I just read recently that some geo is installed using 1 1/4" pipe in the ground instead of the standard 3/4". From what I've read here, for each ton you use approx 400' of the 3/4. How many feet would you use for 1 1/4? Is there any advantage or disadvantage of using either one? Do slinkies come in 1 1/4?

WRT thermal conductivity, there's very little difference
between 1.25" and 0.75" pipe. The 1.25 has greater
surface area -- but it also has greater wall thickness,
and (in principle) those effects "cancel out" exactly.

However, at any given GPM, the 1.25 pipe has less
pressure drop -- so it might let you get by with fewer
circulating pumps and/or lower pumping costs.

OTOH, if you have to ask that question, you'd really be
well advised to seek the services of an experienced
loop designer.

...you pay for what you get,

LK

What the hatter said........
Your question mentions 400' ton of 3/4/ton- I don't know how you surmised that. That would be small if not a pond or vertical loop (in most applications).
Aside from commercial apps, and header pipe, the only folks who use 11/4" pipe for their loops are DIY's. It is considerably harder to work with than 3/4".

At the end of the day it doesn't matter what you use as long as you use enough of it.

Joe...I meant to say 600'. Is this correct?

1.25" pipe has bit over 11% more heat transfer capabilities compared to 0.75" pipe, so we like them in vertical loops, since they save drilling costs. But many cons as well, especially for horizontal loops. It is true that the more in surface area is counter acted by thicker wall, but not fully.
The cons are:
Much more expensive, much more to wrestle with, about 3 times the volume than 0.75", thus much more antifreeze costs. Less turbulent flow. Be aware of 3 times as much pumping power needed to purge the air out, so you have to be careful to not run too many in parallel. Many disadvantages, the only 2 advantages I can think about is the 10% higher heat transfer per linear foot, and the higher volume acts almost as a buffer within the pipe. You have to really know what you are doing when you use them.

Don't you get the increase in heat transfer only if you increase the pumping volume? A foot of 3/4" pipe holds a volume about 5 cu in and has a surface area that the liquid is in contact with of around 28 sq inches. For 1 1/4" pipe that would be a volume of around 15 cu in and area of 47 sq in. So the surface area per volume of water in the 3/4" pipe is almost double that ratio in the 1 1/4" pipe. So, all things being equal, for the same flow, wouldn't you get better heat transfer out of the 3/4"? Wouldn't you only want to go to the larger pipe if you needed the extra flow? Not an installer here...just seemed counter-intuitive to me. Please set me straight.

I will leave the mathematics to those that enjoy a good arguement. From a drilling perspective alot of what goes on in the field is based on the size of your tool, and nothing more. Example being rod size. If you are drilling with 4 inch od rods, the smallest hole you can drill is around 6 inch id. If you are hard rock drilling the smallest down hole hammer is 6 inch. The people driling 6 inch holes would gravitate towards 1-1/4 pipe. Those of us with small rods prefer 3/4 inch pipe. The more material you take out of the ground the more expensive the hole is.
Eric

Posted By waterpirate on 13 Jan 2012 06:51 PM
I will leave the mathematics to those that enjoy a good arguement. From a drilling perspective alot of what goes on in the field is based on the size of your tool, and nothing more. Example being rod size. If you are drilling with 4 inch od rods, the smallest hole you can drill is around 6 inch id. If you are hard rock drilling the smallest down hole hammer is 6 inch. The people driling 6 inch holes would gravitate towards 1-1/4 pipe. Those of us with small rods prefer 3/4 inch pipe. The more material you take out of the ground the more expensive the hole is.
Eric

Makes sense to me.  So in the case of drilling, you then design the system to work with the size pipe that the drilling dictates, if I understand right.

Posted By docjenser on 13 Jan 2012 02:46 AM
1.25" pipe has bit over 11% more heat transfer capabilities compared to 0.75" pipe ...
It is true that the more in surface area is counter acted by thicker wall, but not fully.

Do you have a source for the 11% figure? I'm not disputing it,
just wondering where it came from, and under what conditions.

As I alluded to up-thread, ratio of surface area to wall thickness
remains constant (for a given SDR), and that mostly cancels out
the thermal effects (i.e., your 11% thermal transfer improvement
for 67% increase in surface area is rather effective cancellation).

Where the comparison gets wobbly is in deciding what factor(s) to
hold constant ...GPM? ...pressure drop? ...number of circ pumps?

Re: sesmith's analysis of "surface area per unit volume" -- that
makes perfectly good sense for a static liquid (e.g., hot darjeeling
cooling in a tea cup), but for a flowing system it's a rather poor
model. It's better to think in terms of mass flow (lbs/min/area),
and to keep in mind you must not ignore the wall thickness of
the pipe (or the tea cup).

Zeroth Law of Thermo: "all things being equal" never happened.

CM, WF and GLD all give you the same results. Lets say vertical loops for a 3 ton system your bore whole length is 450 ft for 0.75" and 400 ft for 1.25", which goes surprisingly well conform with what we monitor from our loopfields.
So we now go with 1 x 400ft 1.25" instead of 3 x 150 ft 0.75".


Total flow is the same 9 GPM, pressure drop is pretty much the same, thus the circ pumps are the same in the above scenario.


Since we are trying to explain this, does't the increased wall thickness add even more the the outside pipe surface area?

Posted By docjenser on 14 Jan 2012 03:26 AM
So we now go with 1 x 400ft 1.25" instead of 3 x 150 ft 0.75".

Yep, my installer did the same; 1.25" pipe and 1 x 450' bore for 3-tons.
Total length of pipe (including trench and indoors) is about 1150'. That
gives me ~8 GPM with a single UP26-99 circ pump (20% methanol A/F).
KISS: minimum joints, no manifolds, no balance issues, works great!

Since we are trying to explain this, does't the increased wall thickness
add even more the the outside pipe surface area?

I'm satisfied with the explanation as it stands. IMO, an 11%-ish mismatch
between actual field experience and theoretical calculations is about as
good as one could hope for -- especially since the calculations are based
on an incomplete/imperfect model. (If you could predict soil conductivity
with only 11% uncertainty, you'd win the Nobel Prize for Geodesign.)

But since you asked: No, the difference in actual ODs for HDPE DR-11 is
smaller than you'd expect from the ratio of nominal sizes. 1.25" PE-3408
is only 58% larger in OD than 0.75", (specifically, 1.66" versus 1.05" OD).

You mean that the software is flawed and there really is not difference in heat transfer between 0.75 and 1.25 pipe. That would contradict what we see in the field. We measure absolute the same performance between 400' long 1.25" and 3 x 150' (450') long 0.75" pipe.


So it appears to me the increase of pipe surface is more important to heat transfer than the opposing effect that the increase in wall thickness, since they increase linear. I really don't have another explanation.

Exactly. Greater circumference means more contact with more soil/grout even if pipe wall resistance is pretty much the same.

Posted By robinnc on 11 Jan 2012 11:20 PM
I just read recently that some geo is installed using 1 1/4" pipe in the ground instead of the standard 3/4". From what I've read here, for each ton you use approx 400' of the 3/4. How many feet would you use for 1 1/4? Is there any advantage or disadvantage of using either one? Do slinkies come in 1 1/4?

If you tried to make a 1 1/4" slinkie, it would probably honestly be 5' in diameter and weigh ALOT! 

1 1/4" takes some practice to work with in the field!

Posted By docjenser on 15 Jan 2012 03:09 AM
You mean that the software is flawed and there really is not difference in heat transfer
between 0.75 and 1.25 pipe. That would contradict what we see in the field.

No, actual field experience always rules. If that agrees with CM/WF/GLD software,
(as you claim), the software must have "more smarts" than a calculation based only
on pipe OD and wall thickness. My point is that the simple calculation based on OD
and wall thickness agrees with "field experience" -- within about 11%. IMO, that's
"close enough for government work" considering the other design uncertainties.

We measure absolute the same performance between 400' long 1.25" and
3 x 150' (450') long 0.75" pipe.

OK, if you insist. But I'll remain skeptical of "absolutely the same" claims, because
that requires the highly improbable condition of "all else being absolutely equal."

Posted By jonr on 15 Jan 2012 08:19 AM
Exactly. Greater circumference means more contact with more soil/grout
even if pipe wall resistance is pretty much the same.

But the pipe wall resistance is not "pretty much the same." The
wall thickness -- and thus, wall thermal resistance -- is directly
proportional to pipe diameter. The 'R' in "SDR" stands for ratio.

The net/total thermal resistance of the pipe (btu/time/degree, not btu/time/degree per area) is "pretty much the same". it is thicker but there is more of it. That cancels out, the increased soil contact area doesn't. That's why everything shows that larger pipe has a thermal advantage.

Posted By jonr on 15 Jan 2012 05:11 PM

.... innumerate blather ....

"Stand firm in your refusal to remain conscious during algebra.
In real life, I assure you, there is no such thing as algebra."
- Fran Lebowitz

Posted By jonr on 15 Jan 2012 05:11 PM
The net/total thermal resistance of the pipe (btu/time/degree, not btu/time/degree per area) is "pretty much the same". it is thicker but there is more of it. That cancels out, the increased soil contact area doesn't. That's why everything shows that larger pipe has a thermal advantage.

good point

We measure absolute the same performance between 400' long 1.25" and
3 x 150' (450') long 0.75" pipe.

OK, if you insist. But I'll remain skeptical of "absolutely the same" claims, because
that requires the highly improbable condition of "all else being absolutely equal."



We have two houses about 1/8 miles apart in the same development, one on 3 x 150' 0.75 loop, the other one on 1 x 400 1.25" vertical pipe, the first one extracted 84.37 MMBTU during last years heating season, the latter one 83.14 MMBTU. The first ones loop was at 31.9 F minimum, the latter one at 32.0F at the end of the heating season. Heat rejection during A/C was 11.79 MMBTU and 11.66 MMBTU, respectively. Different houses, but close. Please disregard the "absolutely the same" and replace it with "similar". Similar heat extraction and rejection, e.g. similar loads on the loops over the seasons suggests that the performance of the 2 loop systems are very close together.