I need some advice on two different well driller quotes I've received. Quote 1: 2 boreholes, 320' deep each 2 330 ft 1.25" loops Thermo grout, conductivity of 1.2. 1 set of 1.25" header lines from borehole to inside basement Quote 2: 2 boreholes, 300' deep each 1" geothermal piping (length not specified) Bentonite grout, no conductivity specified 1 set of 1.25" header lines from borehole to inside basement The prices are amazingly within $100 of each other. My main concern is around the depth of the boreholes (320 vs 300 ft), and the diameter of the piping (1.25" vs. 1"). Another concern is the grout. Quoter 1 told me bentonite is no good, and that you must use thermo grout. This is for a 4 ton system, BTW. Thanks for any input you guys have.
This is my thoughts with having more information given to me. Quote 1: 160/ton with 1.25 loops is on the large side for a 4 ton system. Thermogrout is not generally used on residential without reason or knowledge of solid rock formations. 1.25" loops with the same size headers?????? Why???? Quote 2: 150/ton with 1" loops. (More common for residential ) Bentonite is standard in my area mixed to 20% solids for a 0.44 conductivity. 1.25" header is fine for a 1loopfield. Without more information (location, soil conditions, etc...) I can not offer anymore information. All I can say is in my area I drill 180/ton with bentonite grout as a standard bid. We generally drill into sands/silts/clays. Rock drilling would change these depths, and without more condition information, this is all I can say.
As far as location - I'm Northwest of Philadelphia. Given that the pricing is nearly identical for these two, I'm wondering if Quote 1 is doing a bit of "oversizing"? I can't see additional conductivity in the grout hurting anything, but I confess I do not know if having a larger diameter pipe down the bore is good thing or a bad thing. I would think it's good, since technically there is more surface area for heat transfer. If Quote 1 is "oversizing" that's fine by me, since his price is the same as Quote 2. The question though is this really "oversizing" or is something else going on here?
A couple of comments: Thermal Grout is just bentonite with sand added. We normally use bentonite with out sand on residential jobs. Larger pipe does have more surface area, but also has thicker walls so heat transfer is about the same. With larger pipe the flow needs to be increased to maintain turbulent flow which is critical for optimal performance. Option 1 seems to be the better deal as long as you can maintain turbulent flow. The extra depth and 1.2 grout will perform better than option 2. I would ask for and check references on both before making the choice.
Turbulent flow? Yep, I knew there was more to this. Option 1 has drilled and installed over 1400 loops. He's been in business since 1962 and has been drilling geothermal wells since 1978. He could be taking an "old school" approach, which sometimes is good but sometimes is bad if technology progresses and dictates otherwise. No clue yet on option 2.
A 4 ton heat pump should have a flow of 12 gpm. If you split this in half for two loops, then you have 6 gpm per loop. This will give you turbulent flow in 1.25 pipe. Sounds like you can have confidence in driller #1.
I have never seen a system under perform due to an oversized loop field. The reverse can not be said. The choice to use thermal grout or not has more to do with the thermal conductivity of the surrounding ground. If you are drilling in rock, thermal grout is an advantage where 20% grout would be a hinderance to conductivity. In sand and silts 20% grout is appropriate while the thermal grout is overkill. All that being based on thermal conductivity of the two different materials. Hope that helps Eric
Quote 1: 160/ton with 1.25 loops is on the large side for a 4 ton system. Thermogrout is not generally used on residential without reason or knowledge of solid rock formations. 1.25" loops with the same size headers?????? Why???? Quote 2: 150/ton with 1" loops. (More common for residential ) Bentonite is standard in my area mixed to 20% solids for a 0.44 conductivity. 1.25" header is fine for a 1loopfield. Please do misunderstand my first post meanings of standard and oversized. I am referring to the loop diameter not the bore depth. For example we here in NJ drill an average of 180'/ton with 1" loops for residential unless otherwise specified in the plans.
It does help. I am on top of rock, not sand or silt, so I can see where the thermal grout is an advantage. I met with Quote #1 (the driller himself) just yesterday, and I feel very confident with this guy. The owner of the company, old as can be, is the one who delivered the quote to me. I can tell he is somebody who just wants to do the job right. His quote is all-inclusive, and barring major unforeseen circumstances I'm sure they would cover any issues that would minorly affect price. Quote #2, that I got through an installer, I could tell was "engineered" as specifically to the need as could be. In his defense, he broke it all the way down to what the bags of bentonite cost, which I appreciate, but at the same time had disclaimers about the use of additional casing which could drive cost up. I'm not saying he's right or wrong, just that Quote #1 to me was offering a fixed price job and Quote #2 was doing T&M. Given they are both the same price, and Quote #1 seems to be giving some additional breathing room to the system, it seems #1 is the right choice. Thanks for the input from all of you!
Sounds like either are fine. Gotta love a fixed price (no sense gambling when they are near identical). Not sure there is truely any system "breating room".
