Non-barrier PEX turned our hot-water loop into black soup

Ran into a nasty one on a small office retrofit: contractor swapped in non-barrier PEX on a heating hot-water loop to save time. Three months later we had black magnetite sludge, pump seal failures, and a fouled heat exchanger. Controls alarms everywhere because delta-P sensors drifted as strainers loaded up. Everyone chased setpoints, but the root cause was oxygen diffusion through the tubing feeding every ferrous part a steady dose of corrosion.

Lesson learned: do not rely on “closed system” and chemical treatment alone. Specify oxygen-barrier PEX with a documented permeation rate, verify the barrier layer in submittals, and reject “equivalents” that are just plain PEX. If polymers are unavoidable, go non-ferrous downstream or add magnetic dirt separation sized for the actual flow. Add explicit language tying pipe selection to pump and valve materials to avoid mixed-metal traps.

Curious what others put in specs to catch this early. Do you require permeation data, and do you pair it with mandatory mag separators, or go all-copper/steel and skip PEX entirely in hot-water loops?