suppose I could be talked into a closed loop, but at this point I am leaning toward open.

    The second question I haven't figured out is where to dump the water. One HVAC technician suggested I have an empty well drilled and dump the heated water back in the acqufier. That sounded great until I spoke with the well company this morning. They said due to EPA/permits/length of drilling, that it wouldn't be feasible. He suggested building a holding pond. Which leads to my next question - We live on a small 5 acre plot and have some land we could build a small pond on and have access to equipment to dig it out to help avoid that part of the cost. But, I am sure there are details I am missing/costs I am missing for that. Is that the best way to dump an open loop? Is there a certain size I need to build or avoid? Would this be the most recommended way?

    Lastly, if all of this sounds good, I would like make sure I am designing everything to eventually handle replacement of my second (downstairs) HVAC unit so it can be on the geothermal as well. I will probably need another pump at that point to handle the additional load, but do I need to accommodate for anything on the geothermal heat pump on the first system? Ultimately, I am just trying to make sure I don't have to redo things once I get the second system eventually going. And any heat pump manufacturers I should look for or stay away from?

    Thank you!
    Jason
     
  2. arkie6

    arkie6 Member Forum Leader

    You might also consider using your well as a standing column well where you draw water from the bottom of your well and then dump the water from your heat exchanger back into the well at the top. This works better with wells having a large volume of water, i.e. deep and mostly full of water. With this type of well, you will likely need to "bleed" of some water from you heat exchanger to outside of the well when the well water temperature gets too cold or too hot. By bleeding some of the water away from the well, you allow fresh water to flow into the well to makeup for the water bled off.

    Do you use your well for domestic water use also? How deep is your well and what is the diameter? How far down to the static water level? What HP pump do you have?
     
  3. Jason Flippin

    Jason Flippin New Member

    Yep, it's for domestic use also. I was told it was a 2hp I believe (20gpm). They said it was 300ft to drill another. So, I am not sure if that is to the top or the bottom or how that is rated. I am not sure of the diameter though, I will try to find out.
     
  4. arkie6

    arkie6 Member Forum Leader

    If you are running a 2HP (1.5 kW) well pump at 50% duty cycle to supply a geothermal unit, that will severely impact the overall operating costs of the geothermal HVAC. How far down from your home to the top of the water level in your well? The operating costs to lift water are not insignificant.

    What does your current HVAC system use for heating? Natural gas? Propane? If you have natural gas heat and electric A/C, you are unlikely to see any significant operational savings by going to geothermal HVAC being supplied by a 2HP water pump.
     
  5. Jason Flippin

    Jason Flippin New Member

    We are on electric heat pump and propane. Ok that makes good sense. I have an electrical panel monitoring device (Sense) and it doesn't appear that my pump costs much to run. I understand the load will increase significantly if I was pumping for the geothermal but right how its an estimated 13$ a year cost.

    Here's the conundrum at least the way I understand it. It sounds like using a pond is ideal so I don't have to deal with the EPA for dumping water back down into the well. If I were to build a pond, I need to fill it (my wife is enamored with the idea of a swimming pond for the kids at this point, so I am not sure I can change this plan). So, the two ways to leverage a pond in my understanding would be -

    1. Use a glycol type closed loop solution and run it to the bottom of the pond for heat removal. Would this work the same as an underground closed loop in the winter or would the water temperature be significantly lower? I would still need to fill the pond and maintain the water level. I would assume I would need to use the well for that? The advantages I see would be from a trenching perspective. I wouldn't need to go as deep as I wouldn't have a liquid that could freeze in the lines.

    2. Build an open loop solution and dump into the pond. This would (hopefully) maintain the water level. Might be an issue in the summer due to the evaporation. The disadvantages being I would need to trench 5' to prevent freezing. The other disadvantage would be water quality that could affect my geothermal heat pump and additional wear on my current well pump/electrical costs to pump the water.

    And I guess the 3rd option would be build a closed loop system 5' underground and not use a pond at all but I would need 500' of line (5ton unit) to temperature absorption (not sure if thats the right term). One question I don't know the answer to is I have another system I would like to eventually flip my other hvac to geothermal. So, would that mean I would need 1000' line? I probably should have a better understanding of how to size geothermal. I'm not sure if 5ton is the right size, that was just what was recommended as more of a general go to. Today my systems are (2) 2ton outside condenseners

    Appreciate all the help and advise!
     
  6. nc73

    nc73 Member

    I would go with option 1. That is my plan and you still get the tax credit for the pond. Make sure there is enough watershed for the pond and you won't have to worry about keeping it full. You will need a depth of at least 8ft.
     
  7. Jason Flippin

    Jason Flippin New Member

    Does the pond depth depend on the region? How do you size that? I am in Nebraska and winters can get extremely cold below freezing and summers extremely humid. Does that factor in?

    Honestly, I was starting to lean toward the close loop underground the past few days, but I was told that I need (4) 3' trenches wide, 6' deep and 100' long today to handle a single 4 ton system. Considering I would eventually like convert both systems to 4 ton geothermal units, I would need to double that. I am starting to think the pond is the way to go again, but I am not sure how to determine how big / deep the pond needs to be to handle that.

    When you say watershed, are you referring to water run off from rain / excess sprinkler, etc? I am just really concerned about building a pond and then having to constantly pump water from my well so it isn't bare. But, I don't know much about it.

    Does an open loop system into a pond not make much sense? Should I rule that out indefinitely?
     
  8. nc73

    nc73 Member

    According to waterfurnace they recommend 1/2 acre or something like 3000 sqft per ton. yes watershed is runoff, etc. If you have enough clay, you should not have issues but you never know. Open loop will depend on water quality and it is the cheapest route in the short term. You also have to consider pumping costs. Long term i would do closed loop, pond or trench.
     
  9. Jason Flippin

    Jason Flippin New Member

    Ok, appreciate the info. We are definitely full of clay out here, so that shouldn't a problem. When you size a closed loop system, is there a general recommendation per ton? I had been using the recommendations of this geoexcel site (http://www.geoexcel.com/OpenLoopVsClosedLoop.htm). They recommend a 2' wide 6' deep pipe with 3 pipes buried at the bottom of the trench and 3 more a foot higher. Is that the right way to do it? Or is the slinky method a better way? It sounds like 4ton would require 400' of total trench, but does that count as 200' to the end of the trench and then back to the house for the other 200' on the other side of the trench? I hope that makes sense, but I am having a hard time comparing what makes the most sense for our situation since we would have to construct a pond or dig a trench(s).
     
  10. nc73

    nc73 Member

    For closed ground loop it depends on soil type at 6ft or so. There really isn't a rule of thumb. You can design for any type of slinky or 3 pipe. 4 pipe buried. I forgot which program i used. There is a free one out there that will help you calculate what is needed for soil type, temperature, etc. Open loop is easiest, pond being second. If not those 2, you should really get a pro. I think GeoJerry will sell you their design for a reasonable fee.
     
  11. arkie6

    arkie6 Member Forum Leader

    Climatemaster has a geothermal loop sizing program on their site that you can download called GeoDesigner Software. It is under their Residential Professional page under Sales and Sizing Tools. They also have a Pressure Drop Calculator tool (spreadsheet) which is useful also. I remember the GeoDesigner tool does require you to input your location and soil type and depth for loop pipes, but I don't recall if it has inputs for pond size/depth. I suspect it does, but I wasn't going that route so I don't recall if it provides that. With this tool you will have to input a model of Climatemaster geothermal unit for sizing the loop. If you are not using a Climatemaster unit, you will need to figure out which one closely matches your selected unit capacity and efficiency.
     
  12. arkie6

    arkie6 Member Forum Leader

    A general rule of thumb for horizontal closed loop at >6' depth in reasonable soils (not completely dry sand or clay) is 600' to 800' of 3/4" loop pipe per ton of HVAC. Assuming 600' of pipe per ton, for a 4 ton unit you would need four (4) parallel connected circuits of 600' of 3/4" pipe connected via headers to minimum 1-1/4" supply and return pipes (if >100' to the loop circuits then larger supply and return pipes such as 1-1/2" or 2" need to be considered). These individual 600' 3/4" pipe circuits can be configured in different ways. One common way is a 2' wide backhoe trench 6' deep and 300' long. Pipes are placed at the bottom of the trench on each side and back filled. Each trench generally needs to be kept ~10' away from the next trench. Another option is to do a 600' slinky where you can typically use a 6' deep x 3' wide trench that is ~150' long with the slinky laid flat in the bottom of the trench. Or you can do a 8' deep trench of 2' width or less (a chain trencher works here) and install the slinky oriented vertically.

    If you have a area suited for a pond (naturally low spot that receives run-off from rainfall in the area, i.e the water shed) that is an option. I wouldn't consider a pond for this application if it doesn't receive natural rain fall run-off as the amount of water needed to fill and maintain the pond full from a well would be enormous just from evaporation. You would need at least 1/2 acre of pond surface with an average depth of 8' or more for a pond loop. Expect the pond water to get uncomfortable warm in the summer if running A/C unless pond is very large/deep and/or spring fed. With a pond loop you can get by with shorter loop pipes, but I don't have those numbers off-hand. If you don't have that pond depth available, you could install the ground loop pipes in the bottom of the pond and lightly cover with dirt prior to filling the pond. You would treat these loops just as ground loops as far as length of pipe is concerned, but you would be assured that the soil remains fully saturated which helps with heat transfer from the pipes to the earth.
     
  13. Jason Flippin

    Jason Flippin New Member

    Thanks for all of the detail! I think we are going to move forward with the horizontal closed loop system. I am trying to locate someone who can directional bore into our utility room and pop out in our back area somewhere. From there we should have plenty of space to dig and fan out in different trenches with a backhoe as deep and far as we need. For that utility room to the area we would split out at, are you referring to that as the header? That would probably be about 75' from the utility to the header spot. Would it be safe to just install a 1-1/2 or 2" for this? Just to keep my honest, I would install a 1-1/2 or 2" from the utility to the header and than split out into 2 300' trenches at least 6' deep with 4 runs of 3/4? Or would it be better to install the slinky? Any pros or cons to either method? I have access to a track hoe, but I could rent a trencher if that is a better way. Thanks again for all of the info.
     
  14. arkie6

    arkie6 Member Forum Leader

    There is no performance downside to the 1-1/2" header pipe. Bigger pipe = lower flow resistance = less pumping power required. That isn't necessarily the case on the individual loop circuits as you want to maintain the flow high enough to get out of the laminar flow region and into or near turbulent flow which increases heat transfer (Reynolds number >2500 or so). A downside to the bigger header pipe and fittings is that it is heavier, costs more, and is just harder to handle during installation.

    In my rule of thumb example, you would need four (4) trenches 300' long and 6' deep. You would install one 600' loop of 3/4" pipe in each trench (300' down and 300' back). The advantage of the slinky is that it reduces digging (shorter trenches) and the footprint of the loop field. This reduced footprint of the loop field is also a downside as it is the earth that is the limiting factor in the heat transfer and the more earth that you have the loop pipes exposed to, the better the transfer of heat.

    Using the straight pipe example: 4 trenches, 300' long, 2' wide, 6' deep, with 10' spacing between trenches yields a loop field area of ~14,400 sq ft.
    Using the slinky pipe example: 4 trenches, 150' long, 3' wide, 6' deep, with 10' spacing between trenches yields a loop field area of ~7,800 sq ft.
     
  15. Jason Flippin

    Jason Flippin New Member

    I assume it wouldn't be a problem to run 300 feet in one trench, loop around and head back 300 in another trench (effectively doubling the number of trenches I would need). The reason I ask is I can borrow a trencher that will go quite a bit faster, but it's only going to get me 6-8" maybe. I have a field I'll be running the trenches into so the extra trenches shouldn't be an issue if it makes sense. I wanted to see if I could get your thoughts on that prior to doing anything.
     
  16. arkie6

    arkie6 Member Forum Leader

    Down and back in separate 300' trenches is fine, probably better in fact because it allows wider spacing of the down and back loop pipes. Just allow plenty of room at the ends to make the turn. And by 6-8", I assume you mean 6-8' (feet) deep? That should be sufficient.
     
  17. Jason Flippin

    Jason Flippin New Member

    yep, sorry that is what I meant. OK great, I appreciate the guidance.
     
  18. Jason Flippin

    Jason Flippin New Member

    It looks like 63" may be the max depth this trencher is capable of. Is that deep enough or should it be at least 6'?
     
  19. arkie6

    arkie6 Member Forum Leader

    What is your frost depth there?
     
  20. Jason Flippin

    Jason Flippin New Member

Share This Page