Sorry for a question that has probably been answered many times here, but what is the best tubing for a horizontal slinky loop.  Following the advice of GeoDan from a previous post, we are planning to install four ¾” loops at 500’ each (see post Subject: Horizontal slinky on small lot http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/13/postid/35928/view/topic/Default.aspx )  The heat pump will be installed in the future by a professional.  For now, we are just getting the loops installed while we are replacing our foundation.  And yes, our plumber will pressurize them during backfill to determine if there are any leaks.

Thanks,

Brint

The most common pipe used and the only one recommended is HDPE pipe.

Can anyone tell me where one buys 3/4" HDPE in 500' or greater lengths?  I've now contacted Keller and Ferguson plumbing supply and a local irrigation supply house here in Portland, OR, and the longest lengths they carry are 400' lengths.   We can special order longer lengths, but the shipping more than doubles the cost the pipe - though the pipe is relatively inexpensive to begin with, so that may not be a terrible thing.  I talked with our plumber (who knows nothing about geothermal and does not want to be involved in the actual pipe laying) and he mentioned a barbed fitting with stainless hose clamps as an easy DIY solution.  He said the joint would outlast the pipe and would not significantly affect water pressure.  Not understanding the physics of pipe size, it seems to me that the joint’s ID restriction would increase the required pump pressure needed?  Would there be any advantage or disadvantage in going with a 1" pipe with a joint in this case – the extra cost is minimal?

<!--[if !supportEmptyParas]-->Thanks,
Brint
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In my quest to find longer HDPE pipe lengths from a local supplier, I just talked to a very knowledgeable and helpful local Portland, Oregon HVAC contractor that installs geothermal systems, and he gave me some disheartening advice on slinky loop systems after I told him my project’s likely total trench length and loop lengths – (4) 70’ trenches with 500’ of pipe each, 2.5 tons heating. 

His advice:
1. He said that slinky loops run up to 8 degrees cooler than straight runs with longer trenches, which means that the coolant in the loops will likely be well below freezing at the peak of the heating season and could very well approach 15 degrees (with the heat pump shutting off at 15 degrees).  This would mean that we would likely have a freezing “bump” in our yard and that under no circumstances could the trenches be closer than 10’ to the house because they will freeze the soil and soil expansion could impact the foundation.  
2. Do not run the slinky loops to a common manifold inside the house.  For two reasons:  First, they will rob your house of heat and thus lose considerable efficiency and you cannot get enough insulation around to protect against this;  Second, because they will be well below freezing they will sweat and, again, no amount of insulation will protect against this and the head is impossible to insulate.  Instead, fuse them at a common head ten feet away from the house and run them directly to the heat pump. 

<!--[if !supportEmptyParas]--><!--[endif]--> This is the first I’ve heard of such low temperatures with water loops.  Most user reports I’ve read about seem to have return temperature in the low 40s to high 30s range – GeoDean reports that his own house’s system in a much harsher climate than Portland runs a low of 37 degrees.

<!--[if !supportEmptyParas]--><!--[endif]--> The contractor still thought even my small system would be more efficient than an air-source heat pump in the Portland climate and was not trying to talk me out of my plans --  just cautioning me.  He said that after doing two slinky installations that both performed relatively poorly, he does not recommend them.

Does anyone have experience with slinky loops freezing soil or manifolds sweating too much?

Thanks,
Brint

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I mis-quoted GeoDean's lowest return water temp -- 41 degrees.

Hmmm. Some good advice here but also several points that are completely wrong.

1. In general, yes, slinky loops will require more feet of pipe for a given set of conditions (load, soil type, climate) than other configurations. The energy absorbed by a loop field is a function not only of the total length of pipe in the ground, but also how the total area that the field can draw from. For a given length of pipe, denser configurations will not perform as well as a more spaced out layout. That is not to say that a properly designed slinky won't work - they are a very popular design and most work just fine. As you said, pipe is cheap, so generally increasing pipe density while decreasing excavation costs will save money. It needs to be properly designed for your system and conditions - don't go by rules of thumb. Also, slinky configurations are slightly more difficult to backfill properly - this is critical to good loop performance. Again, not a problem if you know what you're doing.

I would absolutely expect the loop temperatures to drop below freezing at the peak of the season in most heating dominated climates - this has nothing to do with the type of loop, it is just the way most horizontal loops are (should be imo) designed. As the ground freezes it gives up latent heat, which helps keep loop temps near or slightly above freezing. Try to find a Canadian geo guy to ask about this! As your guy said, for this reason you don't want the loop field too close to the house, 10 ft sounds like a reasonable number. Again, proper design for your particular situation is critical. You need to make sure you have enough heat gain in the summer (solar, groundwater, ac loads) for your loop field to recover.

2. I will say that an outside manifold is common practice and makes the indoor plumbing much simpler and neater. It is a fine way to do it. HOWEVER, there are also strong points in favor of indoor manifolding, espscially for the DIYer. All outdoor connections should be fusion welded for reliability, period. I think you mentioned barbed fittings and hose clamps above - don't do it! A loop field filled with methanol or other antifreeze 6 feet down is a little more problematic than a water line to a hose pipe if one of the connections should fail. This is just about the only point I've ever seen virtually every geo guy agree on - only fusion welded connections underground. So, if you don't want to rent a fusion welder and get trained on how to use it, you need to bring your loop ends inside so there aren't any joints underground. The other advantage is that you can valve each loop and if it is set up correctly (Again, lots of other design considerations here) you can fill and purge the system with the loop circulator - otherwise you need a purge cart. Also if you do develop a leak, that loop can be isolated leaving you with partial capacity.

The biggest drawback of indoor manifolding is, as your guy said, the difficulty in insulating the assembly. It is by no means impossible, but it is fiddly and labor intensive. I have no idea where he got the idea that the manifold will take a significant amount of heat from the house. Maybe if you set it up in the middle of your living room with no insulation and a fan blowing across it! If I feel ambitious I'll do some calcs tonight to show how ludicrous this statement is. Pipe sweating is really the only issue to be concerned about and as I said it is simply a matter of insulating properly which CAN be done. Also, indoor vs. outdoor manifolding is a completely seperate issue from what type of loop field you have.

Getting back to your original question on where to find pipe, most installers will spec "geothermal" HDPE, which as far as I can tell is virtually identical to regular HDPE but is manufactured to a higher standard and has a longer warranty (50 years in some cases). It also comes standard in longer coils, is marked by foot, and is usually pre-pressurized so you know as soon as you cut the end off that the coil has maintained its integrity. Will regular water HDPE work? Probably, but in most cases the pipe is a pretty small part of the system cost, so you might want the peace of mind of using the real "geo" stuff. Eagle Mountain (radiantmax.com) sells geo pipe, but shipping might be a killer as you said. Note that pipe speced for geo systems is typically SDR11, which is an OD-controlled pipe. Barbed fittings designed for regular HDPE water pipe, which is ID controlled, will fit too loosely to get a good seal. I found the barbs would work for testing with a double hose clamp, but I'd never trust them as a permanent solution.

Hope this helps somewhat - keep doing your research and if at all possible at least get a pro to consult - the pipe is cheap but you don't want to be digging it all up in a year!

41 degrees is very good for a return temp in a cold climate, but as I've said loop design is very dependent on regional climate,soil conditions, and economics. Around here the ground temp gets down to 41 degrees in February 6 feet down, so it would be impossible to maintain that temperature. My loop bottomed out at about 34F incoming and I was very happy with that, although I want to see a couple of years of operation before I'm completely satisified. I designed it for 30F. Google a program called GS2000 - it is a free loop design program put out by the Canadians. It is not without its issues, but it might help illustrate the relationship between loop types, average temperatures, and ground conditions.