g/hvac • Posted by u/freon_frank • 3 days ago

Nitrogen purge pressure for brazing — 2 PSI or 3 PSI, and why?

Simple question that gets answered differently depending on who's in the room.

When brazing copper refrigerant lines, what nitrogen purge pressure are you using, and why that number?

I've heard 2 PSI, 3 PSI, and once "whatever keeps the line from collapsing." The goal is preventing copper oxide formation (mill scale) inside the line. But I'm curious if anyone has actually tested oxidation levels at different purge pressures, or if this is all tribal knowledge at this point.

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u/boiler_bill • 3 days ago

Any positive pressure prevents oxidation — that's the physics. The "exactly X PSI" debate is a bit tribal. What actually fails installs is guys who set up nitrogen at the start and don't maintain it through the whole braze, or don't flow it at all on short joints because "it'll be fine." It won't always be fine.

u/duct_doctor • 3 days ago

I run 2 PSI confirmed with a gauge, not a regulator estimate. The reason 3+ PSI gets recommended sometimes is as a safety margin for guys who don't verify — if you're at 3 PSI you're almost certainly maintaining positive pressure even with some leakage. 2 PSI with a gauge is more accurate. Either works if you're actually checking.

u/hvac_hero • 3 days ago

2-3 PSI is the industry standard range. The specific number matters less than maintaining positive pressure throughout the braze. I use a flow meter set to about 3-5 CFH — that gives me consistent 2 PSI in 7/8" tubing. Too high and you're just blowing nitrogen out the open end fast. Too low and you might drop below positive pressure when positioning the torch. The key: flow must be continuous, not just charged and shut off.