Hi all,
Recently picked the ear the man who will likely be drilling my wells for the geothermal. He seemed very knowledgeable when I quizzed him with some of the things I learned from this forum. (He uses geology samples to determine hole size, fuses the pipes, goes reverse return, etc). However, he said his standard installation includes some kind of webbing around the piping that is filled with dirt, sand and other stuff that's good for transferring heat and cold. A local school is slinkying geothermal and I think I see the same thing sitting around the construction site. Looks like netting of some sort. He said they ensure a good compaction around the piping and if they can't (like lots of water disrupting it) they use grout.

He doesn't have a grout pump but rents one when needed. Says grout will cost $3 a foot and I need 600 ft (3 ton vertical system) to be on the safe side. In that case, grout will cost me $1800 minus (30%) or around $1200. Sounds like I should get grout based on everyone seeming to suggest it as standard. Anyone have any thoughts? Also, I'm not trying to revive the debate about the need for more piping. I'm getting it because of the rebate and will live in this house for decades. I want the option of adding a 4 ton heat pump if I decide on an addition.

My limited understanding of this issue is that grout is only applied where needed to conform to government regs or to isolate deeper high quality aquifer from lower quality surface water.

If you really think you'll do the addition, definitely consider the required extra loop length now. Installing it upfront would slightly increase the efficiency of the initial smaller geo unit.

Ask yourself if handling the addition load by whole house load reducing improvements such as radically better windows or adding sprayfoam insulation might make more sense and allow the then existing unit to handle the extra SF.

Another possibility is to go with an advanced minisplit ductless heat pump on the addition

Just a few thoughts.

See if you can find a product name for the "webbing".
j

I don't know if you want a non-professional opinion, but here it is. One person who I spoke with here in Upstate New York had a serious problem with their geothermal one year after it was installed. It was installed in 2007 and in 2008 they could not cool the house in the summer. After quite a bit of investigation, they found that the drilled well was packed with a sand mixture instead of grout and that the sand mixture had washed away so there was a lot of air around the pipe and no heat transfer. I'm not sure if the webbing that your driller is referring to will stop this, but this experience is something to consider.

When I had my geo installed, grout was just part of the package.

Here in PA, we are one of the only states that doesn't require that wells and other boreholes be grouted with a low hydraulic-conductivity (generally bentonite) product to prevent groundwater contamination. For geothermal boreholes, for best performance and per IGHSPA guidelines, you should use a thermally-enhanced grout that is a mixture of sand and bentonite to increase the thermal conductivity. Some charge extra for thermal grout but plain old bentonite grout shouldn't cost extra. -Adam Hydrogeologist

Ona, I'll take any nonprofessional opinions too ;-). That was my concern though, that the pipe would become exposed, and the installer said that could happen. He said if they didn't get a good compaction they would use grout. He did say some soil types are better for thermal conductivity than plain old grout but the thermal grout could negate that. If there's no negative reason to using grout I don't mind paying a few extra pennies on the dollar for the installation. Thanks guys.

In general--and increasingly important with larger-diameter boreholes, I believe that you should choose a grout that is close to or slightly more conductive than the surrounding formation to ensure the best possible loop performance per foot. Standard bentonite (20% solids) has a thermal conductivity of about 0.38 Btu/hr ft °F. This is reasonably close to the TC of an unconsolidated silt or sand. However, if the loop is in rock such as a sandstone or shale then you are probably looking at a thermal conductivity around 1.0 In this case, adding 200 lbs of sand per 50 lb bag of grout results in a thermal grout with a TC = 0.88 Btu/hr ft °F. Now, if it's cheaper to just drill deeper or put in extra borings for some reason, that is another way to go but it should be modeled in a loop design software.

-Adam

I have never heard of a product like this.  See if you can find out more about it.

I think he means that he just backfills the hole. I thought I heard him say he wraps the pipe with something, too. I'll call him up and find out more.

IGSPA standards= grouting the entire borehole from the bottom to the top to facilitate heat transfer and the thermal link. Secondary is the enviromental issues associated.
hope this helps
Eric

Why wouldn't any grout cost extra? It introduces material cost and uses expensive equipment to inject?
If you mean it should have been cost in already, I agree.

We've heard of many backfill strategies. Grout is named (as mentioned) by IGSHPA. I think a driller worth their salt is going to have to promise whatever is used will not wash away (if grout is not specifically mentioned by local law).
 So to the poster, you might suggest to the installer that you are indifferent to what he employs so long as he guarantees good conductivity which obviously would be part of any geo drillers bid. If base price doesn't include such guarantee then check the IGSHPA list for a local pro and hire a different driller.
Joe

Next to having a compotent HVAC designer / installer the ground heat exchanger is the most important part of the system. Once installed you can pretty well forget making changes to vertical ground loops. Top quality grout that serves the GSHP need and protects ground water can only be installed once, no second chances available. Don't try to save money there, it's key to your systems long term satisfactory operation. If you want to save $$$ do it someplace you can add quality later. IGSHPA specifies using either a bentonite or cementitious grout pressure pumped from bottom of the bore. Most State regulations say somethng similar.

To do a play on "engineer"; without contact, you have no thermal exchange.