Posted By docjenser on 15 Jan 2012 11:20 PM
Please disregard the "absolutely the same" and replace it with "similar".

Not a rational comparison. Why would you expect two vastly different
bore depths to expose the pipes to "similar" geology and hydrology?
The stuff coming up from my 450' bore wasn't even close to uniform
for the entire depth. Mostly rock (in MANY colors), several pockets of
sand, plus two or three underground streams (according to the driller).

BTW, if jonr's calculation-free handwaving constitutes a "good point,"
please explain why the 1.25" pipe shows only an 11% advantage --
rather than the 60%-ish advantage that his [pardon the expression]
"model" would predict?

Sorry, but the calculation based on the (constant) ratio of diameter to
wall thickness already accounts for diameter effects once. You can't
count it a second time, or you're gonna end up with HUGE disparity
between calculated and actual results. ...as jonr ably demonstrates.

Which model would you trust more? ...off by 11%? ...or off by 50%?

Posted By Looby on 16 Jan 2012 12:29 AM

Posted By docjenser on 15 Jan 2012 11:20 PM
Please disregard the "absolutely the same" and replace it with "similar".

Not a rational comparison. Why would you expect two vastly different
bore depths to expose the pipes to "similar" geology and hydrology?
The stuff coming up from my 450' bore wasn't even close to uniform
for the entire depth. Mostly rock (in MANY colors), several pockets of
sand, plus two or three underground streams (according to the driller).

BTW, if jonr's calculation-free handwaving constitutes a "good point,"
please explain why the 1.25" pipe shows only an 11% advantage --
rather than the 60%-ish advantage that his [pardon the expression]
"model" would predict?

Sorry, but the calculation based on the (constant) ratio of diameter to
wall thickness already accounts for diameter effects once. You can't
count it a second time, or you're gonna end up with HUGE disparity
between calculated and actual results. ...as jonr ably demonstrates.

Which model would you trust more? ...off by 11%? ...or off by 50%?

I can't account for everything, nor do I want to. In this development the driller hit bedrock after 4 ft down, water after 27 ft, and very similar geology all the way down. I cannot exclude that other factors are there heating up the deeper pipe more. All I said was that what we can measure goes conform with the software. If you have any data to question the accuracy of the software I am the first to listen!
While the heat transfered into the pipe should be the same if added surface area and the added wall thickness cancel each other out. However, the same amount of heat is still extracted from a larger volume of ground due to the larger pipe, so the soil catches up quicker and you don't run the temp right next to the pipe down as much if you drawing the same amount of total heat from a larger volume of soil.

Some people just can't understand that there are multiple factors involved in the heat transfer. For example, pipe wall conductivity and soil conductivity (and many other things) . And no, they do not proportionally track each other. For example, one can put two pipes in a hole (clearly 1/2 the pipe wall thermal resistance per foot of hole) and no, it doesn't have 2x the total performance. Use the software (and ignore people who think they have it all figured out because they know what SDR stands for).

Posted By robinnc on 12 Jan 2012 10:10 PM
Joe...I meant to say 600'. Is this correct?

Generally for horizontals 600-900 feet with many provisos.

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. 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.

I have another explanation:
Design software relies on average depth as a componet to equate performance of all loops.
Your 11/4" loop has a 200' average depth while 3/4" has only 75' average depth.
Software awards greater performance the deeper you go, due to a percieved higher EWT.

Not a student of deep earth temp variations between 150' and 450' but it certainly is a feasible explanation.

Another thing that impacts loop performance, is system design, i.e. if one house has a 70mbh load and another an 80mbh load and both have a 6 ton heat pump installed with like size loop fields, guess which loop field will appear to perform better based on EWTs and COPs.
j 

Posted By jonr on 16 Jan 2012 07:58 AM
Some people just can't understand that there are multiple factors involved

Some just can't understand the difference between "big" and "small."

My initial statement was: "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."

That agrees rather well (i.e., within 11% ) with docjenser's real world
field experience -- and also with his geo software (according to doc).

OTOH, you have contributed generous rations of bombast and bluster,
while carefully avoiding anything remotely resembling a quantitative
statement or prediction.

If you can't show the math, you don't know what you're talking about.

It seems that if you look only at surface area and wall thickness, sure the proportions might cancel each other out. The larger pipe is still "connected" to much more ground which.

Joe, yes with a deeper pipe you do not have as much pipe subject to surface swing. Yes, the software does not account for it. Yes the load changes everything. Try it yourself with the same load on the system, just change the loop diameter with auto sizing.

Posted By docjenser on 18 Jan 2012 03:35 AM
The larger pipe is still "connected" to much more ground ...

Not really. With 1.25" pipe, the mass of rock & soil that sources/sinks all them
geo BTUs has "the advantage" of being 0.25" closer to the heat exchanger.

OR, you could model it as an "affected zone" that's 0.25" larger in radius.

Either way, unless your soil has poorer bulk conductivity than a solid block
of HDPE, the pipe-to-soil contact radius should have no significant effect.