If you believe you could do anything to NOTICEABLY improve my situation, I'm all ears...

Ha ha. I thought it was a terrifically useless statement at the time, but John S. even says that the time to do something about the tubing location is when it is installed. It is really difficult to alter the location after the fact....My guess is that he has seen this discussion before.

I see a lot of argument about how you couldn't/wouldn't/shouldn't have placed tube in the upper half, but the fact remains that doing so improves efficiency (and response time, too, it would appear).

I think he has drawn a conclusion here that is a bit wide of the mark.

You forget that he has developed his "opinion" with actual models and that they are easily supportable by known physical processes. To date, I have not seen an adequately controlled experiment which shows that there is a "non-noticeable" difference in tubing placement. Oh, sorry, I forgot that it would be an impossibility to find someone who could place ten bucks of chairs underneath mesh to situate it. Not to be snarky, and people are different everywhere, but to say it can't be done because people won't do a little thing like that is just outside my experience.

you are being very snarky. obviously I'm not suggesting you move my pipe. I'm saying you can't significantly improve the performance I've already got. key word "significantly". To continue to ignore the intent is just becoming tiresome and it's bordering on trolling at this point.

His models disagree with my slab. I will take my slab over a computer model any day. and EVEN IF HE WERE COMPLETELY CORRECT, his own numbers with further inspection do not justify this now 5 page thread's worth of discussion. the effect is small. if it's easy and cheap to lift, do it. if it's not, don't.

And just to join you on the overly pendantic parade, you quoted me as saying he had an "opinion" which I didn't say. I said he had a "conclusion" that was wide of the mark. You can have good facts and different conclusions.

You can wave your hands all you want, but the fact is no matter how hard I beat this drum, the vast majority of installers ignore it because chairing mesh makes the concrete install much trickier it would be if they were chairing rebar. But since you obviously know scads of concrete guys who are willing to do this, please do be sure to post all their contact info so we can all refer to concrete guys who are willing to work with radiant installers to maximize performance. I'm sure everyone would love to have access to such a valuable resource.

At this point I'm just repeating stuff you continue to misquote, mischaracterize or purposely turn into straw men. So have a nice day. feel free to reread any post I've made on the matter: I made my case as completely as necessary to anyone with average reading comprehension skills.

protip though: I'm not saying no one should lift pex. never did.

His models disagree with my slab.

It's hard to believe that you can't understand how a model might differ from any given example, yet still have value. You don't know what he put in the model and he doesn't know how yours is constructed. The point is that the physical processes at work can be expected to be similar. Plus, I have to point out that you know intuitively that putting the tubing down by the insulation creates a higher density of heat there which can only cause higher heat loss across the insulation and lower amount delivered to the radiant surface.

the vast majority of installers ignore it because chairing mesh makes the concrete install much trickier it would be if they were chairing rebar

I don't really care what the reason is. The bottom line is that they have about ZERO interest in improving the efficiency of your radiant slab.

you obviously know scads of concrete guys who are willing to do this,

They will bid and execute the contract as it is laid out or someone else does the job. That is how it has always worked.

The entire 3 pages of discourse didn't discuss the R value of the concrete. I read all three pages and kept waiting to read about it. Did I miss it? So I went to Wikipedia and looked up the R value of concrete. If correct, it's 3.9 (air-entrained) so raising the PEX is the same as adding R 3.9 of insulation for each inch raised and removing that R from above the PEX.

I think you are confusing autoclaved, aerated concrete (AAC) which is produced in a factory and is actually a foamed version of concrete containing mostly air and "air-entrained concrete" obtained from your local ready-mix plant, containing about 6% air. The air-entrained concrete from the ready-mix plant is for pouring slabs that will be subjected to freeze-thaw cycles and less suitable for concrete slabs that will be troweled. In any case, you will be using the ready-mix concrete for your slabs and the r-value is the same as any other old concrete, about 0.08.

Just in case anyone is looking for a cooperative contractor Central NY State who uses both rebar and mesh in his slabs and is happy to chair the rebar to bring it to the height you want -
Dave LaFramboise - http://www.manta.com/c/mxjkxh1/dave-laframboise-construction-llc

-Rosalinda

Just to highlight: everyone chairs rebar. rebar is easy to chair. Mesh is a different story, and when used without rebar, it is rarely chaired.

We used chairs called Mesh-ups, 1 1/2" lift. One of the concrete guys (boss's son, know-it-all) didn't like them, was stomping on them. The distributor we got tubing and manifolds from said we should use them. Will never know how close to the middle of the pour the tubing ended up.

ChrisJ

And since PEX is of no consequence in the vast majority of residential radiant slabs--or snow melting systems for that matte-- it is really a moot point. If you want a stronger slab lay the wire or bar on top the PEX as we have been doing for a couple of decades now.

Or...you could have the concrete guy design your radiant slab...I have always found them to be real deep.

ok, I just read this whole thread...It seems like a bunch of head butting over nothing.....Lets say having the pipe in the middle of the slab is better. Lets say it saves $5 of energy over the other way. Can ICF then brag about being right? Yes. but Big deal!...is it technically better yes, but is it going to save hundreds of dollars....NO.......Its like the geo thing...I could put a geo in my new house, And lets say its a crazy 70% better than a different system. WOW sounds great right?...But wait.....I'll spend $15,000 dollars more , so I can save $350 bucks a yr....Lets see..........$350 a yr savings into $15,000 dollars spent....How many yrs to break even?????.......40 yrs!!!!!!!!!!!!!!.....But the system will be worn out and replaced long before that...Oh crap.....NO way to ever come close to save even a penny......just sayin...

but is it going to save hundreds of dollars

I guess it depends on the particulars of the system we are talking about.

We are talking about any residential system with a 4" slab. After a few million feet you tend to get to the important points and ignore the the things that it doesn't pay to know. Well said SIP.

Wondering if you have had enough time yet to come up with actual data that could refute Siegenthaler's treatment of the subject. I'll remind you that his FEA analysis supports the expected physical findings that it does make a difference.

how about the utility bills at my net-zero shop? that good enough?

Please read SIPs response.

His point is well taken, it just doesn't matter in real time and dollars. But you can have the point if you think it is important. We just don't want the layman to be mislead and focus on the minute while missing something really important, e.g. PEX tube length, spacing or design water temperature, that can have a measurable impact on comfort or fuel consumption in a residential design.

It is not whether the tube placement in a 4" slab has an effect, it is whether it is sensible or not. Having designed and installed many radiant slab systems, I have come to the conclusion that it does not. If the people are comfortable in the space, the thermostat is satisfied, and the condensing boiler is always condensing, there is nothing to be gained by raising the tube.

I never attempted to refute Siggy's treatment of the subject. In fact, it as easy to understand as it is to ignore when it is not pertinent to the application. Thankfully, Siggy did not write a book on religion and I have found him to be infinitely flexible where the numbers don't add up to dollars.

Of the dozens of concrete slab radiant panels I designed last year, not one will operate above body temperature, making them all perfect candidates for any heat source. What would I gain by raising the PEX in the slab on these systems? If you add nearly any kind of floor covering the resistance makes the slab temperature nearly uniform negating any improvement from raising the PEX in the slab, real or imagined. In fact, raising the PEX in the slab can decrease the uniformity of the surface temperature and increase the thermal striping we designers are always trying to avoid in our residential radiant floor designs.

To that end, this study by Rutgers University addresses most of your concerns and perhaps will encourage others to read more than one book on the subject. All due respect to Siggy.

http://aesop.rutgers.edu/~horteng/ppt/papers/FLOORHEATING4.PDF

how about the utility bills at my net-zero shop? that good enough?

Yeah, that's the same tack you took earlier. I'm not attacking your chosen method. The point is, quite simply, that however efficient your system is, it might be MORE efficient placing the tubing higher in the slab. With a Daikin air to water heat pump, my system was designed to be slickly efficient and that meant the lowest possible emitter temperature (because of the heat pump). I know you like boilers :-) and they may have a sweet spot in terms of temperature delivery that might make sense for you. HOWEVER, low temperature emitters are more efficient for heat pumps, and then, there is the nagging little problem of heat loss out the bottom of the slab. The lower you put the tubing, the more you will lose out the bottom, insulation or no. You can't escape it. It is physical fact.

You quoted me, not badger the boiler guy I am using an altherma in my shop, will be the end of year 3 next month. my delivery temp can be set at the minimum output for the altherma (86 degrees F) and heat my shop to 70 all day long, because the effect you are talking about is ... say it with me now!... so incredibly minimal. when I do need more than 86 degrees, what is setting it is my upper floor (which is covered by plywood, thus needing the high high temp of 95 degrees F to hit design output), not my slab. There is literally nothing I could do to my slab to improve my altherma efficiency. and that holds true for nearly every uncovered slab radiant system in america... you can pump out up to 20-25 BTUs/sq ft without being able to take your daikin off of minimum.

only downward heatloss has any effect you could even mention with a straight face... and even that is.. one more time... very small. more and more minimal the lower the water temperature you are using. Especially if insulated to my, and apparently Dana's, 3" minimum recommendation, you would be VERY hard pressed to measure the difference. ESPECIALLY if you're using a heat pump and dividing what paltry loss there is by 2 or 3. Pay yourself for the time you've spent thinking about this, or even just typing in this thread, and you would outstrip any savings you'd ever see by a few orders of magnitude, just to put it in perspective.

You might the Rutgers study instructive as it does address the downward flux and noted that it did not change with tube placement and as Rob suggests is of no consequence as the design water temperature goes lower. This design water temperature is dependent on the heat load, thus where you place the tube would depend on the job. In most cases it doesn't matter.

Interestingly enough, the Rutgers study found a 9% improvement in upward heat flux when the tubing was placed mid-slab. Siegenthaler's FEA runs show maybe as much as 20%. Are you guys both saying that you have no interest in improving the upward heat flux by what may be 10% -20% simply through tube placement?

I am a bit suspicious at this point of the Rutgers experimental setup as they found essentially no difference in heat loss to the soil with tube placement. I wonder what happened to the upward flux deltas?

given the already very excellent output characteristics of slabs, no, I would not do anything harder or more expensive to increase upward heat flux, because.... one more time... the room for improvement is so small it's a moot consideration in practical application. I can already make a slab too hot to walk on comfortably, cranking out 40 BTUs/sq ft, with about 105 degree water, without even compressing my tubing spacing beyond 12" o.c. you understand diminishing return, right? we're way past it here.

the upward deltas probably improve because the average resistance is so small across the concrete that more minor changes are noticeable. just like R-value... when the values are low, small changes are more noticeable. tubing at the midpoint has half the resistance upward as tubing at the bottom. but, that resistance is still tiny and even though the upward flux change is noticeable and expressable in impressive double digit percentages, the "volumetric" or "absolute" effect is STILL tiny.

and if you put any r-value over the top of the slab and the difference in position becomes unnoticeable. it would be like debating whether you want 1/2" or 9/16" foam on the outside of your R20 wall cavity... negligable difference.