I am wanting to know the pros and cons of the different methods of placing tubing and wire in concrete for radiant heat.  Placing the wire on the foam insulation first then attaching the tubing with plastic ties seems to be common,  but this leaves the tubing exposed to being walked on by the concrete workers, and pulling the wire up as they go could result in uneven placement.  Then there are clips and staples which hold the tubing to the insulation, which could be faster and more easy than all the plastic ties, and if the flat wire panels were placed on top  of the tubing, would provide some protection for the tubing during installation, and place the wire up in the concrete evenly .  I have read where the tubing is best placed in the middle of the slab, but that would put it closer to being hit by and interior wall anchor if my measurements were not just right. Is the lag time in temp.  change  that great, between tubing placed in the bottom of the slab vs middle of the slab?  I was thinking once the home gets to the desired comfort level, it would be best to leave the thermostats alone.  I was thinking that the wire up in the concrete in better than on the bottom for strength.  The "chairs" for the wire would place the wire up evenly, but I don't know how far they would sink into the insulation while being walked on during the pour.  There may be other methods out there too.   Seeking knowledge !!

You are correct.

The tube is stapled to the EPS, then the wire if required. Performance is enhanced when tubing is suspended in the slab, but unnecessary in most residential applications.

The tubing is tough but it should be under 60# air pressure during the pour and you should be there to watch for big guys with sharp shovels etc. No smoking while pouring by the way.

Tubing is placed 6" from the exterior walls.

We don't even consider tubing placement in our designs. any difference is small. leaving the tubing at the bottom solves an awful lot of jobsite issues. Foam stapling to EPS, a product like Crete-Heat, or tieing to mesh you don't really need (left at the bottom) are the options.

Foam staples work great, wire on top is fine... best wire to use is road mat, 6x6 squares and the sheet comes in a flat 20'x7' 6" piece, lays flat, better than unrolling a 50x4 foot wire mat. Next if wire is required think about it like each end of the wire is a knife point there are hundreds, you do not want these to spear your pipe, pay attention. Alternate is fiber mesh in the pour, check w/ your engineer.
Dan

I don't smoke, but curious at to why no smoking while pouring.........

I was wanting to use both the 6x6 road mat plus the fiber mixed in the pour.  Maybe overkill, but want a strong, crack resistant slab.

Wire and fiber together will work, overkill, but will work. Keep in mind fiber will prevent a smooth finish.

We set the rebar and wire on small concrete bricks cut in half, to hold it at the mid slab level for the pour - so with the PEX tied to the wire it was also at the right depth. This is the way my contractor does his slabs, so no pulling on the wire mesh during the pour. He also uses rebar, wire and fine fiber in the concrete, and he power troweled the concrete to a beautiful smooth finish, a real work of art. As I have mentioned before I was hyper about those pointy bits penetrating my PEX during the pour, so taped all of them. It took 16 hours and was probably an insane thing to do, but I slept better not having to worry about it.

go with stapled to foam more cost effective, performance difference to wire mesh with tube suspended vs stapled is very minimal especially in residencial applications, In my opinion it's a waste of time and money to use mesh, keeping the tube lower in the slab also decreases the chance of puncture from wall installation as well. Once the slab is warmed up it realy does not matter where the tube is, spacing is more important and so is floor covering

Wow - lots of concrete misinformation here! Regarding micro fibers - these are only good for one purpose - the prevention of plastic cracks in the concrete. They don't make concrete stronger, they don't affect Pex, and they certainly don't prevent the attainment of a nice, smooth finish. I know that because we used fibers for about 1000 square feet of 5" concrete on three different levels. Crack free. If you want to avoid the altogether different structural cracks, or structural problems, you should talk to an engineer - we used #4 rebar on 12" centers above gravel above well compacted dirt - no sand - not so bad - and tied the Pex to the rebar with plastic twist ties. You should also use a good expansion joint plan, but in order to do that, you must know where concrete tends to crack. (From interior corners, where the depch changes, etc.) Pay a good concrete guy $200 to get some advice on ths subject.  You'll never get a guarantee, but there's really valid, valuable information out there.

Regarding placement of the tubes - sure it matters if you have a thick slab. From a heating perspective, closer to the surface is always better (assuming no punctures) because the time that it takes the surface of your concrete (from where the heat eminates) to adjust to the outdoor temperature will be increased in proportion to the depth of the tubes below the surface. If you bury your tubes 4" below the surface of your concrete, you'll never get anything better than something like a 24 hour response time. In addition, even concrete has temperature gradients within its cross sections. If you have 90 degree water with two flat surfaces - one 80 degrees and one 75 degrees - wouldn't you rather have the warmer edge at the surface rather than at the insulation? Not only will you get better heat output at lower water temperatures, increasing efficiency, you'll also have 5 degrees less of conductive loss through the bottom of the slab. Nothing to sneeze at there.

Concrete is hard enough to deal with as it is - thick slab wise. Don't make it harder by just letting your pex rest on top of the insulation if that insulation is 4" down. Use small concrete cubes or some other methods to raise the tubing if you have a thick slab. Be sure about where your Pex is. Take pictures before the concrete is poured, like this one:



and more detailed pictures like this one:



It's not rocket science, but it takes a bit of engineering and a lot of preparation.

And oh, one more thing: If you use spray paint (like we did) to mark cabinets and the like, or walls, be sure to find something that doesn't eat away your expertly placed insulation.  It'll drive you bananas!

Jeff

Jeff,

I have a 4" slab downstairs right now with tubing on the bottom that responds much, much faster than "24 hours" from a cold start in mild weather. Leaving tubing at the bottom really isn't a design consideration for residential thickness slabs. Midpoint would be slightly faster... but remember, until the whole mass is heated up you won't be emitting much heat. cold sections of a slab will suck heat out of areas that are warming up very effectively. Ask anyone with solar gain on a slab who initiates a heat demand after the bottom half has cooled... takes quite awhile to get that heat back no matter where the tubing is.

in very thick slabs, perhaps this is more of an issue. it will slightly raise the water temperature requirement since you are losing some tubing contact to concrete, but the effect is very small.

Right on concrete, off on tube placement.

As Rob points out, response time is not an issue in most residential applications.

Special loads such as high glass to wall ratios may be met by tubing placed higher in the slab (again mostly commercial applications). Though I have never needed to raise the tube on a residential application, I have had my tubing saw cut, jack hammered, drilled and melted with an errant cigarette!

In other words, knowing what to worry about is the difference between the pro and the novice.

As for sharp wire ties and re-bar rub, show me a wire tie punctured tube or a PEX tube rubbed-through by direct attachment to rebar.

Re-bar rub??? Is this is new.... RPA???
First, Jeff nice work, you did well.
The fiber mesh will leave hairs. My company has done 1 1/2 light weight for years. I say BS to not leaving tell tail signs of its use on a finished slab. Do not get me wrong I like the product.
The fiber mesh is not a solution to engineered re-bar design, will assist with cracks. assist.
if you need to elevate your re-bar or wire mat look for DOOBE blocks, these will come in thickness variables 1" 1 1/2" 2 " and on, a cube with a bag tie wire in place.
I do not understand why anyone would zip tie a pipe system down, needle in thread approach is slow, use a bag tie and bag tie tool. cheep, fast, the ends are loops and will not present sharp edges.It is the edges on a wire mat I have concern with, those little.....
If you do a lot of pipe or form work there is a spring loaded bag tie tool.
Enjoy your day,
Dan

Dan is right again.

I use nice clean yellow PVC clad ties. Slick and fast.

Dan / BadgerBoilerMN:
Do you have more info on the bag tie tools you seem to use? Brands, links, etc?

Many thanks,
Robert

For DIY applications, simple wire ties will work well.

almost every manufacturer out there offers wire ties and a tie tool as an option. nothing special about any of them, really, unless you do this for a living and get an 'autorachet' tool or the like.

Thanks both!

Rob, I just sent you a PM.

The math tells me that yes, a couple of inches deeper in the slab does make a detrimental difference. Can anyone confirm?

this is an article on it. John is one of the best in the industry. Some of the modelling shows things contradicted by reality and common sense, but for the most part, mid position is best. John advocates for it in the article.

http://www.esmagazine.com/Articles/Feature_Article/7761fb74a2da8010VgnVCM100000f932a8c0____

I think the holes in the modelling are significant though, and I believe (not fact, belief) it's just not best by enough to really make much of a difference in the end based on what I see from slabs and his numbers as well (10% of the downward load is a very small number indeed), and it's nearly impossible to get a slab installed with tubing at the midpoint in a residential slab in any kind of reliable fashion.

In short: there is no final definitive and clear proof of the final numbers. but I can say I have never had to raise a system's water temperature because of a "bottom of the slab effect". I did, once, see all of a client's tubing in a garage cut by a concrete saw when it was lifted too high during the pour. And I was ignored, now I think rightfully, for years by clients who were unmoved by my performance purist point of view and left the tubing at the bottom against my advice. All of their systems work well. could the heat load be very slightly lower? perhaps. but I don't think by even 10% of the downward loss. something less than that. and that's a very small number indeed.