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Partial Pex repipe

Burbman Member Posts: 7
Our house is a little over 100 years old and has the old fashioned cast iron radiators (hot water heat, not steam). The radiators are piped to a main supply and a main return pipe in the basement that connect to the boiler. These main pipes are 1-1/2" and in one spot 2" cast iron pipes that hang down below the floor joists...you know, the ones you always bang your head on in the basement. There are 8 radiators in the house, and they connect via 3/4" black pipe that goes up through closets or boxed out pipe chases. One exception is the kitchen radiator that is connected to the main loop with 1/2" copper.

Adding to the pipe maze is the fact that the new boiler was moved across the basement so it could be better exhausted via PVC, so there are 1-1/4" copper pipes that connect the supply and return on the new boiler to the old heating loop. Here's a quick drawn diagram of how the pipes run.

[img]http://i4.photobucket.com/albums/y134/Burbman/Heating diagram_001_zpspho1cwzr.jpg[/img]

I was reading on another site about using pex for heating and wondered if it would be possible to partially repipe my set up in pex. What I am thinking is eliminating the main heating loop in the basement and replacing with a pair of manifolds that have separate pex runs to each radiator. Leave the existing iron pipe in the walls and only attach the pex where the iron pipe enters the basement. This would avoid any demolition and repiping inside the walls.

Benefits are that I could add a second circulator and split the house into 2 zones, upstairs and downstairs. There is already a t-stat in the upstairs hall for the a/c, so I would just to run a wire to it for heat. Plus I would add shut offs at the manifold so I could disconnect radiators one at a time. Right now, when I need to disconnect a radiator to repair the plaster behind it, it's a whole process of draining the system, removing the rad, capping the pipes, and then refilling/bleeding the system. Then do it all over again when I replace the rad.

Right now the kitchen rad that's connected with 1/2" copper gets hotter than the rest, so I am thinking 1/2" pex would be good for this.

Thoughts? Ideas?


  • Zman
    Zman Member Posts: 7,569
    A house that old would have had a gravity system if it was original to the house. If it was a retro it could be monoflow piped or conventional. More pictures of the piping and tees up to the radiators would help determine what you have.

    If you truely have the ability to re feed every radiator with a pex homerun, it doesn't matter how it is piped. Chances are, 1/2" pex would work just fine. I would suggest performing a room by room heat loss and have someone take a look at the radiator sizing before you commit.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GRIZ
    GRIZ Member Posts: 23
    Sounds very similar to the project I am about to start. Here are the details for my project:


    and here is a thread I found on here that offered some good basic introductory information.


    I have done a lot of research on this, and I feel like a have a decent handle on it, but I'm not experienced enough to give much advice. I will share what I have learned in my research along with references if you find that helpful. I have had 4 different plumbers come and quote my project, but everyone of them quoted Uponor hePex instead of Pex-AL-Pex becuase they dont work with it. Not enough boilers in my area. I'm convinced the Pex-Al-Pex is the way to go.
  • delta T
    delta T Member Posts: 884
    I have done several conversions of old gravity systems to homerun pex systems, and they work fantastically well. If you are going to all the trouble of doing this, I would highly recommend a constant circulation system, using thermostatic radiator valves and forget about zoning. Every radiator will become its own zone. Make sure to set up and outdoor reset for the boiler, and leave a thermostat hooked up just as a high limit. I hope there was a heatloss performed when the new boiler was installed....If your boiler is seriously oversized, (very common) then you will have trouble getting the unit to operate efficiently. Old cast iron radiators are PERFECT for a home run trv system with constant circulation......as long as the math is done first :)
    Paul PolletsSuperJRich_49
  • EYoder
    EYoder Member Posts: 60
    I've done a few home run 1/2" pex type systems to old refurbished cast rads tied to an outdoor wood boiler. This was with a t-stat starting a circulator. Flow can be balanced to match each rad, each loop with valves. It worked great. A very large rad might benefit from 3/4". I like the TRV/∆p circulator idea but didn't tackle it.