I recently had a system installed with two 400 foot vertical loops.

Does anyone know much about the importance of the type of grout used to seal the holes (most specifically its thermal conductivity)? I would expect this to be _extremely_ important. The system fundamentally works by exchanging heat with the ground and dissipating that heat through the ground, and the grout is a critical medium through which that heat is exchanged.

(Can skip this paragraph if you're not interested more details on what I believe the reasons for the importance are...) In particular, a grout with a high thermal conductivity should be able to dissipate heat quickly through the sealed whole into the rock and the earth. This is not too important when you run the system for a few hours or maybe days (?). (It will take a little while to heat/cool all that grout in the first place.) However, it would become very important on numerous consecutive, cold days (which are very common through the winter), for example, when the system needs to be able to dissipate the cold that it's pumping into the ground to continue operating efficiently. (It doesn't get a break in this case. If the grout and ground can't dissipate the cold, the grout will remain very cold itself and thus can't do it's job to warm the cold fluid in the pipes, i.e., pump heat into the house.)

Perhaps there are studies that show things like a 250 ft well sealed with a grout with high (x BTU/hr•ft•ºF) thermal conductivity has the same efficiency as a 400 ft well sealed with a grout with low (y BTU/hr•ft•ºF) thermal conductivity?

I recently learned that the contractor who built my 35K system used Baroid Quik-Grout to seal the holes. (It's not designed for thermal applications at all.)

Any thoughts?

Very appreciated.

Yes I have a thought. Surrounding soil TC is generally the limiting feature, not the pipe or grout. Grout IMHO is about protecting aquifers and ensuring good contact with surrounding soil.

It is more important to me that grout was used than what kind.

......which is also why I'm largely unimpressed with fancy loop systems.

Posted By joe.ami on 27 Jul 2014 11:50 AM

... It is more important to me that grout was used than what kind.

Yep.  And properly installed. 

I've got normal, nothing fancy grout (see http://www.pbase.com/neukranz/image/97429902 ).  I've studied numerous reports that show that the incremental benefit of higher TC grout is minimal for residential systems.  What is important is tight coupling of the pipe to the surrounding soil.

Best regards,

Bill

Insulating materials around your loops always has a negative effect - more thermal conductivity is always better. The studies are out there if you use google, but don't expect huge differences (in COP).

Do you guys have examples or studies? These comments aren't especially compelling while some of the things I'm reading are (in the sense that they highlight the importance of grout with high thermal conductivity). (Grout is for sealing the hole from a requirements perspective, but the heat has to pass through it to get from the pipe to the earth and vice versa. Intuitively, it makes sense that if it's and insulator, it's a disaster. If it's like copper it's great.)

E.g., GHP Systems "Fundamentals of Commercial Geothermal Wellfield Design" ( http://www.ghpgeovault.com/wp-content/uploads/2010/05/Fundamentals_of_Commercial_Geothermal_Wellfield_Design.pdf ) says "The backfill or grout used in the borehole and the positioning of the u-bend pipes within the borehole ... significantly contributes to the VHE performance... The two most significant breakthroughs in the VHE design include thermally enhanced (T.E.) bentonite grouts" and "The primary drawback of using straight bentonite grout in VHEs is that it has a poor thermal conductivity (K = 0.4 Btu/hr-ft-degreeF)." They show a graph on page 8 that shows that you need about 30% more borehole when using standard bentonite grout vs. about one with thermal conductivity of .8 Btu/hr-ft-degreeF (which is good but not extremely high - some go to 1.6).

Similarly, in GeoPro's "Importance of Grout Thermal Conductivity" ( http://www.geoproinc.com/resources/documents/GeoPro_Importance_of_Grout_TC.pdf ), they go through numerous equations that indicate the importance of grout conductivity and state, for example, that you need 79.5 more feet of borehole/ton when you use standard bentonite grout vs. (the top of the line) 1.6 BTu/hr.ft.degreeF thermal grout, when the ground has a thermal conductivity of 1.2 Btu/hr.ft.degreeF.

I'm in Columbia Maryland, where I understand the ground is solid rock after you get through about 50 ft of overburden. It seems unlikely that the thermal conductivity of the rock would be anywhere near as bad as that of grout which is kind of an airy substance (although I'd love to hear numbers if you can correct me). In any case, the thermal resistance is additive so every bit decreases performance, although you're right joe.ami that the resistance of the grout matters less if the earth has a very high thermal resistance (low thermal conductivity).

Is there a way I can see if my system was designed properly? As mentioned, I'm supposed to have 5-tons of borehole. I live in Columbia MD, (west of I-95, apparently east of I-95 it's a different composition). How much borehole am I supposed to have? (I'd like to be able to run some design tool or compute myself, but if that's not possible, I'd appreciate hearing what someone else computes.)

This was a _huge_ investment for me, and I'd like to know it's right. ...also maybe more importantly if I should recommend my contractors. Many of my neighbors and people at work are asking about this. There are a lot of contractors around.

(Not sure what was meant by "fancy loop systems".)

Thanks again.

Also see here. But note that what you care about is length needed and overall performance, not just borehole thermal resistance.

What makes you think commercial design has any bearing on a small residential project? I love when an opinion is solicited; "thoughts?" and subsequently challanged; "Do you guys have examples or studies?"

I'll give you the longer answer and you may google whatever study you like. The most efficient loop or grout or heat exchanger is generally the most expensive. The most efficient loop does not change your operating cost, it changes the amount of feet of pipe in the ground. So you can buy more expensive materials and more laborious to instalationsl or you can buy more pipe. Guess which is cheapest.

That's why in residential projects we always start with the least efficient loop system available (horizontal). We literally have to put twice as many feet of pipe in the ground vs a vertical system. Why would we do that to our customers? Because it costs less to install and the operating cost is the same. So again it doesn't matter what the material is as long as you have enough pipe.

In my kneck of the woods systems perform very well with 150'/ton of vertical bore and a 75' average depth. Don't take the observation of a designer that's talking about systems with dozens or hundreds of bores and compare it to your resi system.

Generally we tell folks to get the best installer in their area and the rest will take care of itself. Hopefully you did that. If not that ship has sailed. Why not just wait and see how the system performs versus challange the design at this stage of the game?

Posted By joe.ami on 28 Jul 2014 10:57 AM
What makes you think commercial design has any bearing on a small residential project?  ...

It does not.  Good quality hole drilling and tight coupling of the pipe to the earth are all that's required.  Beyond this other expenditure for fancy TC grout or quad U bends or ... is not money well invested.  For residential systems.

I do regularly work with the largest commercial driller in the DFW area on monitoring projects.  I'm familiar with their design process, and that familiarity includes being aware that they pay (almost maniacal) attention to the grout's TC.  And select from different grouts depending on project parameters.

But these are projects that easily have over 1000 boreholes to multiple thousand boreholes.  Test holes are first drilled, different grouts are first evaluated, heat transfer is carefully studied, all as part of the project's design effort.  Spacing of holes is looked at, too, down to the inch.  When you have a parking lot of fixed size, and you know you have to get X amount of heat transferred, borehole size, length, grout TC, hole spacing, pipe diameter, etc. all are critical parameters.

Best regards,

Bill

Exactly. 2 boreholes do not justify test bores or high performance anything if driller is already clear on what will work. In the case of dozens, hundreds or thousands of bores obviously the project will be much less cookie cutter.

Posted By joe.ami on 28 Jul 2014 10:57 AM
I love when an opinion is solicited; "thoughts?" and subsequently challanged; "Do you guys have examples or studies?"

To be fair, my original post also requested studies, but yes, in general I am convinced primarily by evidence and/or reason. Skepticism is healthy, and imho, completely justified through 95% of life. For example, I recently asked about the most efficient place to put my water buffer tank and got several contradictory, definitive answers. The only way to identify the right one is to get down to the reason.

The reason I would think that commercial systems would have something in common with residential ones is that the physics are the same, regardless. What I think I'm hearing is that the commercial systems have a space constraint and capacity requirement that residential systems typically don't have (which I know), and that increases the relative benefit of using higher TC grout. However, I don't understand why that would be. Perhaps the cost would be amortized in different ways due to the fact that you're drilling and filling over and over again, but it's not clear.

What you're saying about there being a tradeoff between various parameters (some of them being the thermal conductivity of the grout and some being the depth of the wells etc.) and the result potentially being different systems with the same performance makes perfect sense. Thank you. I am surprised in my area though, where I'm paying $14.375 per foot of borehole, that it's not advantageous to use higher TC grout.

I tried to get the best installer, but that's not an easy task. (Hopefully I did.) Each of the installers with whom I spoke gave me doubts. (As you may be able to tell, that's not hard to do :) ). (For example, this guy wanted to go with a different size system than everyone else, but he had the best reputation from everyone I talked to.) As you said, the ship has sailed.

I want to know if my system is designed correctly for a few reasons. First, pure curiosity that may kill me... Also, while the system has practical benefits, it's also like a hobby for me (as is much of my house). I get visceral enjoyment from it's cool design and want to know about it. Lastly, if it was designed incorrectly, it is better to bring it up now. Even if I can't prove it for 6 months (when winter comes), the earlier I bring it up, the better my chances of getting them to do something to correct (although who knows what that would be). It's much harder to come back to anyone 6 months after the fact.

If anyone can help me verify the design by pointing me to some free software or using some, for example, I would be extremely appreciative.

Thanks again for your insights.

Posted By jonr on 28 Jul 2014 10:53 AM
Also see here. But note that what you care about is length needed and overall performance, not just borehole thermal resistance.

Interesting. Thanks! Will have to examine more carefully later.

It depends very much how the conductivity of the surrounding rock/soil is, and for economical reasons how much you pay for drilling. If you have water bearing rock with a high conductivity it might be worth to invest into higher conductivity grout. Sometimes it is cheaper to simply drill more to get more performance, high conductive grout can be expensive. I never found it economical or performance wise useful to use grout with a higher conductivity of 0.40 in residential applications.

RW,

You continue to miss the fine points.
In this exchange:
"Also see here. But note that what you care about is length needed and overall performance, not just borehole thermal resistance.


Interesting. Thanks! Will have to examine more carefully later."

You focus on the trees (the study) and miss the forest : But note that what you care about is length needed and overall performance, not just borehole thermal resistance.

With this comment:
" For example, I recently asked about the most efficient place to put my water buffer tank and got several contradictory, definitive answers. The only way to identify the right one is to get down to the reason."

You demonstrate the propensity to believe there is only one "right" answer.

And here:
"What I think I'm hearing is that the commercial systems have a space constraint and capacity requirement that residential systems typically don't have (which I know), and that increases the relative benefit of using higher TC grout."

You miss the point again. It's not about space constraints (you can always drill deeper) it's about drilling less.
By comparing commercial to residential design you again are showing a fundemental lack of insight as to what is important to you and your system.

This is the failing of many very bright geo designers.

In your case for instance if you were to have purchased a WF series 7, with Wilo variable flow center, TE grout on high performance PE or "Twisters" or some other fancy loop, with no antifreeze (too inefficient) since you dug a third 400' bore, then for only about $10,000 more you would easily save $50 to $75 each year in operating cost! What if we were to add on to your basement so the water heater, geo and buffer could all sit next to each other? That'd be more efficient.

You know if we pay $2,000 for a test bore and TC test, we might be able to shave a 100' or so of bore (saving $1,500)

What about load share? My wifes in the bedroom right now and I'm in the office. It's warmer in there so with proper zoning (another couple thousand) I could move heat from the bedroom to the office since it's kind of chilly this morning.......

Even the money is different. Many big projects are paid for with bonds or bean counters use LCC. So we just have to pay for the equipment in about 15-20 years right?

Most of the studies you see are going to be by engineers who dis-associate themselves from "bang for the buck" or manufacturer's that want to present their products in the best possible light.

One engineer presents an under the foundation design where his loops enjoy the benefit of conditioned space above adding heat to his loops to heat the house with. Voila! Momentum geo.

Those of us who sell (myself, engineer and Doc) who have weighed in, may not dis-associate ourselves from first cost and ROI or bang for the buck (I've been schooled in use of ROI in this context). Therefore that will be the context of our response (by and large). Bill is a very educated in user with similar perspective and Jonr is a good researcher who asserts the value of superior TC while still reminding you it's not the single most important thing.

.....deep breath.....

In resi-design here's what's most important.

1) what is available? acreage? I can go with the least efficient, most bang for the buck horizontal loops. Backhoe? well an excavator is better for loop installation but it will be less expensive if I don't have to move a 30,000 lb tonka toy. Enough space for the geo, waterheater and buffer tank together? No but anywhere is better than nowhere.

2) sizing. One thing you didn't ask about and one that is often screwed up. Many "expert" geo designers brag their systems don't need auxiliary heat. problem is it is higher first cost and often higher operating cost. You mentioned your guy used a different size than others suggested. If his was smaller your odds are better of best bang for the buck.

3) Bang for the buck. How long will it take for the savings of geo vs other fuel to recoup your additional investment.

4) Warranty/dealer. Did you get the best dealer? Will he cover the things excluded in the warranty? Will he work for the factory allowance or charge you the difference? Do you know what's not covered by your warranty?

5) Performance measuring. Will you know if the system performs as designed? Do you have a monitoring system?

6) Comfort. Are you comfortable? Do you no longer compromise between budget (lower set point) and comfort?

7) Envelope. Did you first attack the envelope and minimize load and thus system size?

Aren't those questions more interesting than grout T/C? Maybe not.

joe.ami, I'm not sure how you infer so much from "Interesting" and "Thanks".

I think I get it. It's about the whole system. There are tradeoffs. (I.e., you can trade cost upfront for more efficiency. There are diminishing returns... You can trade length of pipe for thermal conductivity... One is likely cheaper than another.)

Also, remember however, I am interested in this stuff and it's a sort of hobby. My goals may not be the same as yours. (To be honest, if all I wanted was the most cost-effective thing, I doubt I would do geothermal at all. I might just invest my money in stocks. Over the long period of time it will take me to payoff a geothermal system, the risk of an index fund investment would actually end up being minimal anyway.) But I think the design of the geothermal systems is cool, fascinating, and I'm an environmentalist. Of course, cost and savings are still extremely important or it's not even an option, but I enjoy having a system with various properties that excite me, which don't directly relate to the expense and savings.

Anyway, I agree with what you are saying. You make many good points and state them well. (I still disagree that one should believe what people are saying without reason or evidence.)

In any case, my question is about half of a forest. Is my well field designed well for a 5-ton system accounting for my location (Columbia, Maryland, 21044), west of I-95 where they drill through rock), depth of my two vertical wells (400 ft each), the thermal conductivity of the grout (Baroid Quik-grout, maybe 0.4 Btu/hr-ft-degreeF), the diameter of the hdpe pipe (1 1/4"), and anything else that would need to be known. By designed well, I mean am I to the point where any significant improvements would save less than $10-$20 a year or so?

Some of the contractors who gave me estimates use TC-enhanced grout around here and drill to the same depth or more (although I didn't think or know about the TC-enhanced grout at the time). It makes me curious. I want to know how well I spent my hard-earned savings.

Thanks.

In regards to the well you are describing, judged from a distance, it sounds you are spending your hard earned savings well. Nothing I can think about (in regards to your loop field) would give you a much better bang for the buck.

My inference came from "Will have to examine more carefully later." which focused on the link not the follow comment.

Holy cow 11/4" ubends in the wells. Pipe diameter is another piece of the pie when discussing loop capacity and this is the first I've seen it brought up.
That is a very robust GHX( I wouldn't be suprised if that could carry 8 tons). We go 150' of 3/4" around here (per ton). I would say your field if anything is overengineered and improvements would offer no operating cost savings (let alone bang for the buck).

With that answered, if this is sport for you, have you no questions about the rest of the design?

"But I think the design of the geothermal systems is cool...."

Good so what else can we help you sort out? Personnally I think over sizing and over-engineering are amongst the biggest dangers to geo-shoppers.

>If anyone can help me verify the design by pointing me to some free software... I am interested in this stuff and it's a sort of hobby.

You might get the free evaluation of "Ground Loop Design" software. Probably full featured enough for learning about the effects of different grouts on loop length. And if you really want to have fun, rig up a water heater and do thermal tests (which aren't hard, the problem is that drilling a hole and then waiting a few days to drill more isn't so practical).

You could also buy "Grouting for Vertical GHP Systems" from IGSHPA. Also see table 4 here (roughly ~15% length reduction for .8 grout and another ~15% for double U-tubes).

This document seems to say that standard bentonite should only be used with saturated, unconsolidated soils (not in rock). Low thermal performance? Bentonite shrinkage when dry?

> Some of the contractors who gave me estimates use TC-enhanced grout

I suspect that what they are doing is cost effective, but who knows what their costs are.

As I recall, Bill uses 300' per ton - at almost 2x your loop, that heavily influences the need for other enhancements.

"As I recall, Bill uses 300' per ton - at almost 2x your loop, that heavily influences the need for other enhancements."

don't forget pipe diameter matters too. 11/4" utube 200' long may have more capacity than 3/4" 300' long. Soil TC affects this as well of course.

Also don't forget when we talk about foot per ton we want to clarify whether the feet we are talking about are of bore or pipe (as bore feet are 1/2 what feet of pipe will be).

Posted By joe.ami on 31 Jul 2014 11:14 AM
"As I recall, Bill uses 300' per ton - at almost 2x your loop, that heavily influences the need for other enhancements."

don't forget pipe diameter matters too. 11/4" utube 200' long may have more capacity than 3/4" 300' long. Soil TC affects this as well of course.

Also don't forget when we talk about foot per ton we want to clarify whether the feet we are talking about are of bore or pipe (as bore feet are 1/2 what feet of pipe will be).

Correct.  I'm in a very hot climate (Dallas).

I was designed at 300' of *bore* / 600' of *pipe* per ton, using 1" HDPE pipe, with boreholes at minimum 20' away from the closest neighboring borehole, for clay soil.

Best regards,

Bill