Replacing Cast Iron HW Radiator Distribution With PEX-Al-PEX - HVAC - DIY Chatroom Home Improvement Forum

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Old 02-07-2012, 11:25 AM   #1
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Replacing cast iron HW radiator distribution with PEX-Al-PEX

I am looking to change out all of the original cast iron pipe on the heat distribution system in a 100+ year old 3-story building that I am renovating in Easton, PA. The distribution system is on two NG boilers with feed pumps (1 for 1st floor; 1 for 2nd/3rd floors). The house was not properly winterized by the previous owners and several pipes and fittings burst. I saw a video on This Old House (you can view here) where they reconfigured a hot water radiator heating system with PEX-Al-PEX off of a manifold. If running the PEX-Al-PEX in this "home run" design, what size tubing would be required with dedicated supply/return to each radiator? I know there are practical requirements for flow and pressure, so I am wondering how small I can go on the ID. Can I get away with 1/2"? 5/8"? Or would I have to go all the way up to 3/4"? I would prefer a smaller diameter over a larger to reduce some of the costs, but not if it means installing an inadequate distribution system.

Thanks for your help!


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Old 02-07-2012, 02:39 PM   #2
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Replacing cast iron HW radiator distribution with PEX-Al-PEX

Probably nothing smaller then 5/8". Might need 3/4" depending on how long the run is.


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hot water heat , pex-al-pex , radiator

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