4.2 Joining Methods, Soldering, Brazing & Mechanical Fittings

Soldering, Brazing, Flaring and Listed Mechanical Joints

Edition rule: the 2018 IMC permits soldered refrigerant joints when they meet § 1107.5.5. It does not impose a blanket soft-solder prohibition, and its brazed-joint paragraph does not contain the later code's explicit inert-purge procedure.

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1. Soldering and Brazing

Soldering joins metals with filler that melts at or below 840°F; brazing uses filler melting above 840°F while the base tube and fitting remain solid. Both depend on a clean surface, correct joint clearance, uniform heat, capillary flow, and a filler compatible with the base metals, refrigerant, pressure, temperature, and vibration.

A good joint begins before the torch is lit:

1. Cut the tube square.
2. Ream without allowing chips into the circuit.
3. Clean the tube and fitting without removing excessive metal.
4. Dry-fit and confirm insertion depth.
5. Protect valves, insulation, wiring, and combustible construction.
6. Select filler and flux from the filler-metal and equipment instructions.
7. Arrange fire watch, ventilation, extinguishing equipment, and inert purge where required.
8. Heat the assembly evenly and feed filler at the joint rather than melting it in the flame.
9. Allow it to cool naturally, clean flux residue, and inspect before pressure testing.

2. Brazing Filler Metals

Copper-phosphorus filler metals in AWS BCuP classifications are common for copper tube. The phosphorus supplies self-fluxing action on copper-to-copper joints. BCuP alloys differ in silver content, melting range, flow, ductility, and cost. A higher silver percentage can improve flow and ductility, but no one alloy is automatically correct for every compressor, reversing valve, or line size.

When a joint includes brass or bronze, the self-fluxing property on copper no longer means “no flux.” A flux approved for the filler and base metal is generally required. A BAg silver-brazing alloy with matching flux is another common choice and can cover a broader range of dissimilar-metal joints.

Do not memorize the false rule that BCuP can never join copper to brass. Conversely, do not use phosphorus-bearing filler on steel or nickel alloys unless the filler manufacturer specifically permits it; brittle phosphide formation is a known concern. Follow the equipment and filler-metal instructions.

Flux goes only where needed and is kept out of the refrigerant circuit. Excess internal residue can retain moisture, create corrosive contamination, or move downstream. After brazing, remove external residue as directed because some fluxes remain chemically active.

3. Inert Purging During Brazing

Heating copper in air creates internal oxide scale. Circulating refrigerant and oil can carry loosened scale to a metering-device screen, oil passage, or bearing surface. Flowing dry oxygen-free nitrogen or another approved inert gas displaces oxygen and keeps the internal surface clean.

The 2018 IMC § 1107.5.1 requires cleaned brazed-joint surfaces, approved flux where the filler manufacturer requires it, and AWS A5.8 filler. It does not state a numeric nitrogen flow. Manufacturer instructions and accepted installation practice commonly require an inert purge, and later IMC editions added an express code purge requirement.

Use this sequence:

• Connect an inert-gas cylinder through a regulator and suitable flow-control device.
• Provide an open outlet so the piping cannot become a sealed pressure vessel during heating.
• Establish only enough flow to displace air without blowing molten filler from the joint.
• Maintain flow through heating and initial cooling as the governing procedure directs.
• Never use oxygen, shop air, or fuel gas as the purge.
• After completion, carry out the separate strength/leak test at the pressure required for the system.

Values such as 2–5 SCFH are common field starting points, not a universal 2018 IMC mandate. Piping volume, length, outlet arrangement, and manufacturer instructions determine the actual procedure.

4. Soldered Refrigerant Joints in the 2018 IMC

Section 1107.5.5 of the 2018 IMC states that surfaces to be soldered are cleaned, flux conforms to ASTM B813, and solder conforms to ASTM B32. The code therefore does not prohibit all soldered refrigerant joints.

Code permission is only the first screen. The equipment listing, manufacturer instructions, refrigerant pressure and temperature, joint location, vibration, local amendment, and project specification can require brazing or a listed mechanical joint. A solder joint unsuitable for a modern high-pressure unit does not become acceptable merely because the base code contains a solder paragraph.

Later IMC editions introduced additional limits, including refrigerant group and pressure restrictions. The Maryland Master examination currently permits only the 2018 IMC, so answer an edition-specific question from that edition rather than importing a later rule.

5. Flared and Mechanical Joints

Refrigeration flare fittings normally use the 45-degree SAE geometry, but the tube, fitting, flare tool, and torque must be compatible. Cut square, ream, place the nut before flaring, make a concentric flare without cracks, and tighten with a torque wrench and backup wrench to the equipment instructions. Apply oil to a flare only if the manufacturer directs it; indiscriminate lubricant can alter torque or contaminate an oxygen or specialty-refrigerant service.

Listed press-connect refrigerant fittings create flame-free joints using a dedicated tool and sealing element. Verify refrigerant compatibility, pressure and temperature rating, tube preparation, insertion depth, jaw profile, and tool calibration. A plumbing press fitting is not automatically listed for refrigerant service.

Threaded refrigerant joints use the approved thread form and a lubricant or sealant compatible with the refrigerant, oil, and piping material. Under the 2018 joint rule, thread lubricant or tape is applied to external threads only so material is not pushed into the system.

6. Inspection and Failure Analysis

After joining, inspect for complete filler flow, overheating, pinholes, cracks, misalignment, damaged valves, and residue. Pressure testing proves tightness; appearance alone does not. A leak at a braze joint can result from inadequate cleaning, wrong clearance, insufficient or excessive heat, unsuitable filler, movement during solidification, or internal pressure caused by a blocked purge outlet.

When repairing a failed joint, recover refrigerant to the applicable federal level, isolate the component, remove damaged filler and contaminated tube, reprepare the joint, protect nearby components, and retest. Never heat a closed refrigerant volume.