I feel the same about your attention to detail.

Still, cruel, just cruel man...

The wife won't let me take the new siding off!

"When you put the bars on chairs, you can tie your tubing to the bars and the tubing ends up higher in the slab, where it belongs."

Strictly a matter of opinion and certainly job specific. Raising the tube is not a the panencia nor credentials to advanced design. If you can sell it at premium where it will lend little, if anything to the performance, more power to you. If tube depth improves performance we have to define performance. Most of my slab-on-ground designs would not be measurably improved by "raising the tube" in a four inch slab, but the cost would go up in every one.

When you start designing and installing radiant floors and snow melting systems, full-time , for a living, you will want to write a book.

The same goes for re-bar. It is for crack control in the vast majority of the small structures mentioned here.

Speaking of tube depth, I just ran across Climatemaster's take on it:

• Tubing should be placed at approximately 1/2 the slab depth below the surface, ...

Leaving the tube at the bottom of the slab can increase the required supply water temperature several degrees Fahrenheit. This will decrease the heating capacity and COP of the heat pump.

Let's say "several" equals 3 and you have an efficient geo system that is running at a delta-T of 40F. That's worth about 4% in COP.

When you start designing and installing radiant floors and snow melting systems, full-time , for a living, you will want to write a book.

If you will recall this conversation from another thread, John Siegenthaler did write that book and his conclusions on tube placement are identical to mine.

It is simply irrelevant in the vast majority of radiant slab where efficiency is the main concern. Performance in terms of output may be effected but would have to be required before rational consideration given. Comfort is often compromised when tube is raised in a slab, much like thermal striping of the old 'staple-up" sub-floor systems or the copper and steel tubing, both under and in-slab, popular with Prairie School home and Levitown.

Even when snow melting driveways, a higher tube place will increase output to a point and then with a bit of wind the furrows of snow stand proudly between those high output elevated tubes. It sounds smart until you look at the empirical evidence and do the heat loads where you find that very few new buildings needs the heat output "modeled" in Siggy's "theoretical" study.

Like condensing boilers the once you have reached "the" efficient return temperature, accounting for other variables--that vary--you will get diminishing returns. The DIY crowd, having done their "research" are all about "raising the tube" as a badge of elite radiant designer...yawn.

When you do not value your time or think the extra effort negates the need for experience or a valid Manual 'J', which would actually give you a clue as to the practical necessity of raising the tube, anything goes.

When you do not value your time or think the extra effort negates the need for experience or a valid Manual 'J'...

Not only do I have an advanced engineering degree, but I hired some of the premiere energy efficiency experts in pursuit of designing a reasonable and efficient home. I can guarantee you that tube placement figured in the calculations and contributes to the efficiency of MY system.

However, it does not melt snow. That was more the concern of the HVAC guy down the road when he saw what I was building. Luckily, we didn't get any snow this year.

Wisdom and intelligence are often confused.

The "time" wasted is seldom in design but rather in application. Since I have spent much time in both, I resent waste in all it's forms. I have bigger fish to fry. But if you are happy, I am tickled pink :-).

Hopefully all of it can be discussed in terms of tradeoffs and numbers vs right/wrong.

Yeah, well, we had the wisdom of a number of energy engineers with over thirty years in the business designing both large and small systems, half of them "hippies" who have spent time trampling mud.

Sorry jonr, falling back to my humble roots on the river.

It costs roughly three times as much to pay people to "raise the tube". A factor not seriously discussed in any of Siggy's books or articles. The theoretical evidence refers to EWT and outputs we seldom see in residential and never see in commercial buildings. The lower the AWT the less tube placement matters.

Furthermore, floor covering can easily negate any modeled improvement, stratifying the most carefully controlled tube placement. When placing tube in a residential garage or basement we would never consider the unwanted cost and unmeasured theoretical gain. I have seen no empirical evidence to report the benefits in the majority of real-world applications, though I have produced evidence to the contrary, in our debates of the past. Most notably that downward flux is not decreased as you raise the tube.

More info on framing factors than I care to read:

http://www.energy.ca.gov/title24/2005standards/archive/documents/2001-11-14_workshop/2001-11-07_FRAMING_FACTORS.PDF

It costs roughly three times as much to pay people to "raise the tube".

How do you figure? We just used molded chairs and a kid did a couple thousand square feet in about 20 minutes.

Posted By ICFHybrid on 22 Apr 2013 10:52 AM

It costs roughly three times as much to pay people to "raise the tube".

How do you figure? We just used molded chairs and a kid did a couple thousand square feet in about 20 minutes.

It is comforting to know the hyperbole is not the sole domain of the profane and under-educated classes.

It is comforting to know the hyperbole is not the sole domain of the profane and under-educated classes.

Maybe you could be a little more specific about how it costs "three times as much".

Posted By jonr on 22 Apr 2013 10:43 AM
More info on framing factors than I care to read:

http://www.energy.ca.gov/title24/2005standards/archive/documents/2001-11-14_workshop/2001-11-07_FRAMING_FACTORS.PDF

Thanks for that bit of reference!

Yes it's bit dense for a full read, but skimming on down to the plotted trend lines on the "Wall Net Framing Factor" figures in section 4 tell the story of why 25% framing factor can be used as a typ number, even though careful framing designs can take it below 20%, and sloppy ones can exceed 30%. See figure 4.4, p. 21 (P.31 in .pdf pagination.)

Table 4.7 on p.28 (p.38, pdf) claims that the wall framing fractions will come in somewhere between 24.9% & 27.3%, with 95% statistical confidence.

With 25% of the wall being R1-R1.2/inch wood, the net heat loss from that 25% fraction swamps that of any high-R cavity fill.  The dumbed-down 2-D model is good enough:

R whole = 1/U average= 1/( 0.25/R stud + 0.75/R cavity )

In 2x6 framing with R1.2/inch wood and 5" of R6.5/inch foam, the Rstud is about (5 x 1.2=) R6, and the center-cavity is about (5 x 6.5=) R33.

So the net whole wall R (not counting siding & gypsum)of the stud layer is about  1/( 0.25/R6 + 0.75/R33 )= R15.5

With R1/inch framing (like Douglas fir), its even less- slightly under R14.

The 3-D modeling bumps the outcomes slightly, but it's fundamentally similar.  With a full cavity fill you can count the R value of the other half-inch of the wood, but in anything other than a full cavity fill the lower-R wood is effectively a heat-sink presenting a higher surface area to the interior space, which makes it effectively lower than if it was a strictly 5" stud flush with the closed cell foam.

If you add an R1 allowance for the sheathing & siding & gypsum (valid for wood sided wood sheathed houses) you end up around R16.5 whole wall, with R33 cavity fill- literally half the cavity-fill value.

No amount of cavity-R will reduce the heat transfer through the thermal bridging 25% of wood by much, no matter how you slice & dice it, which is why the foam budget is better applied to the exterior, where it can block the thermal bridging.

Posted By ICFHybrid on 22 Apr 2013 10:52 AM

It costs roughly three times as much to pay people to "raise the tube".

How do you figure? We just used molded chairs and a kid did a couple thousand square feet in about 20 minutes.

I used broken scraps of "quartz" counter top material out of the dumpster for free. Also took about 20 minutes to chair all the rebar.

My slab is done and worked out great.

1100 sq.ft.
$300 for 10M (1/2") rebar on 24" centers.
$200 for 6" mesh.
Final concrete price not in yet but probably around $2000 (10m3 30MPa, 13 cu.yards 4350psi) (fiber reinforcement, small aggregate, pump mix).

John

I am very meticulous in my design and installation, but also frugal. What ever we waste could be put to better use elsewhere. Chairing is bad enough but tying or "zipping" tube can't be done as fast as stapling tube to rigid insulation or snapping it down to Creat-Heat or Thermo-Snap. We our work, both local and national radiant slabs are mostly in new construction and basement renovation. Neither present high heat loads nor require demanding response time (the only valid arguments for suspended tube). The same goes for rebar and in most cases welded wire. We are talking about residential whether DIY of professional installation.

Yes you may lower the design water temperature and eek out another .5 COP, but at what cost? Comfort won't improve, response time isn't an issue and if the saw nicks your PEX you will not be a happy camper. Commercial radiant floors are a different animal, not covered here.

If you hire people to install tubing you will pay more. If they do it for free, you are all set.

It is hard to argue with folks emotionally attached to their systems, once formed in the mind and then into concrete ego overpowers logic. I find this especially true of one time installers. I have made mistakes but fortunately get to make new ones everyday and improve on design and installation. It is such a small market I fear all the detail puts the layman off. Making radiant floors harder to install serves no one.

Designing radiant floor heating is a complicated business and as Rob correctly points out, takes some experience. Doing the math is only one of many skills necessary for perfect comfort.

Tube should be placed in the center of a slab to achieve maximum heat transfer efficiency. This is a scientific fact and has been well documented. I would agree it takes more time, cost, and hassle to accomplish this. Why is that folks on this forum who clearly didn't pass high school algebra class and have shown that they clueless when it comes to heat transfer and fluid dynamics (and have even stated that they don't bother with it when doing residential designs) always try to make their lame case based on how experienced and wonderful they are? Sounds like BS to me.

Maybe we should just agree that the right tradeoff of small amounts of efficiency vs time/cost/hassle/risk depends on the circumstances/numbers and there isn't an "always right" answer.

high heat loads nor require demanding response time (the only valid arguments for suspended tube)

You keep neglecting the third one, which is radiator efficiency.

eek out another .5 COP, but at what cost?

Recently, you have said the cost is "three times as much", but, despite requests you haven't clarified how that is.