if that soil next to the pipe does not get enough enough BTUs transfered to by the ground further away, it will cool down, reducing EWT and therefore COP.

Except that the EWT decrease caused by the insulating effect of HDPE exceeds that. The net effect is that more conductive pipe, in a system that is otherwise the same, always outperforms thermally (higher COP). GLD software confirms - you won't find any data, research paper or modeling software that says otherwise.

Practical matters, like cost and "HDPE is pretty good", I have no argument with.

Thanks for all the discussion, Newbies in the field (pun intended) of GSHP learn a lot from this. Actually site issues have changed the whole thought anyway! Originally I was going to use an excavator for the ground loop, which as you know takes some time, but would leave a large enough trench to use the cable. Since then I have found a rental trencher with a 5 foot digging depth, which will dig the trenches in a fraction of the time. Any reason a trencher is not as good as a wider trench?

If you have an insulating effect of the pipe, how can EWT increase? We are talking about heating mode here, where the water temperature in the pipe will significantly decrease if the heat exchange with the surrounding ground is decreased.


Sure, all things are equal, copper will always outperform HDPE, but there is a saturation curve, especially with geo systems. A good loopfield is always designed maximize heat exchange, so it is large enough to have the fluid inside the pipe take on pretty much the same temperature than the ground at the end of the loops. The temp sensors we have beside the pipe in the ground pretty much show the same temp as EWT at the end of the heating season.
How much better would it be with copper?

The extreme example is running down the ground to lets say 15F, when your loopfield is way too short, then it does not matter anymore if you have copper or HDPE, your are done, beyond the current limit of the refrigerant circuit in heat pumps. If you oversize it, it will also be right where the ground is, the extreme example is the open loop.

You simply need a certain amount of ground to pull a certain amount of heat out. Plus, if you reach that level, you can double the size of the heat exchanger and you get a slightly higher EWT, but not a higher efficiency over the year. As Joe always points out, there is a diminishing return here, you can spend an extra $5000 on a larger loopfield, and save $50 in operational costs. And it simple does not matter anymore if you have copper in the ground or not. The ground is the bottleneck.

Posted By whirnot on 03 Oct 2012 12:52 PM
Thanks for all the discussion, Newbies in the field (pun intended) of GSHP learn a lot from this. Actually site issues have changed the whole thought anyway! Originally I was going to use an excavator for the ground loop, which as you know takes some time, but would leave a large enough trench to use the cable. Since then I have found a rental trencher with a 5 foot digging depth, which will dig the trenches in a fraction of the time. Any reason a trencher is not as good as a wider trench?

No, just 5ft is kind of shallow, at least in my area. You might need some more ft in the ground. Where are located?

Central Oregon (Bend Area) I could also go a foot or so with a loader then trench, pretty easily, I was told to shoot for 4 to 5 feet.

In a milder, more balanced climate like Oregon, you can have a bit more leeway with the depth. You can compensate for it by lengthening the loop. This fundction is almost exponentially, meaning that going 2 ft shallower from 8 to 6 ft has much lesser of n impact than going from 6 to 4 ft, which needs almost an increase in loop lengths by 60%. so keep it as deep as you can, 5ft being much better than 4ft.

Posted By jonr on 03 Oct 2012 12:00 PM
The net effect is that more conductive pipe, in a system that
is otherwise the same, always outperforms thermally ...

Yes, in exactly the same sense that a solid silver 4 AWG line cord
"outperforms" pedestrian 14 AWG copper in providing an electrically
more conductive path between Hoover Dam and your DVD player.

...but I gotta admit, it *does* work wonders for MonsterCable Inc.,

Looby

whir.....a trencher will probably take longer than if you dug a wide trench and at 5' deep, it's gonna make just as much a mess.  A wider trench, you can put several pipes in.  I've installed a number of irrigation systems and putting all that dirt back into a narrow trench is a real PITA!

Just as a note.

A trencher that will attain a bury depth of 5' is no small machine and if it does not have a 3 way backfill blade I would be surprised and keep looking. I investigated the feasability of plowing in to a depth of 5' and found the cost and size of the machine counter productive to vertical IMOA. That machine cost nearly as much as a vertical drill rig.
Eric

Imagine if you did spread all that steel cable to the surface. Then imagine getting a dozer or a backhoe onto the field to fix a leak something. It sounds like a recipe for disaster IMHO.

Posted By whirnot on 27 Sep 2012 10:55 AM

I read somewhere that the majority of heat transfer between the ground and the ground loop occurs within 6 inches of the ground loop.  Since it is common knowledge that any temperature conductor can be made more efficient by adding more surface area, similar to the fins in a radiator, it would seem to me that if a temperture conducting medium such as steel cable were  installed serpentined, next to the ground loop it would create a more efficient thermal transfer.

See any reason that would not work?  Obviously if you had to buy the cable it would not be cost effective, but if you had thousands of feet available...........?

I was thinking, that at a rental rate of $550 per week, the 5 foot trencher would easily dig all the trenches in a couple of days. I can also rent an excavator but it would take a whole lot longer. I am dodging trees constantly (600 trees per acre) so I can wind around them with the trencher. The trench would be 5 inches wide, I can see the back filling being a little of a PITA.

Posted By whirnot on 10 Oct 2012 10:42 AM
I was thinking, that at a rental rate of $550 per week, the 5 foot trencher would easily dig all the trenches in a couple of days. I can also rent an excavator but it would take a whole lot longer. I am dodging trees constantly (600 trees per acre) so I can wind around them with the trencher. The trench would be 5 inches wide, I can see the back filling being a little of a PITA.

"Easily" becomes a very relative term if you really have that many trees. I trenched about 300' of drainage tile 2-3' underground to tie my downspouts into. I only have probably 20-25 trees per acre; and I still had a lot of issues with getting into tree roots. Tree size plays a part in that too, though. Mine are 30-60' tall; so the roots obviously extend quite far.

Posted By whirnot on 10 Oct 2012 10:42 AM
I was thinking, that at a rental rate of $550 per week, the 5 foot trencher would easily dig all the trenches in a couple of days.

I think you might be in for a big surprise.

Posted By geodean on 10 Oct 2012 02:57 PM

Posted By whirnot on 10 Oct 2012 10:42 AM
I was thinking, that at a rental rate of $550 per week, the 5 foot trencher would easily dig all the trenches in a couple of days.

I think you might be in for a big surprise.

You think I am underestimating the trenching time?  The ground is primarily sand and gravel according to the well drilling report. Trees are all pine trees, mostly 15 to 25 feet. The area I will put the field has been thinned but the stumps are still there.

Good luck. Let us know how it goes.

Well since at least two of you with Websites that include the word "Geothermal" in them and displaying skeptcism, methinks I should make sure I have an alternate plan.......

I am just saying that seldom are things as easy as they seem, especially if you have not done it before.

Since then I have found a rental trencher with a 5 foot digging depth, which will dig the trenches in a fraction of the time.

Is that what someone told you or have you trenched hundreds of feet in that soil with that machine before?

Any reason a trencher is not as good as a wider trench?

I think an excavator is the preferred tool, particularly if you don't have a lot of experience with ground work and all.

Unless you think it will be "fun" to do the groundwork yourself, you can probably hire a skilled operator with his own machine and do it for around the same it will cost you to muddle through with a rental excavator.

One would think that the company renting the trencher would have a very good idea of how well it performs in various conditions. But what's the worse case - you return it after one day?

Posted By ICFHybrid on 11 Oct 2012 10:16 AM

Unless you think it will be "fun" to do the groundwork yourself, you can probably hire a skilled operator with his own machine and do it for around the same it will cost you to muddle through with a rental excavator.

Field size factors length and depth. Deeper you are the shorter you can go. So depending on your requirements, that trencher may cause you to install a larger field, sand and gravel don't mean you can get a pipe 5' down with a 5' trencher. So you are marginal to begin with and that's if you max this thing out.

I agree with ICF you might find an owner operator costs less than the trencher or a few hundred more but you don't have to rent it, gas it up, tow it.......

You can still have fun watching a pro work the tonka toy.