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Black pipe to Cast iron

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Are the compatible? Working on a 100-year-old heating system. I am going to go from old cast iron to black pipe if it will work. After that copper to the new furnace. And what's up with malleable iron pipe?

Comments

  • Cast iron is typically DWV pipe, I have not seen threaded cast pipe? The fittings may be forged type however. Most fittings are malleable steel now also.

    Common black steel pipe schedule 20, 40, and 80 is the same OD and will take a NPT thread.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • scatgo
    edited 1:54AM

    So the pipes going to the cast iron radiators are not cast iron? But the elbows and such might be? I think people have suggested the fittings will crack if that's what it takes to remove them. Maybe I misunderstood?

  • hot_rod
    edited 2:14AM

    Here is what the fittings typically look like. The heavier, thicker hub is an iron that you can split with two hammers.

    The lighter version is a steel, it will not crack, you need to saw a notch or two and peel away the fitting, or nipple inside.

    If you plan on splitting fittings with force, gloves, face shield, hat maybe the hard type, long sleeve clothes, pieces can fly.

    You can find videos of plumbers splitting fittings. An Instragrammer recently showed a concrete anchor gun being used to split cast fittings!

    Screenshot 2026-09-03 at 8.12.55 PM.png Screenshot 2026-09-03 at 8.02.21 PM.png Screenshot 2026-09-03 at 8.02.48 PM.png

    https://www.instagram.com/reels/DGI5R1tRQTG/

    Screenshot 2026-09-03 at 8.11.07 PM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    scatgo
  • scatgo
    edited 2:23AM

    Got it. I did see similar videos. We used to call them Hiti guns. Brand name of course but it's the type of nail gun that uses what we called shots. Basically, like a blank that would be use in a pistol. We used them to drive nails into steel beams on one job we did. Those things are no joke. Thank or the help.

  • The fitting used on steel pipe systems are usually malleable iron or cast iron as @hot_rodmentioned. Cast iron fitting are more brittle and can be broken with 2 hammers if you know how. Malleable iron you can't crack with hammers. You can use either type of fitting on hot water or steam heat. Malleable is more common now and some suppliers don't stock both. In the old days CI was more common for heating systems. CI cannot be used on gas.

    You may run into some brass fittings or galvanized fittings as well but not as common except for brass valves.

    The pipe has always been steel or wrought iron. Wrought iron was used more for underground use as they say it is more rust resistant. You don't see it much anymore. Steel pipe is commonly sold and is what every place stocks either at a supply house or a big box store.

    scatgo
  • it seems like a ricochet could be deadly using the desa or remington…

    scatgo