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# Large diameter pex for radiant

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Member Posts: 2,542
is potable water rated tube extruded in Canada. Not exactly sure why they only use this tube, but I suspect it has to do with the fact that they consider it "plumbing" and not "heating" wich would throw them into a whole new code category, for which they are not equipped.

That is one of the problems I've run in to. They sell this stuff to the unsuspecting homeowner who knows no difference. The HO hooks it up to a cast iron or steel boiler and things disolve. When confronted about it, they make claims like "If the tube is operated at temperatures below 140 degrees F, diffusion is not an issue. Yeah, right...

There are more than one i-net retailers from Vermont. I suspect they all came from the same barrel.

ME

• Member Posts: 6,106
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the use of 7/8 pex for radiant a bit lately. Seems to be a hot topic

Using a flow rate of .5 gpm with 3/4" id pex, my calcs, using Siggy's Design Studio show a velocity of .4 feet per second.

Reading Chapter 6 of Modern Hydronic Heating I learn a velocity of 2- 4 fps is desirable in a hydronic system. This has to do with the ability of the fluid to carry along entrained air. As we know if the air is not carried to the purger we end up with noise, and possibly undo wear at the pump, caviation.

But more important is a calculation known as Reynold's Number. Who ever the heck he is (was):)

If the Reynold's number falls below 2300 flow in a pipe will be laminar. In laminar flow the fluid slides along the tube with a very slow moving layer against the tube's outer wall, as I understand it. This slow moving layer creates a thermal resistence hampering the ability of the fluid to transfer it's heat to the outer wall of the tubing.

It's designed and used by engineers when moving fluid long distances, between buildings with minimum heatloss!

In Modern Hydronic Heating Siggy states " laminar flow is undesirable if the objective is to move heat between the pipe wall and the fluid stream."

Check my math someone

RE# = vdD divided by U

v being flow velocity in fps times d internal diameter of pipe (in feet) times D fluid density.

divided by U fluid's dynamic viscosity.

v, d, and D can be found in the Design Studio Fluid Properties module.

Velocity #'s can be found in the Pipe Sizer Module. The B&G System Syzer Calculator confirms my nunbers at .4 fps.

I come up with a Re # of 492.2. Way below the 2300 threshold number desired for heat moving turbulent flow. transfer.

I propose this to be a major bottleneck in the heat transfer ability of 7/8" (3/4") id tubing used for radiant applications, when designing around .5 gpm flow rates. If the BTU's aren't getting to the wall of the tube, then the are not getting to the transfer plate, and hense not to the floor and space to be heated.

Could this be considered a constipated hydronic design?

What say you? Am I up in the night, again, with my math and theory?

Larry Drake also wrote about Reynold's numbers, but I can't seem to find his article. Was it a RPA newsletter article? I'd be interested in his take on this subject also.

Any thoughts?

hot rod

• Member Posts: 477
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Nothing wrong with the math

But why use such large tubing for such a small flow? I am commonly using 3/4 pex for large commercial radiant heating/cooling slab systems and the typical flows are in the 2.5 to 3 gpm range for the outputs and temperature differences needed for thermo-active cooling/heating slab systems. Your math is correct and it clearly points out why 7/8" tubing at low residential flows will not work due to the insulating laminar flow boundary layer in the tube. The only place 3/4" and larger pex tubing should be used is in large commercial applications where the balance between large tube zones/tube lengths vs delta T and output are required, and the flow rates are typically high.
• Member Posts: 418
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Why .5 gpm?????

Using HDS if I figure 4-200ft runs of 3/4 pex, which is what the one guy said he had...(I believe)I get 2.76 gpm using a 007 pump.....
that using the pipe sizer mod. shows me 2.4 ft/sec and a re# of 31,371......

Also using some real life experience I can tell you that at least some heat can be had from a very low flow tubing application...I had some 5/8 tubing laying around and my kitchen floor was cool..(wife unhappy)....so I hooked it to a 2 loop manifold and hooked that to one of the 1-1/4 X 1/2 monoflo tee's in my main loop for my house... the tee's were there because I cut the CI baseboard off that runs behind the fridge....couldn't see trying to heat and cool the fridge at the same time.....soooo now kitchen is cool and wife unhappy.....anyway...I try to measure flo through the tubing with my new Truflo flowmeter and get .25 gpm....I figure no way will it help......
Guess what.....wife is happpy!!!!! her piggy's are warm and I'm outta the dog house!!!!

Wooof, wooof!!!!

Okay HR, long story, but I hope I made my point.....and...still wondering.... how did you arrive at the first assumption of .5gpm?????
• Member Posts: 6,106
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The data

is from the fellow below with the "internet system" I'm trying to come up with a reason they supply 3/4" tube for 16" on center residential applications.

Maybe I misread the .5 gpm flow in his system design.

hot rod