1/2" or 3/4" PEX in slab?

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  #1  
Old 06-03-10, 03:35 AM
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1/2" or 3/4" PEX in slab?

Hi,

I am planning the radiant slab installation in our new house in NH. Slab is approximately 1,500 sf.

I have to decide between 1/2" PEX spaced at 12" (5 rolls of 300ft) or 3/4" PEX spaced at 18" (2 rolls of 500ft). What do you think?

Thanks,

JF
 
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  #2  
Old 06-03-10, 09:51 AM
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I'd split the difference and use 5/8 inch tubing and get as much in the slab as possible. I don't recommend 1/2 inch because you have to use several loops to reduce the "head" (resistance to flow) or else use a "high head" pump and that will cost more to operate. Generally speaking the loops should not exceed 200 feet with a larger diameter of tubing having a bit more leeway.

Be sure to properly insulate the slab, both underneath and the perimeter or you will be wasting a fair amount of heat.
 
  #3  
Old 06-03-10, 12:53 PM
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My recommendation would be that you acquire pieces of 1/2", 5/8", and 3/4" Pex and see which one you can work with by yourself. If you can't lay the Pex down, it won't matter that you have a lower pressure drop. My guess is that as a DIY'er, you won't be able to manipulate 3/4" Pex at all to your liking, and even 5/8" will be a significant chore.

Also, the size of the Pex should have little or not effect on the spacing. If someone has told you that you can get by with 3/4" Pex on 18" centers while 1/2" Pex requires 12" centers, you have been sold a bill of goods. The heat transfer differential is minimal - the only advantage to running larger diameter Pex is that the loop lengths can be somewhat longer, but when you factor in the increased pressure because of the increased flow rates that you must run to get the water moving more quickly, even much of that advantage is lost.

And finally, with 1/2" Pex, you can plenty well go up to 300 feet. But, you must do the a heat loss calculation and a hydraulics calculation to figure out what the head loss will be. If you are going to be pumping 1/2 gallon / minute through a 300 foot loop, you'll have a pressure drop of less than 4 feet, which is perfectly fine. If you are pumping 3/4 of a gallon, you're up to almost 8 feet, which is significantly worse. You can't know until you run the numbers.

Jeff
 
  #4  
Old 06-04-10, 05:27 AM
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I would do an actual design. Otherwise, it's a total crapshoot. Figure the load, output needed, tube size and spacing, water temperature, pumping requirements, temperature drop, etc. etc.

John Siegenthaler's Modern Hydronic Heating is a good resource. Also check out the Uponor/Wirsbo guides.

To echo what furd said, slab performance is HUGELY affected by insulation detail. Underneath and especially edges. The idea is to heat the building, not the surrounding earth. A good radiant design guide will show what and how.

Radiant is not hard to get right, as long as you do your homework. But it's very easy to get wrong, which is expensive, frustrating, and poor-performing.
 
  #5  
Old 06-06-10, 08:53 PM
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I think you should go to the people who is sellign you the pex, not the supply house but the name of the company on the pex i would you 1/2 inch unless its outside..
 
  #6  
Old 06-09-10, 07:15 AM
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Smaller - shorter runs....

The closer you space the tubes gives you more even heating. I would even look at 200' runs of 3/8 spaced closer and see how that works out.

Tim
 
  #7  
Old 06-15-10, 05:58 AM
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Have your system designed by the supply housse you are getting your material from. As a manufactures rep, they have the radiant programs to design it correctly. Most manufactures want no more than 160-200 ft lengths and depending on your layout, it may require less. I've done this for 30 years and never had a design that was wrong. If you want to install it yourself, thats fine but I would pay for a professional to consult and help you with the layout. It will cost you more in the long run if it's designed and installed wrong to begin with.
 
  #8  
Old 06-15-10, 07:52 PM
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Originally Posted by 1maytech View Post
Have your system designed by the supply housse you are getting your material from. As a manufactures rep, they have the radiant programs to design it correctly. Most manufactures want no more than 160-200 ft lengths and depending on your layout, it may require less. I've done this for 30 years and never had a design that was wrong. If you want to install it yourself, thats fine but I would pay for a professional to consult and help you with the layout. It will cost you more in the long run if it's designed and installed wrong to begin with.
Sorry, 160-200 foot loops of 1/2" pex is not cost effective.
Unless your a manifold manufactorer, or using geothermal heat pumps as your heat source I see NO reason not to run 300 to 330 foot runs of 1/2" pex.
 
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