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Repiping my cast iron radiators

A couple of years ago I did almost the same thing, a 1922 gravity HW system with CI radiators and head-banging pipes.

I ran new copper loop mains starting at 1" and reducing to 3/4" at the far end, but connected loops. The branches were 5/8" PEX-AL-PEX (PAP) with Kitek couplings to bushings where I connected to old 1" iron risers. Easy.

I also installed TRV's on all but a couple of radiators.

Your manifold and home-run is equally applicable and was my Plan B. The copper loop made the installation neater than what would have been two manifolds. Same manifolds as for radiant by the way (with flow meters) and with 5/8" PAP fittings also.

If you know your heat loss and can establish your flow rate, do so. You may be surprised that even a reduced flow (to even half) will still give you about 90% of capacity. It is a forgiving variable indeed.

Not knowing a thing about your house, I suspect with six radiators at maybe 5 to 8,000 BTUH, it is a small to medium size structure. A Grundfos 15-58 or other 3-speed circulator by Wilo or Taco will give you all of the flexibility you need.

Pipe it up to protect your boiler from low return temperatures until you get the new one in place. The switch-out will be pretty straight-forward and ought to please you.


  • Marson
    Marson Member Posts: 3
    Repiping my cast iron radiators.

    I have an old heating system in my 90 year old house. Presumably, the piping dates from an old gravity system. Two loops, supply and return, and the supply starts as 2" (or maybe even larger) and then necks down as each radiator splits off. Vice Versa on the return side. Each radiator has a 3/4 supply and 1/2" return. At some point, someone installed a boiler with a circ pump and tied into the system with 1" copper from the boiler.

    The system works fine. I love my cast iron radiators. Just two problems: I bump my head on the pipes because they are hung so low from the floor joists, and I need to add several radiators. So I'd like to repipe the whole works. I'll keep the boiler I have for a few years.

    I'm thinking about converting this to a home run system. I'd like to hook onto each radiator with 1/2" pex run back to a manifold. I'd like to do this all in the basement--I like the old pipes coming up through the floor, 14 coats of paint and all. All of this seems pretty straightforward--twist off the fitting below the floor, transition to copper, then to pex. I'm a building contractor and have done a bit of plumbing so the mechanics of the install don't scare me.

    I have a few questions though. Does the home run system make sense? I have only six radiators now (some previous idiot disconnected two radiators and replaced them with an electric baseboard). I'd like to add 3-4 new radiators, meaning I would need a 10 loop manifold. Should I buy this ready made? I've seen heating guys build manifolds out of rigid copper and ball valves--would I be crazy to consider building my own? If I buy a manifold, would it be the same as is used for infloor heating systems?

    What about supply to the manifold? Ten 1/2" pipes seem like they would need more than 1" to feed them, yet the whole house is now heated through this 1" pipe feeding the current system.

    How about pump size? If I go to a manifold, it seems like friction will increase.

    Or perhaps I should just go to a conventional system with monoflow tees?

    Sorry about the long and complex question. Any comments are appreciated.
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