sorry, last I knew the National Sanitary Foundation only inspected potable pipe. but if you are potable approved with barrier pipe, that would explain it. my apologies for the mistaken assumption!

the point of contact is NOT the same with either pipe, however. Onix does deform more than pex, and the contact area is easily 2 or 3 times as large as PEX's direct subfloor contact. you can easily see this with a picture of the onix installation. that is how you get more surface area. and that's direct contact, not indirect contact through a lightweight plate.

I am not buying the pipe deformation argument, what causes the deformation? If it is staples what is the pattern, how is it enough to make a 2-3 time difference on surface contact? A circle or diameter of a 3/4 inch pipe will only present so much contact where it touched the sub floor, unless the pipe is fuzzy to extract warm? Radius point contact is radius point contact. Where is the additional surface area contact coming from on this Onix pipe? I could buy it if the pipe was an oval.
With the heat transfer plates we securely capture 70% of the exterior diameter of the pipe to spread against the floor. I understand there remains the question of thickness and effective value. But I do not support the concept that onix pipe is equal to a plate system.
Perhaps we are closer to agreeing with each other than it appears.
Dan

Dan- U-value of the PEX itself may be lower than Onix, so it may not NEED as much surface area contact for the same conducted heat flux, and it's likely to be more emissive, making small air gaps less insulating as well. (It is black & dull after all, not nearly as bright & shiny PEX, eh?)

Thin plates have nothing like 70% actual contact with the PEX- the micro-gaps of air will reduce the heat transfer, which is a major factor in why the grippier extruded plates perform better.

Bottom line, when you're talking about materials as dramatically different as PEX and EPDM, you shouldn't assume too much about how they behave relative to surface area contact and proximity. Nothing quite beats actual measurement- they may be more equivalent than your gut tells you, but any equivalence is likely to vary with temperature. It's not a simple model.

I think it is like garden hose you can flatten the bottom by pressing on the top enough.

hold on, Micro air bubbles ??????
Beg to differ our heat transfer plates snap on fairly tight, full contact on 3 sides you have to press them on. I do think that this is subject to a wide varity of temps and situations, for instance we specified Onix on a heliport in Alaska, I did not want to worry about the potential of freeze failure or UV, the pipe was exposed. The pipe has a place, but it seems way expensive for day to day heating work.

Danna, another often overlooked aspect of our pex..... the red color of the pex makes the pipe hotter. (not really),
Good evening to you all,
Dan

I'm not a big fan of EPDM tubing by any means (it has a crappy track record in the biz- they SAY the problems have been fixed but...) . Yet is different enough that you can't make direct comparisons with PEX. I'm thinkin' the reason they spec radiant barrier to get the performance out of Onix installations may have to do with higher emissivity relative to PEX (just a guess- no data.)

For yuks I poked around with an infrared thermometer last night- PEX isn't super low-E- far more emissive than the bronze manifold or copper pipe, but not as emissive as the paint on the pump (unless there really IS significant delta-Ts in between the flange on the pump, and the stub of copper to the manifold, and the (white/clear Wirsbo 1/2") PEX, which I sincerely doubt- especially since the PEX down stream of the copper & bronze reads ~20F higher than the copper, but ~5F lower than the pump flange. :-) The pump motor reads a bit hotter, but I took the reading right at the flange to factor that out. I doubt that the water lost 5F in less than 2 feet, but it could be the U value of the PEX was enough to account for that much surface temp difference- it's hardly a refractory material.) I s'pose I could spritz some black paint on the different materials to even out the emissivity and re-test sometime to see what true surface temp differences are... I don't have any EPDM in the system to compare it to.

No plates are gas-tight, but extruded versions are usually a hammer-fit (OK, rubber-hammer-fit), not as easy a snap-in. There are some high-E anodized versions too, but I haven't seen any data to suggest how much (if any) improvement that makes in the heat transfer. Bare aluminum has extremely low emissivity, and the heat transfer is largely conductive and (micro) convective, at both the PEX/Al interface and the Al/subfloor interface.

...and so now you're tellin' me I can stop specifiying red PEX??? ;-)

no, I don't think onix does have a place. I think it's junk.

but the fact is it deforms more than pex. if you smoosh pex against a subfloor you will not get the same amount of "flat tubing" against the subfloor as the rubber pipe will give you. you need to find a picture of an onix install to see what I mean, but pex is simply more rigid in that respect. it also is more of an insulator than the rubber pipe is. between those two advantages, I think it's quite reasonable to see the enhanced direct contact for the rubber pipe would give it a bit more output, and that puts it in light plate range.

Your plates have good contact for lighweights, but they are still limited in the by their thickness in their ability to transfer the heat outward, and they are still transferring it from PEX instead of rubber, and the contact is not "firm" in the same way that it's "firm" in extruded plates.

no no, keep specking that red hot pex, unless of course you are looking at our white hot pex C,

I think I will break out the heat gun as well, the comparison from Iron, brass and pex is interesting,
Dan

Yes it is! Iron's pretty emissive, aluminum copper & bronze not- an infrared scan will under-temp copper plumbing in the heating system by quite a bit.

And, putting radiant barrier paint on the exterior of your house will make it look like you have R50 walls, according to the mid-winter infra-cam shot (something that should give paint insulation additive & radiant barrier snake oil sales droids major thrills! :-) )