I've heard both a Waterfurnace distributor and my builder complain that grout reduces heat transfer since it keeps the borehole pipes away from direct contact with wet soil. I think I read in an ACEEE (check their site - you can waste DAYS reading about interesting energy saving technologies) paper that geothermal systems could be improved if a more conductive grout were found / used.

Water management district likes grout because it isolates crummy surface water from good deep aquifer water.

Currently, there are bentonite based grouts developed that yield up to 1.4 thermal conductivity. These work very well. Also, there are cementuous grout mixes one could use that will get even higher conductivities. These both satisfy the water management and the thermal conductivity needs of boreholes. All geothermal drillers better be using these type of grouts. Pure bentonite grouts, without sand, will yield about a .4 on thermal conductivity.

Very good info - Thank you.

As my wife and I were discussing Tuff Guy's (hope you don't mind, Clark) post on thermal conductivity of grouts, she asked why not incorporate chips of recycled aluminum cans into a grout for better heat transfer. As GSHPs are a relatively new technology for residential use, W would expect grout R&D should be looking at cost effective improvements to keep pace. We are planning for a vertical closed loop and will certainly expect a grout with a good heat xfr rating.

How much will a 0.4 thermal Conductivity (TC) affect the heat transfer process vs a 1.4 TC? Would it be enough to have to make your loops longer, increase GPM and pump sizes, etc.? In other words would it be important enough to know the TC when designing the loops or do you just adjust the system to account for whatever TC grout is used? Dave

Bill,

If your boreholes were grouted with pure Benseal, ( no added sand ) then this would account for the inability of your loop to effectively transfer heat to the surrounding ground. As Clark mentioned, pure grout has a TC (thermalconductivity) value of .4.

On our commercial jobs, we routinely mix grout with high grade silica sand to achieve TC values of 1.07 to 1.2. This requires mixing 300 to 400 lbs of sand to every 50 lb of grout. This is not an easy process and getting this high TC grout down the hole presents some real problems.


In defense of your drillers, we seldom mix high TC grout on residential jobs, but then our heating and cooling loads are more balanced.

I have attached two bore hole designs where the only difference is the  type of Grout Thermal Conductivity used.

One report uses a grout TC of .4.  This design calls for 1475'  of borehole to meet the cooling needs with keeping the EWT at 85°

The other report uses a grout TC of 1.2.  This design calls for 1003' of bore hole to meet the cooling needs with keeping the EWT at 85°

By using a high TC grout, you can reduce the loop length by 1/3 in this example.     To be this precise you really need to know the TC of the gound where you are drilling.  A TC test of the ground costs upward of $15000.  It doesn't make sense to run a TC test for a house.

If you wanted to keep the EWT below 80° with 1.2 grout,  then you would need 1209' of borehole.

You can see all of the input parameters that are used to calculate a vertical bore field.

It is very  interesting to watch the loop length change as different parameters are used.

These reports were run on the GLD Loop Design software which is the leading software package on the market.

So in answer to your question,  unless you have an abundance of drillers in your area,  you are pretty much stuck with what grout the driller is willing to do.  If you can get them to use a higher TC grout, you will be better off.  Just realize that most houses are not lucky enough to get high TC grout.

This thread is exceptionally helpful, interesting, and educational.  Thanks to everyone's contributions.

There's obviously an interesting, complex, and dynamic business model associated with creating a borehole field.

For example, it looks like it's just not cost effective to drill/grout a small residential field the same as a commercial field.  In fact, it looks like some approximations have to be made for residential fields to keep costs down.

Such as what kind of grout to use.  Yes, my holes were grouted with pure Benseal (no added sand).  But, there's no way I could have afforded a commercial-style grouting - it was already expensive enough as it was just to do it the way it was done. 

At least now I know why my EWT varies at any given moment on really hot days.  This morning is was around 78° - it got up to around a high of 83°.  But today was a high of 105° here in Dallas, and I'm probably going to dump close to 600 KBTUH into my borehole field before the day is finished.  The ground can only absorb the heat so fast, and for a residential job, the grout is going to slow it down further. 

My drillers were exceptional.  I'm quite confident that I got the best compromise of performance and cost.  Below is an image of the grouting machine used.

Best regards,

Bill

You should advise your driller that they could grout their holes for less money, and with a higher conductivity by using masonry sand.

Dewayne mentioned that a high cost sand is used on commercial projects. On residential projects we use masonry sand. Depending on that sand, we've had tests between .9 and 1.1 thermal conductivity. It ends up taking less bags of grout, and the sand is relatively cheap. They'd be able to grout the holes for less cost of materials. The only question is if they have a grout pump capable of pumping sandy grout mixes.

Here is a MN company website with costs for vertical loop materials, http://www.geoclip.com/savings.html .   Their T.C. Enhanced Grout (K=.85 btu/h-ft-F) costs about 4x more than the regular bentonite grout (K=.4 btu/h-ft-F) .   For a 150 ft borehole that would be an additional $85 ( T.C. Enhanced $117 - regular bentonite $32).  But with the increased TC, as Geodean's attached sample loop designs affirm above, you should be able to reduce the loop length.  Check out the example at the website. 

Clark...I talked to one of the few GSHP loop drillers in KS earlier this year, but wasn't armed with the info you provided.  I'll be better prepared when next we talk.  thx,  Dave

Forgot to add that the company sells a clip that keeps the supply and rtn tubes separated to allow better heat transfer. That may be another thread.

Farmboy, not to throw cold water on your parade, but geoclips are a joke. We have tried to install them and after hours of cursing gave up. Every driller we have talked to has the same opinion of them. The idea looks good but does not work in the real world. At least not in our world.

Geodean, say it's not so!! Any other way to keep the tubes separated?

My 1" HDPE pipe was simply pushed down into the hole - no spacers.  (A lot of hard work, by the way - don't think I'd be strong enough to do what was done for my installation.)

My guess is this is standard procedure, in the Dallas area at least, and at least for residential work.

Best regards,

Bill

Not that I know of.  We have installed 1000's of loops.  The engineers who design the systems realize that there is no practical way to keep the loops separated.  We had them come out when we tried to install them and they soon realized that  geoclips are junk.  The systems are designed accordingly and the loops perform fine.

So don't lose any sleep over this.....geothermal systems work fine with out geoclips.   If you notice on their website,  there are no pictures of an actual install.

Only pictures taken in a lab type envrionment.

Geodean! Thanks for the benefit of your experience.