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Black PEX:

icesailor Member Posts: 7,265
Lately, it seems that every heat system I look at that has a Cast Iron boiler, copper tube and PEX, has all the PEX being black inside. It isn't water PEX. It all has HePEX and "not for potable water water" marked in the tube.

One job, I replaced a boiler with a W-M WTGO-5 and fed the old copper baseboard. An air handler was installed on the first floor, all in copper. Then, a air handler was installed in a third floor attic to heat and AC the second floor and fed with Heat PEX. I noticed that one side of the supply/return was turning black. Now, they are both black and the water is getting nasty.

What thoughts?


  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    I had the very same thing happen to a system

    I installed. It consists of an outdoor wood boiler then insulated pex running underground to the house where it first goes through a flat plate heat exchanger that serves 1st floor radiant and second floor H2o coil. Then it makes it's way through a second heat exchanger that preheats domestic water as it enters the water heater. Then it scoots over through the wall into a newer addition where it runs through a 3' copper baseboard and has a radiant garage ceiling tapped into it before finally making it's way back to the boiler and completing the primary loop.

    All the pex is HePex from Uponor. It is rather clear so it was quite obvious when it started turning a blueish black color. I took apart the pex and swiped the inside of the pipe with my finger. It reminded me of the soot on the globe of a coal oil lamp. It even caused a circulator to seize.

    Having thoroughly flushed and purged the boiler and piping I can only assume that when the cast iron in the boiler heated up it released some kind of mineral deposits into the water.
  • kcopp
    kcopp Member Posts: 3,815
    Happens all....

    the time w/ cast iron boilers. I notice that when installing SS mod/cons I don't have that. The only way I could see that NOT happening is to fill the boiler heat it up and then flush it out. Then add a cleaner... and let it run for a few days and flush it out again. I believe that Rhomar would say all boilers should have this done to maximize the heat transfer.... I also don't think that PEX has anything to do w/ this and that if you cut out a copper line you would see that same layer of iron oxidish ooze.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837

    PEX with any oxy barrier tube does NOT stop oxygen ingression, it only slows it down.

    I've seen systems with S.S. boilers get the same coating inside, but they had cast iron scoops and pumps in the circuits.

    I've also seen situations whereby the O2 barrier was accidentally scratched by a floor joist hanger, and it had a BUNCH of grey/blackness occurring within a month.

    Is it a problem? Possibly, if pump bearings get locked up. But I've seen bearing lock up occur in all metal systems too?

    Will water treatment stop it? I doubt it. Oxygen never sleeps. Rust never sleeps. It might slow it down, but mother nature despises any differential in oxygen, pressure or heat, and will by God make all things equal.

    I know for a fact, at one point a major manufacturer of PEX tubing considered putting a colorized version of their PEX out so that you couldn't see the oxidation occurring. They never went thru with that plan, so I suspect that the byproducts seen on the walls are not a major concern, but only time will tell for sure.

    Now rubber hoses are a completely different beast...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • LarryC
    LarryC Member Posts: 331
    Black rust

    As I understand it, when iron rusts in the presence of air, it produces the soft red rust.  When iron rusts by dissolved oxygen in water, it forms a hard black rust or magnatite.  In commercial power plants, magnatite plus copper leads to a hard scale on heat transfer surfaces.  That is why most power plants work very hard to remove all copper and brass from their piping systems.

    Perhaps the black buildup seen is magnatite and the presence of copper is helping it precipitate on the tubing surface.  Will it significantly affect heat transfer?  My gut feeling is, not significantly.  Can it cause bearing clearance problems?  I do not know.

    Does Perry still hang around this site?  He might be able to answer the questions.
This discussion has been closed.