Having a 6" slab poured for a 26 x 36 garage in a couple of months.  Intend to put 1/2" Pex spaced 12" (6" at perimeter); and want to lift the Pex up in the slab with 3" tall stand-offs.  I know there are mixed opinions on whether or not to move the Pex higher up in the slab; and I'm not wanting this thread to become a debate about that issue.  Just looking for suggestions, from those who do lift the Pex, on what "stand-offs" you have used and like.  My initial thought was stand-offs made for the Pex to clip into and then secure the 6x6 wire mesh on top of the Pex to help protect it during the pour; but I'm also open to supporting the mesh on pedistals and zip-tying the Pex to that.  The slab will be poured on top of 2" thick 25psi EPS, if that matters.  Have found different spacers for the mesh (like the link below); but not finding a lot for the Pex.  I have heard there are some made that screw into the below insulation; but I must be using the wrong search terms or something.  Thanks in advance for the help!

http://www.pexheat.com/3-Meshup-200?sc=7&category=156

http://www.rebarplastics.com/tichair

Raising the PEX in a 6" slab sometimes makes sense, depending on the loads.

If you must, the wire or rebar is supported on chairs and the tube wired to the reinforcement. If you were using the slab for heat storage raising tubes would work against you. We prefer PVC clad wire ties dpending on the size of the job. Few slab-on-grade radiant floors require variable tube spacing, since the technique is mainly used to overcome extraordinary loads e.g. where a full field pattern will not be employed (perimeter heating) or where human occupant comfort is critical, such as a residence where seating may be near a large exterior window.

We recommend at least 2" of cover and have our people watching every minute of the pour.

We like to get the PEX 1/3 the slab thickness above the insulation. In theory, going 1/2 the slab thickness would be better for heat transfer and result in a more efficient system...but as you said, this subject has already been discussed ad nauseam.

We use mesh pedestals and zip tie the PEX to the top of mesh. This works well if you use equipment and have a crew that can pour the slab without trampling the PEX/mesh. We like putting the PEX on top of the mesh because this makes it easier to repair any PEX that gets damaged during the pour or that develops a leak sometime in the future. The PEX should of course be pressurized prior to and during the pour to allow monitoring and identification of any leaks. You should also minimize putting PEX where contraction joints will be cut (typically 1/3 the slab thickness) and sleeve the PEX in these areas if you do.

As Badger indicated, variable spacing in a given zone should not be necessary. We typically design the PEX circuit layout so the initial PEX from the manifold (i.e. the hottest temp in circuit) is routed in the area where the heat loss is greatest (i.e., near exposed slab perimeter, windows/doors, etc). This makes for a more uniform slab temp that a more discerning barefoot residential customer might notice and appreciate.

Hadn't thought about ease of "on the fly" repairs; but that is a good point.  Will definitely be placing the pex on top of the mesh if for no other reason than that.  I know that reinforcement mesh is placed 1/3-1/2 the depth of the concrete; so that is why I had opted for 3" depth on the Pex... I can do 2" just as easily if that would work better.  I'll be using the 6x6 mesh sheets over the rolls for a nice flat surface.  I just need to be able to walk on the sheets to lay out and zip-tie down the tubing.  That is why I'm looking for suggestions on what kind of "chairs" to use.  The ones in the first link I posted above claim to spring back when walked on.  Sounds good in theory; but I didn't know if they worked as advertised or not.  Here's a like directly to the product home page:
 
http://www.buymeshups.com/site/page


BTW, here are the manifold and Pex I'm considering:

http://www.pexuniverse.com/store/product/store/product/radiant-heat-manifold-ssm004

http://www.pexuniverse.com/store/product/pex-tubing-oxygen-barrier-12x300


What you do prefer for sleeving the Pex where a control joint will be?  Especially if it is a curved part of the Pex?

We first zip tie the PEX to the top of the mesh and only place the chairs just prior to the pour...starting from one corner and egressing away from the completed work. If you use a reasonable quantity of chairs, it isn't any big deal if some get broken/displaced during the pour...there will still be enough remaining chairs to maintain the desired location of the mesh/PEX in the slab.

Yes, Everhot makes a good manifold for a good price. Our high-end manifold preference is Rehau. Of course, every installer has their own preference and a good reason for having their preference.

For sleeving we like using 24" long PEX of the next larger size that was used for the associated circuit which has first been been cut lengthwise (to facilitate ease of placing the sleeving during the install) and which is then taped and zip tied at both ends and the middle. The objective here is to allow the PEX the freedom to move longitudinally in this area of the slab given that the slab will move at this joint.

This is another reason I like rebar for slabs, particularly when you are talking about 6" and thicker. Every time a foot goes down on mesh, it is going to get bent and deflected. Only the biggest job site gorillas are going to bend #3 bar when they walk on it and most of the time, your foot doesn't have to go down on it as the spacing is 16" or greater. Bar stays where it is set. I also don't follow the "protecting the tubing" notion as PEX is incredibly tough. On a residential build there should be nothing threatening the PEX except just outright negligence.