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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.