What are the pros and cons of using 3/4"  VS   1 1/4"  HDPE for the ground loops in a horizontal installation?  We are being told that we can use either size but the length will remain the same.

Friction loss and the total length of the loop will will dictate pipe size when it comes to sizing the flow center to circulate the fluid.  The longer the loop, the larger the pipe diameter gets, so that the friction loss is kept at a workable number so the flow center can be smaller and consume less electric.
Hope this helps

For horizontal loops 3/4 is often used because of cost and it's "much" easier to work with. 1 1/4 is often used for the header which connects the loops together and into the home.

Again size of the loop dictates pipe sizing.

Can 1 1/4" be purchased in the same lengths as 3/4"? If not, would more fusion joints be considered a possible drawback to using 1 1/2"?

Posted By geome on 29 Apr 2010 09:29 AM
Can 1 1/4" be purchased in the same lengths as 3/4"? If not, would more fusion joints be considered a possible drawback to using 1 1/2"?

11/4" usually isn't purchased in the same lengths as 3/4". Though I suppose I could find 600 or 800' rolls if I looked, they would be difficult for 1 man to move around and manipulate.
Besides friction loss, pump sizing etc. do not underestimate the impact of P.I.T.A. (pain in the......) on your install cost.
I would consider that the main drawback to larger bore pipe.
J

Sorry. I keep forgetting how spoiled I am. We have a hydraulic driven machine that can accomodate 600' of 3 inch hdpe at a time. It straightens the pipe off the role and feeds it down the ditch faster than you can comfortably guide it.

It is simply a pain to work with due o higher stiffness and bending radius. The amount of heat exchange is about 15% more per linear foot, pressure drop is less. We use it frequently for vertical loops which you don't have to bend, when you want to save every foot of drilling. However excavation is relatively cheap, thus the pipe is suited better for horizontal application is 3/4 inch. So it more for practical then theoretical reasons, especilly when you are doing slinky.

It is simply a pain to work with due o higher stiffness and bending radius. The amount of heat exchange is about 15% more per linear foot, pressure drop is less. We use it frequently for vertical loops which you don't have to bend, when you want to save every foot of drilling. However excavation is relatively cheap, thus the pipe is suited better for horizontal application is 3/4 inch. So it more for practical then theoretical reasons, especilly when you are doing slinky.

Also consider multiple 3/4 pipes or loops in parallel if you need less friction.

Posted By jonr on 30 Apr 2010 07:12 AM
Also consider multiple 3/4 pipes or loops in parallel if you need less friction.

Yep, multiples in parallel reduces pumping losses, but it also reduces GPM in each
parallel branch. That reduces turbulence and WILL seriously reduce heat exchange
efficiency if turbulence (Reynolds number) is allowed to get too low.

Your total pipe length and GPM requirements are dictated by your heating load,
local geology, and loop type, i.e., vertical vs. horizontal, straight vs. slinky, etc.

Once you have established those constraints, loop design is a matter of finding
an acceptable tradeoff of material cost vs. pumping cost vs. Reynolds number
(with the antifreeze type/concentration of your choice, if any).

> WILL seriously reduce heat exchange efficiency if turbulence (Reynolds number) is allowed to get too low.

Actually "minimally reduce" when poor conducting plastic pipe is involved and this is more than made up by the increased surface area of larger or multiple pipes. Plus the lower pumping costs.

Posted By jonr on 30 Apr 2010 12:53 PM
> Actually "minimally reduce" when poor conducting plastic pipe is involved

Proof by repeated strenuous assertion? Please
provide one reason why anyone should accept the
unsupported opinion of an anonymous net.expert
when it directly contradicts the opinions of every
major GWHP manufacturer.

...this is more than made up by the increased surface area of larger ... pipes

Nope, just plain wrong. Larger pipe has thicker
walls, with a constant diameter:thickness ratio,
e.g., SDR-11 pipe has an 11:1 ratio.

Thus, for SDR pipe, BTU/(hr-LINEAR_FOOT-°F)
is mathematically independent of pipe diameter.
The benefit of increased surface area is EXACTLY
cancelled by the increased thermal resistance of
the thicker walls.

"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

Nope, not even close. If you aren't going to learn the physics (such as the effect of increased soil contact area) then run some software that will do it for you. Ground Loop Designer: 3/4 pipe, turbulent flow: 957'. 1 1/4 pipe, laminar flow: 916'.
> it directly contradicts the opinions of every major GWHP manufacturer.
Like Water Furnace? their software is Geolink, 3/4" pipe: 1520 ft, 1.25" pipe: 1370 ft.

I did run it too. You save about 10% in loop lenght with a 1.25 pipe versus 0.75 pipe to get the same heat extracted.

Posted By jonr on 30 Apr 2010 08:30 PM
If you aren't going to learn the physics (such as the effect of increased soil contact area) ...

If you aren't going to learn algebra, physics becomes nothing more than techno-superstition.

Which part of "exactly cancelled" didn't you understand?

...then run some software that will do it for you.

GIGO

3/4 pipe, turbulent flow: 957'.
1 1/4 pipe, laminar flow: 916'.
...
3/4" pipe: 1520 ft
1.25" pipe: 1370 ft.

Those numbers are totally meaningless without the specifics of GPM, temperature,
and antifreeze type/concentration. Your "turbulent" and "laminar" characterizations
mean nothing without the Reynolds numbers. Outputs without inputs are anecdotes.

'Zactly how turbulent was that "turbulent?" ...and how non-turbulent was "laminar?"

We are considering using a WF 3 Ton , Synergy 3D system, which provides forced warm and cool air along with in floor radiant heat. Earth tied slab construction. We have plenty of space and have a back hoe. To keep things simple, I was wanting to run 1 long loop in a 24 inch wide trench at least 6 foot deep. For me that is easier than multiple loops, but would like to know how it may effect the system.

Another good thing about geothermal loop design software is that free versions are generally available for anyone who wants to understand various parameters and their effects.

You will definitely need a bigger pipe if you only have one flow path.

How many feet of pipe are you planning to run?

Posted By jonr on 30 Apr 2010 08:30 PM 
3/4 pipe, turbulent flow: 957'. 1 1/4 pipe, laminar flow: 916'.
> it directly contradicts the opinions of every major GWHP manufacturer.
Like Water Furnace? their software is Geolink, 3/4" pipe: 1520 ft, 1.25" pipe: 1370 ft.

Aren't you the jonr who preached that there is no difference with turbulent flow? Are you sure you want to use a program that disagrees with your premise to support a different one?
j

Posted By Stonelake on 02 May 2010 09:05 AM
We are considering using a WF 3 Ton , Synergy 3D system, which provides forced warm and cool air along with in floor radiant heat. Earth tied slab construction. We have plenty of space and have a back hoe. To keep things simple, I was wanting to run 1 long loop in a 24 inch wide trench at least 6 foot deep. For me that is easier than multiple loops, but would like to know how it may effect the system.

If you are buying a waterfurnace then the dealer can help you with your different loop options. Even if you pay a little for the design, you want him, not you, to own the results.
j