Hi,
This thread is hanging in there..
Here is a link to our site to give an idea about wire ties.
http://www.blueridgecompany.com/radiant/hydronic/687/wire-tying-tool-ties
If you are tying to re bar it is logical to elevate the pipe as high as possible.
RPA may have some info on heat transfer and pipe in slab depth
Link http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=471
Dan

one last trick if you do raise your pipe, and I agree with Rob it is marginally effective but if you tie to structural rebar you need to any way, ah yes the trick, place additional rebar on top of pipe at precise location of cut. this will hold all pipe from floating up and will provide a barrier of bar thickness should you go to deep. cheep insurance. Document accurately on master prints for your cutter.
Dan

Heat load analysis, tube layout,vapor barrier,EPS,tube,bar, concrete.

I would be more concerned about a properly sized heat source, ventilation and controls. Wire ties are irrelevant.

Rob, that's an interesting article about a simulation. Summary - keep the tubes closer to the top and you can run at cooler temperatures and get higher efficiency with geothermal. But it is significantly flawed in assuming a constant ground temperature - as soon as you lose some heat into it, the ground warms up.

the slab with the bottomed-out tubing needs 115 degrees water to yield an output of 25 Btuh/sq ft, compared to only 101 degrees water temperature if the tubing were centered 1.7 in. below the surface.

The tubes aren't water filled during the pour to keep them from floating up?

If you are referring to floating in my previous post, that was a pour word choice on my part, more to the concept that there will be some variation on the pipe when tied to re bar as to how it lays, it is not flat like glass when placed.
Do not fill your tubes wit water unless you are moving forward with the mechanical to heat. Pex left untended in a slab over winter can freeze and ruin your hard work and $.
Dan

Splitting hairs here.

A 28% increase in delta-T needed in a common geothermal situation?

Jonr: if his resuts were accurate, they would be significant. However, that is definitely a wide enough spread that my calculations would be called into question on a regular basis if they were accurate... that is, if my clients routinely needed 10 degree hotter water than I said the did, I'd hear about it.

That does not happen. Thus I question the final recommendations of the article.

I understand that your calculations agree with your tubing placement. But a rough check using the R value of concrete and the expected btu flow indicates that less concrete over the tubes reduces the temperature needed - the resistance to heat flow through the concrete is a significant portion of the total resistance from tubes to room.

None of this has anything to do with downward heat flow - once you have some insulation under it, the R value of the concrete is insignificant to downward flow.

Jonr, if that were the end of the story, thin pour concrete would be the lowest temperature floor system possible in concrete. This is not even close to true. the mix is not the same, which is part of it, but it would be greatly offset by a reduction in material over the tubes if your theory held water.

The R-value of concrete is for all intents and purposes negligible, especially as measured by inches. I can't comment on the math you are doing, except to say that if it tells you this is worth worrying about, it's wrong.

I have found thick slabs slow to preform, and so would argue that at about 5 inches you may have diminishing returns on response to heat and cool. if you are tying to bar raise it, wont hurt, other wise staple to slab and place your bar on top, by far faster and easy. But if you are in the 6 inch or greater, I would opt for bar and raise it to the 2 or 3 inch mark. That's how I would look at it. No sience here though.
There are a lot of ways to do the same thing.
Dan

I have found thick slabs slow to preform, and so would argue that at about 5 inches you may have diminishing returns on response to heat and cool. if you are tying to bar raise it, wont hurt, other wise staple to slab and place your bar on top, by far faster and easy. But if you are in the 6 inch or greater, I would opt for bar and raise it to the 2 or 3 inch mark. That's how I would look at it. No sience here though.
There are a lot of ways to do the same thing.
Dan

I looked into it more and once you have enough concrete to transfer heat laterally, then adding more concrete on top is just like adding anything else on top - carpet, tile, wood, etc. They all increase the water temperature needed. In the case of concrete, the effect is small - R=.38 (for an extra 4" of concrete) isn't much as compared to something like R=2 for some carpet. So around 3F difference for this example.

extra 4"?? if you're talking 6" deep slabs, ok, as I believe we've said before you don't leave it at the bottom for that. though 3 deg F is a rounding error. even for geo that's small. and that's on design day, typically, so really its a "maximum of 3 deg F" if your figuring is good.

typical slab depths though, you're talking about 2", which isn't even worth thinking about.

I agree, according to Rehau's graphs, the effect is there but not enough to matter. I used 10 btu/hr/sqft which should be closer to an average output than a max.

fair enough. I should have known better than to think you'd get caught on that.