I'm in the process of getting my geothermal heating all sorted out, and one thing that concerned me was the borehole separation. I have two 240' boreholes that were supposed to be 16' apart. When they moved the drill for the second hole they seemed a little close to me. I asked if they had measured it as 16' and they said yes, so I went about my business. Once they were done I took my own measurements for my property plot and found that (center to center) the boreholes are only 14'5" across. Is that enough? Is 16' an overly generous buffer? Edit: Just talked to the installer and he says that the real minimum is 10', and since they are farther apart than that it should be fine.

You won't notice that minor difference (see purple line, figure 7). On the other hand, without space constraints, I'd use 25'. And I'd orient a 1x2 array perpendicular to ground water flow (can be worth ~5%).

It's fine.

Posted By jonr on 04 Nov 2014 09:23 AM
You won't notice that minor difference (see purple line, figure 7). On the other hand, without space constraints, I'd use 25'.

This is an exceptionally helpful document to read.  Best attempt to quantify benefit of various borehole sizing's I've seen.  Thanks.  Bill

In this area it is 10 ft separation so I guess it all depends on the location and what us usual and customary.

Assuming all factors being equal, and assuming average conductivity, load etc...The difference in performance between 2 boreholes 14.5 ft apart versus 16 ft apart is about 0.5F at the peak of the heating season, meaning if your system is designed for 30.5 F with 16feet apart, with 14.5 ft apart it will drop down to 30F.

I don't know your loads, but given that you posted before that you have a 3 ton heatpump, the average loop temp will only be affected by a tiny amount. The difference in annual operating costs are less than $2.

since drilled holes never go straight down, it is possible for two holes to intersect under ground. This is reason enough to keep distance. We shoot for at least 15', most commercial jobs go for 25'

They don't go straight down? I've never heard of that before.

That didn't occur to me before, but it makes sense. Steel isn't particularly rigid when you've got a 200 foot column of it that's only 6-8 inches wide, especially when you are pushing on it. It'd be interesting to point a laser range finder down a new hole and see how far down it went before it started to stray.

I have seen a loop 25' away get drilled into . ... here is what you might see

Wow, how deep do you have to go to get that amount of wandering?

Yep I have seen it too.

if the drilling is in an area of large bolders, the drill bit can wander off plumb when glancing of the side of a boulder. Kind of like a ricochet. The holes were 300' deep where this happened.

Doc points out the more practical side. The consequence is a fraction of a percent of the annual operating cost.

The consequence is a fraction of a percent of the annual operating cost.

Unless a hole isn't straight and the spacings of 14.5' and 16' feet at the surface become something like 1.5' and 3' below the surface. Unlikely, but that would make more like a 7% difference - with the overall loss (compared to straight @ 25') going from ~18% to 25%. Use three or more boreholes and spacing becomes even more important.

Unfortunately, it's unlikely one can get a thermal performance guarantee beyond "it works" - so I guess the owner should watch for and investigate corner cutting. But in this case, let it go.

"Unless a hole isn't straight and the spacings of 14.5' and 16' feet at the surface become something like 1.5' and 3' below the surface."

To the question does 14.5' instead of 16' spacing matter the answer "The consequence is a fraction of a percent of the annual operating cost." is correct.

IF the bores aren't straight, IF the bores aren't grouted, IF the header pipe is kinked, IF the bores are too short, IF the bores weren't correctly flushed...........IF...........

Yes many things could be wrong that matter, but the lost 1.5' of spacing isn't one of them and hopefully we can put that concern to rest for a consumer who just spent alot of money vs worry about the boogeyman in the closet.

Posted By jonr on 10 Nov 2014 09:41 AM

The consequence is a fraction of a percent of the annual operating cost.

Unless a hole isn't straight and the spacings of 14.5' and 16' feet at the surface become something like 1.5' and 3' below the surface. Unlikely, but that would make more like a 7% difference - with the overall loss (compared to straight @ 25') going from ~18% to 25%. Use three or more boreholes and spacing becomes even more important.

Unfortunately, it's unlikely one can get a thermal performance guarantee beyond "it works" - so I guess the owner should watch for and investigate corner cutting. But in this case, let it go.

1.5 foot is far away from corner cutting. No reason to assume that there is anything wrong. He should enjoy the geosystem! Jon, please enlighten me, why is the overall loss between 18-25% if the distance becomes 1.5' - 3' below the surface?

At that distance your two holes will transfer to the same part of the ground. I'd expect you to over-saturate the ground, so that shortly in to each season the water temperature would get quite cold/hot for winter and summer respectively, so your unit would have to work very hard to keep moving the heat against the gradient.

In the winter you'd probably end up using Aux Heat a lot as well.

Posted By Yonder on 10 Nov 2014 03:44 PM
At that distance your two holes will transfer to the same part of the ground. I'd expect you to over-saturate the ground, so that shortly in to each season the water temperature would get quite cold/hot for winter and summer respectively, so your unit would have to work very hard to keep moving the heat against the gradient.

In the winter you'd probably end up using Aux Heat a lot as well.

I understand the principles of heat transfer! Just to clarify, are we talking about missing 1.5-3' from the 16 feet, or about the 2 loops only being 1.5-3' apart. If it is the latter, the loss is performance is a bit more than 18-25%. Thus the question how John came up with that number.

Ah sorry, I read him as saying that eventually the boreholes came within 3' of each other, and I assumed that his percentage was based off of an assumption of when the boreholes would get that close together. I would imagine that the boreholes would remain 15' away for at least a hundred feet before starting to wander off near each other, and they would probably approach each other fairly gradually.