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Old McDonald

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Reply with quote  #1 
I decided to search for some UK prices for PEX and the first place I have looked at uses the selling point that "Speedfit pipe is cooler and therefore safer to touch". Not really what we want is it? 

gbwillson

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Reply with quote  #2 
You can always use copper for the collector and PEX for everything else. 

Greg in MN
SolarInterested

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Reply with quote  #3 
PEX is less conductive to heat but also less expensive and it will work OK. From the BIS site:
"Test 1: Copper Baseline vs PEX Collector with Grooved Aluminum Fin:
The PEX tube with grooved aluminum fin collector does 84.2% as well as the baseline all copper collector."

http://www.builditsolar.com/Experimental/PEXCollector/SmallPanelTests.htm

http://www.builditsolar.com/Experimental/PEXCollector/PEXALPEX.htm

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Both temperature rise and airflow are integral to comparing hot air collectors
Old McDonald

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Reply with quote  #4 
I just wanted to make the point that anybody buying PEX for heating purposes needs to ensure it is not one of those "safer to touch" but one that is designed to allow the heat within to be used.
stmbtwle

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Reply with quote  #5 
To be honest I suspect that applies to ALL plastic pipe and PEX. Plastic is just not as good a conductor as metal, but it has other advantages.
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SolarInterested

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Reply with quote  #6 
Quote:
Originally Posted by Old McDonald
I just wanted to make the point that anybody buying PEX for heating purposes needs to ensure it is not one of those "safer to touch" but one that is designed to allow the heat within to be used.

Not sure but I think the claim that "Speedfit pipe is cooler and therefore safer to touch" is in comparison to copper pipe. I believe all PEX pipe has less thermal conductivity than copper.

The Speedfit PEX is described as having 5 layers as does PEX-AL-PEX but I'm not sure if it's the same. For what it's worth, PEX-AL-PEX has better thermal conductivity that regular PEX but that's probably due to the aluminum layer.

http://www.johnguest.com/speedfit/product/pipe/pex-barrier-pipe-in-coils/

http://www.builditsolar.com/Experimental/PEXCollector/PEXALPEX.htm

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Both temperature rise and airflow are integral to comparing hot air collectors
Old McDonald

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Reply with quote  #7 
SI, You could well be correct in the meaning of cooler to the touch. The way I read it was that it is cooler to the tough than other PEX. Since my OP I have wondered if in fact there are differences between different brands (thinking of ones with an aluminium layer) and if so whether this means that different pipe length runs could (or should) be used.

For example if a pipe run is expected to have a temperature drop of x degrees over a 100m/300ft run, and with a different manufacturer's"cooler"  pipe that run could be extended by another 20 or 40%, would it be preferable to have a longer pipe run? Or, as I first thought, does it mean that a "cooler" pipe is not permitting sufficient heat transfer and so should not be used at all?
SolarInterested

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Reply with quote  #8 
OMd, I think you're right in that if some PEX is significantly less thermally conductive it might be better for long runs where insulating is desired (say between remote panels and the house). However, most folks would just insulate these pipe runs anyways. Did a bit of Googling and found this about thermal conductivity:

Copper  401 W/mK vs PEX  0.51 W/mK
The SI unit W/mK indicates the amount of energy in Watts or one joule per second taking into consideration the thickness of the material in meters and the temperature in Kelvin’s
http://www.healthyheating.com/HH_Integrated_Design/Week%205/CPRT192002010.pdf

Another paper shows a much lower value for copper (error?)
Copper  28 W/mK vs PEX  0.41 W/mK
https://plasticpipe.org/pdf/tr48-r-value-thermal-conductivity-pex-pe-rt.pdf

This one also has copper at 401W/mK
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

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Both temperature rise and airflow are integral to comparing hot air collectors
Old McDonald

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Reply with quote  #9 
SI, A good name given your link to an SI unit! I do like the way you give links to provide a tutorial for those who may not know. Anybody wondering about the use of different materials within a collector should at least read the conclusion of the first link you gave. It is particularly significant that at temperatures above boiling they found no distortion of any of the materials. 

We have been using the equation of WattsmetreKelvin right from the beginning of first thinking about using solar. After all, one of the team is an astrophysicist of some renown within the profession - see my OP on the "Direct pumping underfloor heating" under Solar Liquid Collectors. The figures we produced from the use of this is the reason we believe it will work for the project in hand, but we wanted the views of experienced users.

My thoughts on the different thermal conductivity of various makes of PEX was more about the possibility of extending heating runs rather than within a collector or runs from the collector to the distribution of heat. I saw it as a possibility of avoiding a manifold if the right PEX was used and somebody had a run of say 400ft. 
 
stmbtwle

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Reply with quote  #10 
I've always been interested in PEX but the high cost of fittings (and tools) has put me off.
However after inspecting the regular PEX fittings I decided, what about a hose clamp? Unlike the crimp rings it could be removed. The PEX is tough but not all that hard, and it does soften somewhat when heated, so as a last resort I figured I could blast it with a heat gun and then tighten the clamp down hard.

I had to do some repairs to my solar space heater so decided to try it in place of the old CPVC... So far I haven't had to resort to the heat gun.  The clamp seems to be tight enough, and after it's had hot water run through it for a while I'll tighten it down some more.  I'll check again in  a few days for leaks.

I wouldn't recommend the clamps for high-pressure connections or where you couldn't get to them, but for low pressure solar systems I think they might be just fine.  And they're a whole lot cheaper than SharkBite fittings!



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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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