I'm a renter in NYC and our 4-story building has recently-installed (within the last 8 years) baseboard hydronic heat. All of the tubing is PEX, except for from the boiler itself (as required by code) and a distribution panel with shutoffs for each floor, which is all soldered copper.

Earlier this week, our building had a piece of the PEX that runs to one of the floors split and explode, draining the system's contents into the boiler room. A plumber came, spliced in replacement pipe for the split section, and purged all the air from each floor's heating loop.

Here's the split piece of PEX:

red PEX pipe with a 5-inch split

The plumber seemed satisfied with the explanation that air in the system caused an overpressure and split this section. (I don't think the landlord has ever purged the air. The landlord did mention this has happened before at the building. He's not a quick learner.)

Anyway, working in software has left me unhealthily obsessed with root-cause analysis. I noticed another discolored bulge near the split, which looks a lot like burnt/overheated plastic to me:

discolored brownish bulge on red PEX pipe

It looks like there's similar brownish damage near the split itself. It's impossible to tell if the discoloration preceded the failure or not.

There are similar brown, bulging discolorations on other sections of PEX, and each discoloration is adjacent to a section of copper pipe in the same system.

view of red PEX with brown discoloration near copper pipe view of red PEX with brown discoloration near copper pipe view of red PEX with brown discoloration near copper pipe

  1. Is this discoloration likely from when the copper fittings were installed and sweated, or from being directly above these fittings and slowly being exposed to heat coming off them?

  2. Are these discolored spots likely to fail under normal circulating pressure if the system is purged of air regularly going forward?

  3. Should these sections of PEX be replaced? Do they need to be insulated or protected from the copper pipe to meet either code, manufacturer's instructions, or just common sense?

(This is purely a learning exercise: it ain't my building, and I'm just curious in case I ever buy my own house with a boiler and PEX.)

2 Answers 2


PEX pipe does not corrode or otherwise interact with copper or other materials the way dissimilar metals do. It's essentially an inert substance under normal circumstances.

The bulges and darkened areas, however, do appear to be signs of a problem and I agree with your assessment that these were caused by heat, probably from the torch used to sweat the nearby copper pipe joints. The person installing the copper should have taken precautions to prevent heat from getting to the PEX pipes but clearly did not do so.

The damage has been done and the bulges show that failure is likely since it's the water pressure and NOT air in the system that is causing the weakened area to expand. Once that process starts, it will keep "blowing up like a balloon" until it fails.

The damaged areas should be cut out and new sections should be put in. Based on the obviously careless installation of the copper, an end-to-end inspection is called for.


There may be another issue with that installation. Red pex is typically used for domestic hot water, and does not have an oxygen barrier.

Now it is possible that all of those red pipes are domestic hot water pipes, but without more context it is difficult to guess what is for what.

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