'Double piped' well?

I'm in the process of having a new system installed in northern New Jersey. I got two quotes from the two biggest Waterfurnace dealers in the area and went with the one that could meet my timeline + more reasonable cost. I'm getting a 4 ton 7 Series, desuperheater, etc. Calculated heating load is around 3.5 ton.

Well drilling started on Monday. Originally the main contractor told me I was going to get 2 x 375' x 1 1/4" wells. Drilling has not gone well at all, in 5 days they managed to get one ~300' well and have elected to 'double pipe it.' Essentially they pushed two separate U bends down it (I have four pipe ends above ground) and are claiming it's spacer separated + grouted between. I didn't see it go in the hole so I have no way to verify this.

Before I let the crew leave I called my installer and verified that he had signed off on this design change. He swears that he has validated this design and I will still have ample loop but the math side of me is skeptical. If it were this simple it seems to me that every system would be designed this way to cut drilling cost in half.

Is this a reasonable design choice? Anybody have the math to say how two vertical loops with normal 10-15' spacing would compare vs this?

 

Hi and welcome!
2 loops in one borehole is done quite frequently when planned for in advance. The math side of it I will leave to the designers, but two loops in one borehole does not equal the same load as 2 loops in two separate boreholes. Will the 2 loops in one borehole support your load is the question.
Hope this helps
Eric

 

That does help, thank you. The installer claims an install base of ~800 systems, I'll have to trust him (have little choice I suppose). It's just something I had not seen in my fairly lengthy research so I got a bit paranoid. I suppose I will know nearly immediately if I am going to have a problem as we'll be nearly into heating season by the time this is done.

 

You gain about 15-20% by putting in a second pipe, not more....

 

I have not seen a spacer for double pipe before. Doesn't mean they don't exist, but I have heard of them. Not that it would add to efficiency much.

So, your 2 x 375' boreholes are now 1 double piped 300' borehole? The latter is about 1/2 the capacity of the former. Or are they doing 2 x 300' double piped boreholes?

 

That is kind of the range I would have expected.

Maybe they just meant they did their best to keep them on opposite sides of the hole while it was grouted. Perhaps not a literal physical spacer. And yes, 2x375 became 1x300' double piped.

Is this completely outside the realm of workable?

 

If the original design required 2 wells x 375' with one loop per well, and it seems reasonable, then 1 well x 300' with 2 loops in that well is nowhere near equivalent. As stated above, a single 300' well with double loop pipes will perform maybe 20% better than a single 300' well with a single loop in it. With the single well double loop 300' bore, you would get roughly 300 + 20% / (2 x 375) = ~48% performance of the 2 wells x 375' with a single loop in each well. Now if they are installing 2 wells x 300' with 2 loops in each well, then you are roughly equivalent to the original 2 x 375' with a single loop in each well.

 

Met with the installer today, it seems to have been a miscommunication from mixing terms (intermixing borehole depth vs loop length per U in the same sentence basically). Seems due to all the trouble they had getting to bedrock (at 165 feet) they opted to do it all in one borehole at 600'. So 600x1.2 = 720 which is basically where we were originally supposed to be. He agreed that a single 300' with two loops would not be nearly enough, so I think we are all on the same page.

Life looks much better when the math and sources align. Thanks for the math assist everyone, the 1.2 multiplier was what I needed to make sense of it all.

 

No to be a doubter (I'm a believer in trust but verify, especially when a lot of $ is involved), but did the installer or well driller provide you with a copy of the drilling report that typically has to be filed with the state? If not, you should be able to get a copy from the state.

https://www.state.nj.us/dep/watersupply/pw_permit.html
"Once a well is constructed, the driller is required to submit a well record document which is the as-built description of the well."

You can also search for your well drilling permit here:
https://www13.state.nj.us/DataMiner...ategory=y&catName=Water+Supply+and+Geoscience

I assume the permit had to be amended if it changed from 2 x 375' to 1 x 600' ?

 

At one point the crew doing the drilling were talking about going to 600 in one borehole. Given that, my inclination is to believe them, but with the link I'll definitely check on it. Currently there are two permits for 300' each.

I'll ask for a copy of the final well record document both from the drilling company and from the state once it's fully registered.

 

So you have 1 borehole, 600' deep, with one single 1.25" flow circuit installed?
You pressure drop now is pretty high, with Methanol it would be 45ft/hd just for the borehole (versus 7.9 ft/hd with 2x375 boreholes), plus about 15 ft/hd for the rest of the system.

If that is the case, what circulation pump are they planning to use, and who is paying for the significantly higher pumping energy needed for the life of the system?
You picture shows 4 pipes going into the hole. Are they at 300' borehole depth, or is the borehole 600' deep, with (2) x 1200' pipe in there?
You need to figure out precisely what they are trying to do. How much pipe in how many holes, how many circuits and what pipe diameter....

 

My current understanding subject to verification of borehole depth is:

600 foot borehole
2x1200 ft loop
1x32-140 pump
1 1/4" IPS loop (I photographed the spools on the truck)
Ethanol with environal additive (I believe methanol is not allowed here)

I have been following (or at least trying to follow) your best practices carefully. I was pleased with the pipe diameter, pump choice, and coolant selection originally as they tracked your summarized efficient system list as closely.as possible assuming methanol is indeed not allowed. I clearly need to read up on pressure issues as I don't have a good grasp on that currently.

(furious google research)

OK here's what I've come up with. It seems logical to me, but I'm sure I've made mistakes. Please do correct me as I want to understand.

Base assumptions (from your post history mostly):

3 gpm/ton, in this case with 4 ton I'll assume I need all of it so 12 gpm total system flow
Methanol/ethanol are more efficient to pump than water so I'll use water to get worst case numbers and since I don't know the specific mix they will be using.
15 feet of head outside the well itself
0 elevation to worry about since it's a loop starting and ending at the same height
12 gpm should get split in two for a parallel loop (which appears to be standard), we'll assume equal length and materials so each gets 6 gpm

Web calculators vary quite a bit the worst case one I found using 6 gpm, 1200 feet of 1.25" poly pipe resulted in about 12.4 feet of head loss. So double that and round it to 25. Add your 15 and I'm looking at ~40 feet of loss total.

The pump appears to be capable of only ~11 gpm at 40 feet of head but that number dropping would change the head pressure yes? With pumping efficiency gain from ethanol it looks like I'll be right at the edge of the pump's performance envelope.

There's a lot of worst case/rounding up in this math so I think if anything I have erred on the side of caution unless I am just hopelessly misusing the tiny bit of knowledge I've gained today.

 

With two (2) parallel loops, why would you double the head loss from 12.4 ft-hd to 25 ft-hd? Normally, with equal length/size loops, you only need to use the head loss for a single loop in your total head loss calculation when you have parallel loops.

https://www.engineeringtoolbox.com/pipes-series-parallel-d_1787.html

What size header pipe to connect those 2 x 1200' loops? How long will the header pipe be from the well loops to the unit?

 

WITH Ethanol I get about 14 ft/hd for the loop field, and 9 ft go hd for the unit. As Arkie mentioned, it now depends a bit on the pipe length side and outside to the loop field what your final system pressure drop will be.

Ethanol has higher pressure drop than water, not the other way around, and is LESS efficient to pump.

However, even if you only have 11 gpm during the rare occasion when the heat pump runs at stage 12 for full heating capacity you'll be fine.

If that is what your design is, nothing strikes me as a concern.

 

I had found that exact same page and apparently managed to combine the pressure loss formula with the mass flow formula below it. I knew the result should be less than the full flow through a single equivalent length pipe so it sanity checked and here we are.

A few lingering questions:

1) With regard to antifreeze - When you say ethanol is more difficult to pump than water is it a viscosity issue, or a mass issue? Ie, do you have to generate more pressure to pump the same volume or are you forced to pump higher volume to get the same thermal performance? In either case, I think I'm starting to understand that methanol/ethanol mixes are simply better than propylene glycol and since you must use some form of antifreeze they are preferred.
2) No major efficiency issues then with this design? Your numbers are a whole lot better than my attempt and would put me solidly in the middle of that pumps capabilities.

Thanks to both of you for the assistance all around, I'll verify borehole depth and if it checks out at 600 I will stop worrying.

 

I ran some numbers using the Climatemaster Pressure Drop calculator (an Excel file that you can download from their website) using a 4 ton TE_049 2-stage heat pump with internal Magna 25-140 pump and 20% Ethanol. This gives you a freeze point of 18.2 deg F. If you bump up the Ethanol to 25%, it drops the freeze point to around 13 deg F, but also slightly increases the head losses.

I assumed 2 loop circuits of 1-1/4" pipe x 1200' long, header pipe = 1-1/2" total of 120' (10' from unit to wall, 50' from wall penetration to loops), 1" rubber hose to connect unit to 1-1/2" header pipe = 5' total.

This resulted in 25.2 ft-hd total system pressure drop at 12 gpm flow . This was well within the capability of the loop pump (pump can push 12 gpm against ~38 ft-hd). Reynolds number shown is 3,298 which is above the recommended 2500 for non-laminar flow. These numbers would change some based on the head loss of the actual unit heat exchanger used (The Climatemaster software isn't going to list a Waterfurnace unit).

 

It is a viscosity issue, not a mass issue. Ethanol has a higher viscosity than methanol, but much less than glycol.
With 2 x 1.25" circuits you are fine......you have 11 ft of head pressure drop thought the unit piping and the heat exchanger.

 

As an update to this:

System has been operational since late September last year. Aside from some issues related to zone sizing (which were corrected with a service call) we have had no issues. On the coldest day of the year I had to manually enable emergency heat (gas Navien boiler to exchanger in duct) + wildly over-ask on temperature because the zone sizing was preventing the cold upstairs from calling for heat on its own. Once this was corrected I don't believe there was any automatic emergency heat usage (and I was watching it closely). I don't have my own monitoring yet but the lowest loop temp I recorded was 37 F on a ~10 degree overnight temp day.

Now that we're nearing the hottest part of the year I'm keeping the house downright chilly (66 upstairs, seeing as low as 64 downstairs by the primary return) running generally stage 3 cooling @ ~64 F loop temp. Just a couple more months and I'll have a full year of data showing that the system as built does indeed meet my needs near 100%. I may even make it through the typical year with no backup heat at all now that I have completed a full house air-sealing and insulation adventure. I'm on track to be within 5% or so of estimated annual energy use which is pretty good given my over-use of cooling.

So in short, you guys were all correct and the system is fine. All of the input is much appreciated, I'm not sure how much my installer enjoyed having me researching every decision he made but in the end I agreed with every choice he made thanks to the advice I received here both directly and indirectly. I read a lot more than I post, and I read a lot.

 

Whoo Hooo! another happy outcome. Congrats.
Eric

 

I'm in central NJ and just went through vertical loop design in rock. I caution getting too excited this early on in your usage only because in NJ, you are bound to have very unbalanced heating and cooling loads (lots more heating than cooling). So over time, you will tend to cool the ground around the heat exchanger (extract more heat in winter than you put into the ground in summer). This will result in lower winter loop temps over time. You will probably need several years (maybe a decade) of data to realize this but your lowest loop temp of 37F that you got this year may trend toward 32F over multiple years of use because of the heat/cooling imbalance. Its just a product of where we live - not much you can do about it. So I suggest being cautiously optimistic while keeping an eye on winter loops temps for the next say five years.