Vertical Closed Loop Grouting Issue
Having a 5T, two speed, closed loop (vertical), DSH WF-Envision being installed(loop wells are in). Installer has grouted the wells (3x at 250', 5" bore, 16' spacing, reverse return) with 100% Bentonite vs. mix. Issue is per manufacturer specs (found after the grouting was done) that they suggest at least a 4:1 silica sand mix to bring the 100% Bentonite Grout from 0.4 BTU-ft/F* to 1.0 or so.
Earth in bore is nearly continuous limestone (Average rock... 1.4 BTU-ft/F*).
So I have a situation where my first energy transfer is wounded by the installation, and this seems a big deal...
I don't have a functional Ground Loop Design or similar software, but from the demo, this is NOT a good solution.. (dry sandy soil is ~ 0.4 BTU-ft/F*)...
Can anyone let me know if I'm wounded or this is not a factor.. I'd like to know prior to confronting the contractor..
Ground Temp ~ 57*F (nearly solid limestone in cuttings)
Heat Load: 49KBTU/HR
Cooling Load: 80KBTU/HR
Loop piping 1", bore 5", depth 250ft, 3 wells, spaced 16ft, reverse return, closed loop
Flow pump: FC-2, should provide 18gpm (including both loop and unit)
Heat Pump: WF Envision 5T, ECM, DSH, 15kW strip
I'll reply to my own post...
I ran some numbers on Ground Loop Design software (the demo version, used the default heat load, can't be changed on demo).... only change was the grout thermal conductivity...
Net result showed almost a 30% increase in loop (well depth) required as a result of the 100% bentonite vs. 5:1 silica sand/Bentonite.. and this showed the much wider swing in EWT as a result as well...
Waiting to hear from the contractor (still happy with them, they are responsive/listening) and others at this point... Might end up adding one more bore hole/loop (3 to 4 @ 250ft)..
Any 'real world' experience is very welcome..
At what level did they hit water, if they hit water at 50 feet then 200 feet of your loop is in water (or very wet grout) and would transfer better than dry grout. If they never hit water then you may have a concern.
I do not have access to the calculators, only applying common sense enginering here.
Kevin, all of the drillings were dry, only the first well (left overnight) may have had some water in it after some time (they filled the first U-loop with water after installing and left it in the well overnight, prior to grouting). The drill truck was a dry 'air drill' and could have dried the drillings via the high volume compressed air used to remove the drillings from the bore (they were dusty dry). That said, I'm not sure there was any 'fast' water in the wells (dry drillings, and next 2 air pressurized U-Loops could be put in the well without being filled with water...not boyant, all then grouted on the 2nd day).
The grout was water mixed and pumped, just without any sand or drillings added to improve the thermal properties. On the other hand, I would guess the bores/grout would fill with water (to some level).
Therefore, I really DON'T know if there is water or not in the wells (and at what level)..
Thanks for the feedback..
Results are in:
My Contractor has provided a University of Kentucky (ASHRAE sponsored) study that provided data that indicated that while a 100% bentonite fill is a comparative worst case (compared to sand/bentonite -best and limestone drillings-mid performance), the effective conductivity of the limestone 'field' is still 1.4 BTU/Hr*ft*F.. effectively the same as the limestone 'average rock' for the field used in models of loop conductivity. My job site is in Cincinnati, Ohio (might be very similar Limestone)... This provides a measured result of applicable alternatives in grouting.. The differences are not that large (1.68 for sand/bentonite, 1.55 for Limestone, 1.40 for bentonite), so that and water transfer/exchange (site to site variables), I think I'm OK here...
Issue is resolved... FYI to the group and again a thanks to all..
Good contractor with very good (and patient) staff... Moving on with the install...
I am glad your issue is resolved to your satisfaction, however I would like to point out some things.
If the borehole was grouted completely there is no water in the bore, just grout and the loop. The purpose of the grout is two fold, to ensure the formation resists contamination and to enhance the transfer from the earth to the loop. In your case grouting with a 100% bentonite prolly was between 18 and 20% solids content. It actually insulated the loop against the better transfer of the surrounding rock. A sand enhanced grout would have mirrored the conductivity of the rock and not inhibited the heat transfer.
Was the job engineered via a test bore and thermal conductivity test? If it was not you are prolly just fine with what you got, the specs and the field just did not match.
If the job was engineered it would be cause for worry as the engineers reduce footage and number of bore holes based on enhanced thermal conductivity for each hole.
Hope this helps.