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Old 07-14-15, 11:17 PM   #1
MEMPHIS91
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Default DX Geothermal Pond 4 ton Heat Pump

Greetings and welcome to my largest project yet, and by far the most expensive. I currently have a old 10 seer 4 ton air source heat pump that is on its last leg. I had plans to replace with a new unit but considering the cost and how doable this build looks I plan to build a DX (thats right no water pumps) geothermal heat pump (R290 of course) that uses a 3 acre, 17 feet deep pond/lake that is 40 feet from the edge of my house.
I currently have NO parts for this build. Of course I will be buying the copper tubing and I have all my sources out looking for a suitable compressor.
I really don't know where to start with all the data so might as well give a diagram of the house and 3 options I have thought up.

This is the distance that everything thing is apart. The pond at its deepest is 17 feet deep, and I get average temps in the 60's.


This is just a few ideas I had. No numbers are solid just guesses based on just a little math.

1 is 2 long loops buried deep (I have free access to a back hoe, so as deep as I need to go) that come out into the pond about 6 inches from the bottom.

2 is taking 2 larger lines out to the edge of the pond and brazing in a manifold that splits into 3 (or more/less) loops.

3 is long lines from the house to the compressor/s at the pond with a manifold/or long loops.

I'm really open to any and all suggestions.

I am thinking that about 800' of 5/8 copper tubing would be about right. So the smaller runs will be longer.

Also should I have 2 compressors, one 4 ton sized, and one 2.5 ton sized and hooked up with reversing valves so I can only use the 2.5 ton on mid days and for dehumidifying.

Thanks in advanced for all warnings/ideas/designs/complaints/or general help.

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Old 07-15-15, 02:51 AM   #2
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Quote:
Originally Posted by MEMPHIS91 View Post
Greetings and welcome to my largest project yet, and by far the most expensive. I currently have a old 10 seer 4 ton air source heat pump that is on its last leg. I had plans to replace with a new unit but considering the cost and how doable this build looks I plan to build a DX (thats right no water pumps) geothermal heat pump (R290 of course) that uses a 3 acre, 17 feet deep pond/lake that is 40 feet from the edge of my house.
Oh boy! One amazing project after another!!

Personally, I think that the pond scheme could put you right up against the maximum limits of DX feasibility, what with lubrication issues, etc.

I have seen schemes where GSHP systems of the size you are talking about have worked.

Here is how they do it:


The advantages are that all of the copper loops, return to a central manifold. Also, since they are on a slant, the vertical distance (head loss) is reduced, while the linear exposure to the earth (water) is the same.

You might be able to use parts of this general scheme for your mega project.

If it were me, I would go with HDPE piping, which is not such a beast to weld as it might first appear.

In the grand scheme of things, going from copper to plastic results in about 15% reduction of heat transfer per linear foot. The remedy is to put in more plastic pipe.

Once welded, HDPE is eternal. It is very tough, and totally immune to corrosion.

You could run your HDPE up to your compressor shed, and go through a HX. Since it will be closed loop, a correctly sized brazed plate would be a perfect choice.

Also, please note that a 4-Ton compressor unit (and associated copper if you go this route) will contain a non-trival amount of propane and you do not want that to be in, or even up-hill of your house. Definitely outside, definitely downhill, preferably at a modest distance.

Them's my thoughts...

Go for it, however you will.

Best of luck,

-AC_Hacker
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Old 07-15-15, 07:32 AM   #3
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AC, yeah I was wondering about the head of the system. But I've seen buildings that are more than 50 feet high with the site handler on the to floor. I wonder if they are special compressor or over sized to make the climb. I plan to research more about the max head pressure of different compressors and their ability to keep lubricate moving. But it does look like a manifold style system is most common. Even if not at the eaters edge I can go crazy deep 6+ feet and not have much temputure fluxuation.
I have thought of doing it with hdpe and pumps. I just really don't want the extra power cost and the freeze potincial. But as I'm probably looking at over 2 grand for copper it might just be the best option. And doing it in a small pump House would be a great idea.
Yes propane in the set up would all be down hill but still a large risk. In willing to get it charged by a local guy that uses the new stuff called MO99. Seems to work better than R22 but acts more like propane in that in lowers head pressure.
Thanks for your thoughts AC!
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Old 07-15-15, 08:25 AM   #4
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Memphis91

What a great project. I wouldn't be able to find it again but I saw the amount of copper/
ton of heat was 100ft. so a loop of 50 out and 50 back to the manifold. For HDPE its 600ft/ton. I believe for the residential installation they had used 3/8 copper tube brazed to a manifold. When I was installing a ground loop with HDPE 1" the cost was $1.00/ foot and I see that 3/8 tube is about that cost/foot and you will use a lot less.

Look at it this way an outside unit (residential) uses 5/16 and 3/8 Dia copper in their aluminum fins.

There is a copper product now that has a thin plastic skin that will mediate corrosion.

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Old 07-15-15, 11:27 AM   #5
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...but I saw the amount of copper/ton of heat was 100ft. so a loop of 50 out and 50 back to the manifold. For HDPE its 600ft/ton...
It would be a good idea to get several verified sources for these estimates.

When I was tracking this kind of thing down, I found a Canadian GSHP company that was doing vertical holes, and their practice was that DX in copper had a 15% advantage over HDPE & water, in other words, a DX bore hole (copper) could be 15% shorter than the same configuration with HDPE and water and deliver the equivalent heat.

Six-to-one advantage of DX & copper sounds extreme to me. I suppose some testing would be in order here, before ordering materials and hoping for the best.

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Old 07-15-15, 09:07 AM   #6
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How about a compromise (copper pipe vs plastic)? Use plastic for the runs out to and from the pond. Then only use the amount of copper at the bottom of the pond for the true exchange loop.

Seems a waste to use expensive copper for the long runs to and from pond bottom.

One thing is certain, you need to measure the temp at the bottom of this lake. I suspect it is 65-70 F even though the surface water is 95 or so. This allows you to calculate the delta T across your heat exchanger and then work backwards to figure out how much copper pipe to put in. It may be cheaper to buy thinner (1/4 inch) copper and do a few parallel loops to get your total surface area.

The ratio of surface area to volume gets much higher in smaller diameter pipe. That maximizes your BTU transfer.

I second AC's warning about propane. This gas has a density higher than air and when (not if) you have a leak, it could get . . . . . interesting. Well know situations in basements of a propane bottle slowly leaking, collecting in a "pool" on the floor, and then someone turns on a light switch.

In the absence of a dielectric contact, copper does not corrode per se, but develops a surface green copper oxide. In the trade, this is known as "patina". Acid water will corrode copper, but that is a different issue as a pond will not have a pH of < 5!

This sounds great, but look carefully at the costs of this large amount (800 feet, 250 meters) of copper tubing.

I have consulted on several projects that have used closed loop pond loops for geothermal units. If I recall, all of them were HDPE plastic and about 300 feet per ton. Inexpensive to purchase, but would take a LOT of gas to fill the entire volume. I sure would use inexpensive propane to test this out, then use a higher priced gas.

How is the water heater going? What temp can it get the tank water up to?


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Old 07-15-15, 11:48 AM   #7
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I have consulted on several projects that have used closed loop pond loops for geothermal units. If I recall, all of them were HDPE plastic and about 300 feet per ton. Inexpensive to purchase, but would take a LOT of gas to fill the entire volume. I sure would use inexpensive propane to test this out, then use a higher priced gas.
I think that using HDPE for refrigeration gas would be a very, very bad idea.

I have serious doubt about the permeability of HDPE to pressurized refrigeration gases.

I'm concerned about how it would hold up to extreme temperature cycling.

And I also have concerns about how HDPE would hold up to the ravages of pressurized lubricants and refrigerants, whether hydrocarbon or proprietary global warming gases.

If this was a viable idea, it would be possible to find thousands of success stories.

Do they exist?

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Old 07-15-15, 11:56 AM   #8
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AC, that is why I suggested it - a hack! Good points on permeability.

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Old 07-15-15, 12:03 PM   #9
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Just run a pump and dump system, using your endless supply of pond water as heat transfer fluid. You could still run a dx coil indoors if you wanted to. Pipe your suction line underground, and if the pond water is cold, it would pick up some heat underground on the way in. You could even use copper pipe if you wanted more heat transfer. Much safer, less refrigerant, way less expensively accomplished. Spend some extra money on a hx and pump.
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Old 07-15-15, 05:12 PM   #10
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Wow good suggestions and ideas. I'm looking into the good and the bad of all that's been said. But Jeffs ideas of pump and dump just took the cake I think. I didn't like the idea of closed loop with water but I never thought about open loop. This would be so easy that it's almost wrong for me not to do it. I would need a pump probably 8 GPM but that is the only down side. I'm thinking a super insulated 1 inch or bigger PVC pipe run out of the pump straight down 7 feet deep all the way to the pond edge and then 17 feet down to the bottom where the coolest/hottest water should be. And then the dump can just be maybe 2 feet deep to the edge of the pond.
How about building my own 4 ton hx?
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