I've been reading these forums for about six months and I've really learned a lot from all the resident experts.  Now that I'm getting ready to put in my system I've got a quick question of my own.  Drilling starts next week on the holes needed for my new Geothermal system.  They're drilling 4 280' holes for two Climatemaster split units, a TTS049 (4 ton) and a TTS038 (3.2 Ton).  These will be heating and cooling a 4800 sq ft. house in southeastern PA.  The company drilling the holes for the Geothermal is recommending using 'Rice' (small stones?) to backfill the holes rather than regular grout or Thermally Enhanced Grout.  I've asked for a cost estimate to switch to the Thermally Enhanced Grout but I'd like to see if anyone had some advise on whether there was a real benefit to using this grout.  I'd also like to know if anyone has used 'Rice' before and if it's comparable to regular or Thermally Enhanced grout.  Thanks for your help.

I have never heard of using rice for grouting a geothermal bore hole. I doubt very much that it will provide adequate heat transfer unless you are into the water table. If you are into the water table, I doubt that the State Water Quality Department will allow rice to be used.

Have you checked references on these guys? Ask the State who regulates well drillers and check with them.

You can read more about grouting here

I agree.
Grout isn't really an upgrade, it is the proper material.
Do some research on the contractor.
Is horizontal out?
Joe

Croaker,

Here's a thread that in part covers thermally enhanced grout.

http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/13/tpage/1/view/Topic/postid/39569/Default.aspx

Really interested in what "rice" is.  Any word from the contractor on that?  Dave

Thanks for the info.  I talked to my driller and he confirmed that he's always used bentonite but that he had heard from another driller in the area that 'Rice' was as effective as Thermally Enhanced Grout.  Sounds like that is not the way to go, if it's even legal in my area.  I still don't have an estimate on the Thermally Enhanced Grout yet but is the extra conductivity going to give me better performance of the system?  I'm seeing numbers of TC of .4 vs 1.2 for the Thermally enhanced grout.  That sounds like it would be twice or even three times as effective as the regular bentonite.  Does that transfer to an added efficiency of the whole geothermal system? 

I forgot to mention.  Yes, horizontal is out.  We've been playing find the water in our backyard with our builder and we've got two of our four holes drilled for free.  They've drilled two wells to 500' that collapsed to 280' which is how I got started on my geothermal kick.

Better heat transfer between the loop and the earth means you can reduce the loop depth (assuming vertical loops here), so reduced drilling costs. But you need to account for increased grout costs. Does less volume of fluid flowing might mean you can use smaller pumps that will use less electricity? Others can probably answer that. But there are other factors to take into account: design EWT and LWT, existing deep earth temps, ability of deep earth to absorb and dissipate heat, flow rate to maintain turbulence, spacing of boreholes from each other and more. The use of TC is only part of the equation in seeking max efficiency.

Chances are the TC of the ground in your area is around .8 .    Paying extra for 1.2 grout would not be worth it.  Usually on residential systems the TC of the grout is not a big factor at all. 

Theramally Enhanced  Grout is just bentonite with high grade silica sand added.   The sand should be cheaper than the grout so don't get suckered into paying a lot more.  Mixing the sand and grout requires more labor and sometimes a different type of mixer.

Find out what the driller can do and how much then let us know.   

The most important part of grouting is making sure that the hole is grouted the entire length. Some drillers place the grout near the top of the hole and hope that it makes it to the bottom. The proper way is to place the grout at the bottom of the hole and grout from the bottom up.

If there is a water table, no grout is needed for thermal conductivity. A good friend of mine in the drilling industry has done side by side test holes with 1.2 TC grout vs. pea gravel to the top of the water table. Above the pea gravel, he uses the required 1.2 grout. The tests show that it's just as effective as grouting the entire bore hole.

Groundwater protection varies by state, so your driller will know what is required. If there is no water table, definitely DON"T let him not use a sand/bentonite mix. It doesn't need to be high grade silica sand to be .9 TC. We commonly use masonry sand and have had good success with it. It typically tests at .9-1.1 TC, depending on how much sand we put in the mix.

Curiously, we only run 150'/ton around here, I wonder if a loop design expert would require you to drill two more holes (or any). We have also spoken before of mixed vertical and horizontal........
RE the grout, water source in the water table may not need it, but our state for one requires it. Ask for a web site on "rice" and share it with us. It may be a nickname for a different - approved- grout, or it may be something mixed with cuttings and mud that we probably wouldn't recommend.
J

There's an interesting grout I've learned about recently. It's at www.supergroutproducts.com
It's pricey, though.

Clark, Thanks for the www.supergroutproducts.com link.  Good video of boring and grouting DX borehole.  This grout looks like a cement-base grout.  What size tremie pipe do you normally use in DX borehole?

I heard back from my driller and he quote about $1200 for each 280 foot hole, so about 4800. That seems extremely high but based on what I've heard here there's no reason to do this. I asked a few more questions about the 'Rice' solution. This solution mixes the stone dust, or rice, with the bentonite to get a better TC. He said this wouldn't cost anymore since he doesn't have to pay any more for the stone dust. He said it's become very popular around here and a lot of geothermal drillers have switched to this. It still makes me a little nervous that I can't find any info on it online and no one here seems to have heard of this. I think I'll be sticking with the straight bentonite.

Is the $1200 extra just for a Thermally Enhanced Grout? Sounds like a rip off to me.

I would not have any problem with mixing stone dust and bentonite to improve the TC of the grout. It would give you a better grout.

Your original post sounded like all stone dust and no bentonite.

I believe "rice" merely refers to fine gravel / coarse sand - in other words an aggregate sized similar to rice.

A quick Google gave me this "Rice Gravel - A 1/8" nominal size gravel used mainly as a drainage material and often used as a filler in the manufacturing of block and specialty concrete products.
Plants: Belvedere, Perryville"

Another site had a picture, and it looked to be similar to what is sold at a local aggregate vendor as "89"

Most DX drillers are using a 3/4" - 1-1/4" tremmie tube for the grout. We use our drill pipe/casing with our sonic rig.