I'm in the middle of a geothermal install at my house in the Baltimore area.

Two 400' vertical, closed loops have been drilled, the hdpe pipe has been run down them, and I believe they've been filled with grout at this point (as agreed upon and priced in the contract).

The installer left about 45 feet of PVC casing (6" PVC pipe, I believe, maybe 6.5") around the hdpe pipe at the top of one of the wells. This concerns me because the PVC seems like a pretty good insulator, and the entire effectiveness of the system relies on the ability of the pipes to exchange heat with the ground. Prior to installation, a big emphasis is put on digging a well that is deep enough to have sufficient ground with which to exchange heat, and you pay a lot of money for every foot that needs to be drilled! (I think it was several thousand dollars for an additional 80' in each well, which is a lot of money to me, so I want to get the most out of what I'm paying for.) Any part of the well that is insulated well provides little value because it can't exchange heat.

Does anyone have any thoughts on this?

Is it a common practice? It is a reasonable/good one?

Thanks.

Here's a little clarification on my concern, which knowledgeable readers can skip (geothermal 101):

During a cold (analogously hot) spell, the system will pump cold fluid through the hdpe pipes in the wells for days, continually cooling the ground.  The more the ground surrounding the pipes cools, the less effective/efficient the system becomes because the system works by actually using the ground to warm the fluid.  The ground needs to be able to dissipate the cold to the ground around it.  If this happens, the ground surrounding the hdpe pipes isn't cooled too quickly.  I.e., it's warmed by ground around it. However, if that ground is well insulated (possibly by a PVC pipe?), it can't dissipate the cold to the surrounding ground or it does it more slowly and the ground around the pipe is much colder. Thus, it can't warm the fluid in the hdpe pipe like it needs to where the insulation exists. As a result, the system will cool the uninsulated ground surrounding the pipe faster, making the total system less effective/efficient because the ground around the pipe is colder overall, the thing that you want to avoid with a geothermal system.

PVC is a better insulator than your HDPE geothermal pipe. Heat transfer is about half as good through PVC, but since the diameter and heat transfer area is so much larger than the diameter of your HDPE pipe, the heat transfer through the HDPE is the limiting factor. The PVC should be able to shed/absorb any heat that moves through the HDPE.

You GHX was sized correctly for the materials in place or it wasn't. The materials themselves don't matter, the design that employs them does.

Posted By joe.ami on 27 May 2014 10:59 AM
You GHX was sized correctly for the materials in place or it wasn't. The materials themselves don't matter, the design that employs them does.

This is a good point. This isn't a question that can be answered on its own. In this case, I'm pretty sure the contractor just knows that 400' of pipe is recommended for a 5-ton system in this area. (I suspect this was originally obtained via some software or read somewhere). They were not planning to leave the PVC in although they also weren't surprised to leave it there. Do the sources and tools for designing GHX systems usually account for the possibility of PVC?

(When I asked the contractor about the PVC, he was not able to give a compelling/convincing explanation regarding the presence of the PVC, and he specifically did not refer to it in any design. Details regarding PVC casing in the wells did not appear in the proposal/contract from him or any other contractors, or I would have done more investigation beforehand and would have been able to compare.)

You are right - any insulation has a additive effect and in this case, more is always worse (there is no such thing as a limiting factor). But the effect should be minimal - HDPE and grout and soil aren't great thermal conductors either. And the PVC length is minimal.

It's somewhat unfortunate that such wells aren't tested for thermal performance. Then you would know exactly what you got for your money.

From a well drillers stand point here......
You could possible be in a area with known contaminates and your local DEP. May require the any wells in that particular area be cased down to the first layer clay for cross contamination reasons.
There are a few areas in my neck of the woods that have ground pollutants from old gas station tanks that leaked and polluted the ground water. Our DEP requires 60' of 8" PVC and cement based grout to eliminate the possibilities of further contamination.
JUST A THOUGHT.... FROM A WELL DRILLERS MIND.

What Dan said.
Also the western shore of MD is a truly a " soup sandwich " when it comes to drilling a hole. My guess is that the casing was used as a surface casing in the overburden to overcome an obstacle, and it is now stuck in the hole. Worrying about the heat transfer from some pcv used in the overburden is hust well " silly IMHO ".

Does the pvc detract from heat transfer? Yes. So does the grout and the hdpe pipe.
Eric

Eric and Dan know the business and are offering good advice

Since only 10% of your loop has an additional insulator, which really is not the limiting factor for the heat transfer. Your loop seems to be generously sized, there should be no indication why this should not be a nicely performing ground heat exchanger.

Thanks for the comments everyone.

If anyone has any hard numbers, I'd still be interested, but this is good.

In case you want some perspective, this for a Dallas TX southern climate, for residential only (not commercial):

Here's an image of a residential bore hole field at the point of 4 of 8 holes drilled:
http://www.pbase.com/neukranz/image/84286027
.

PCV casing is rarely ever used here.

Nor are the borehole fields 'engineered' or designed as a function of soil type. Instead, they're all simply 300' per ton holes with 1" HDPE pipe in them, and later interconnected with 1.5" HDPE pipe.

Best regards,

Bill