Brazing Refrigerant Lines

A brazed joint on a refrigerant line has to do more than hold. It must seal against hundreds of pounds of pressure for many years, and it cannot leave anything inside the pipe. This guide covers what is different about brazing in HVAC and refrigeration: the nitrogen purge, filler rod choice and protecting nearby parts. Learn general torch handling from a qualified instructor, and treat every braze as hot work.

What You Will Be Able to Do

• Explain why a nitrogen purge is required during brazing.
• Choose a filler rod for copper-to-copper and copper-to-brass or steel joints.
• Prepare, heat and finish a brazed joint in the right order.
• Protect valves and other heat-sensitive parts.
• Check a joint for a leak.

Why Nitrogen Flows During the Braze

When copper is heated with oxygen around it, a black scale called copper oxide forms on the inside of the tube. That scale flakes off, circulates with the refrigerant and can plug metering devices, foul filter-driers and damage the compressor.

Dry nitrogen pushes the air out of the tube, so oxygen is not there to form scale. It is a low-flow purge, not a pressure test. You should feel only a light breath of gas at the far end.

Filler Rods

Higher silver content flows better and handles vibration better, but costs more. Follow the manufacturer's instructions for service valves and compressors. If you use flux, keep it off the inside of the joint, and clean any residue off the outside afterward.

Do not use soft solder on refrigerant lines unless the manufacturer lists it. Refrigerant work calls for brazing, which happens above 840°F.

Before You Light a Torch

1. Confirm the refrigerant has been recovered and the system has been purged. Refrigerant in a flame forms toxic gases.
2. On A2L systems, follow the full recover, purge, evacuate and purge order, and clear ignition sources.
3. Get a hot work permit if the site requires it, and check the area for anything that can burn.
4. Have a fire extinguisher within reach and use a heat shield on walls, wood and wiring.
5. Remove Schrader cores on nearby ports, so the heat does not ruin them. Removing them also gives the nitrogen a path out.
6. Wrap service valves, TXVs and reversing valves with a wet rag or a heat-blocking paste. Keep them below the temperature limit in the manual.

Warning: Wear safety glasses with the right shade for torch work, gloves and long sleeves. Keep the torch away from any part of the system that may still hold refrigerant.

Making the Joint

1. Cut square. Use a tube cutter, not a saw. Saw chips go inside the system.
2. Deburr. Ream the inside edge lightly. Burrs slow the flow, and loose chips travel.
3. Clean. Use an abrasive pad or a fitting brush on the outside of the tube and the inside of the fitting. Do not leave the dust inside the pipe.
4. Fit up. Push the tube all the way into the fitting. A correct gap lets capillary action pull in the filler. A loose joint leaks.
5. Start the purge. Connect nitrogen through a regulator and a flow meter at one end and leave an open path at the other. A flow of about 2 to 5 SCFH is typical, and the manual may say more.
6. Heat the tube first. Move the torch back and forth, heating the tube, then the fitting. Copper turns a dull cherry red at the right temperature. Heat evenly around the joint.
7. Add the rod. Touch the filler to the joint, away from the flame. The heated metal should melt it, not the torch. Capillary action draws the filler in all the way around.
8. Stop when it fills. A small fillet around the joint is enough. Extra rod can run into the pipe.
9. Cool it. Let the joint cool in still air. Keep the nitrogen flowing until the copper has lost its color. Do not quench a hot joint with water, since that can stress it.

Inspecting the Joint

Look for a clean, even fillet all the way around. Warning signs include:

• Gaps or pinholes at the fillet.
• A blackened, overheated joint.
• Filler that balled up instead of flowing.

Do not trust the look alone. Pressure test with dry nitrogen and use bubble solution on each joint.

Repairing a Leaking Joint

Brazing over a leak on top of old filler rarely works. Recover and purge again, heat the joint until it comes apart, clean both sides, and start over with new fittings if you can. Never apply heat to a joint that holds refrigerant or pressure.

Common Mistakes

Check Your Understanding

You see a black, flaky powder inside a cut piece of tube from an old repair. What caused it, and how do you avoid it? Copper oxide scale from brazing without nitrogen. Flow a low rate of dry nitrogen through the line during the braze.

You need to join a copper line to a steel fitting on a compressor. Is a phosphorus-copper rod the right filler? No. Use a silver brazing rod with flux. Phosphorus rods are for copper and some brass, not steel.

You turn up the nitrogen to be safe, and now the joint will not seal. What went wrong? Too much pressure pushes the molten filler out. Keep the purge low.

The joint looks clean. Are you done? No. Pressure test with nitrogen and bubble it before evacuating.

Key Takeaways

• Flow low-pressure dry nitrogen through the line during every braze to prevent copper oxide scale.
• Use phosphorus-copper rod on copper-to-copper joints and silver rod with flux on brass and steel.
• Recover refrigerant, clear ignition sources and protect valves before you light the torch.
• Heat the joint, not the rod, and let capillary action fill it.
• Prove every joint with a nitrogen pressure test.