1/2" Pex for radiator feeds?

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Old 10-14-13, 11:10 AM
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Question 1/2" Pex for radiator feeds?

I will be installing a couple of Runtal UF-3 radiators in my house, one 5' and one 6' unit. They will be about 25'-30' from the boiler and manifold, and each will be seperate home run feed and return. They will be fed with 150* water max.

Is there any reason I can't use 1/2" Oxy barrier PEX for the feed and return? The Runtals have 1/2" pipe threads on them. I know 1/2" PEX ID is less than 1/2", but will it still carry enough flow for one radiator? If its borderline suficent, I can use 3/4" PEX, but figured if its only going into a 1/2" fitting, I could use the smaller PEX.

Thanks for any advice
 
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Old 10-14-13, 02:47 PM
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To really know the answer, you'd need to consider the Btu/hr you need from those heat emitters, then look at their specs for flow and temperature, and figure the total pressure drop compared to the circulator head. Personally, I would just go with 3/4" pex rather than 1/2". Your maximum 150 deg water doesn't help - why so low?

Just because the connections on the heat emitters are 1/2" doesn't necessarily mean that 1/2" pipe is correct. It's the total pressure drop that governs, not the smallest sized connector.
 
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Old 10-14-13, 03:04 PM
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The radiators arent needed for their full rating at 180*. They will be mounted under two large picture windows to suppliment the radiant heat on the coldest days. If I was running it at 180* I could get by with radiators half that length. System is running with a mod/con boiler so lower temp with longer radiators gives more than enough heat.
 
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Old 10-14-13, 03:35 PM
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With copper 1/2", we figure about 15K BTUH is the max load for that size pipe.

Yes, PEX ID is a bit smaller, so figure maybe 10-12K BTUH is the max.

As gilmorrie said, make sure the pump will do the job.
 
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Old 10-14-13, 06:23 PM
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Thanks guys, the larger one will only be in the area of 4-5K BTU's so even if the PEX can only do 9K, I am still well within the scope of possibilities. And the radiators will only be about 3' above the level of the boiler and a 25' straight shot to each. I dont think there will be too much head or friction for a standard circulator to make the required gpm.

Thanks again
 
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