25.2 Brazing, Welding, Flaring, Swaging & Soldering

25.2 Brazing, Welding, Flaring, Swaging & Soldering

The same Trade Area D 14 percent now asks how the tube is joined: welding and brazing copper, silver brazing copper-to-steel, flaring, soft soldering, soldering swaged joints, and welding with filler rods. Open FBC Mechanical 2023 Sections 1108 and 1107.5.1, and keep Refrigeration & Air Conditioning Technology 9th Ed. on the nitrogen-purge pictures. A beautiful ACR run from Section 25.1 is still a failed system if the joints dump copper oxide into the TXV.

Quick Answer: Braze refrigerant copper with AWS A5.8 filler while oxygen-free nitrogen (or helium or argon) flows: five volume changes at ≥100 fpm, then 1.0–3.0 psi during heat (1108.3.1). BCuP is copper-to-copper. BAg silver plus flux is copper-to-steel. Soft solder is A1 and ≤200 psi only (1108.3.3) — not an R-410A line set. Flare annealed tube with a 45° tool. Swage to Table 1107.5.1 cup depth, then braze the lap.

Brazing versus soldering versus welding

Brazing joins metals with a filler that melts above 840°F (AWS) but below the melting point of the base metal. The filler is drawn into a capillary gap by wetting. Soldering is the same idea below 840°F. Welding melts the base metal and adds a filler rod of similar metal; that is how you join steel pipe, not how you join ACR copper on a Florida split.

FBC Mechanical 1108.7: joints in copper tube shall be brazed, flared, mechanical, press-connect, or soldered — soldered only if 1108.3.3 is also met. 1108.6 copper pipe also allows threaded and welded joints. 1108.3.5: weld only after an approved cleaning procedure, with an approved filler metal.

The CILB item “welding/brazing copper” is almost always brazing on the exam. Fusion-welding copper with a steel rod is not a line-set method.

Nitrogen purge — FBC 1108.3.1, word for word in practice

Heat plus copper plus air makes copper oxide scale inside the pipe. That scale breaks loose, clogs the metering-device screen, scores the compressor, and acids the oil. 1108.3.1 is not a shop preference:

1. Clean the joint surfaces. Apply an approved flux only where the filler-metal manufacturer requires it.
2. Purge air and fill the pipe with oxygen-free nitrogen, helium, or argon. The inert gas must be directly connected so ambient air is not sucked in.
3. Pre-purge for a time equal to at least five volume changes through the piping, at a minimum velocity of 100 feet per minute.
4. After the pre-purge, keep the inert gas flowing during the actual braze at not less than 1.0 psi and not more than 3.0 psi.
5. Braze with filler conforming to AWS A5.8.

A tiny nitrogen trickle “so the gauge barely moves” is the 1–3 psi hold, not a firehose that blows filler out of the cup. Never use oxygen as the purge or the pressure-test gas (Section 25.4). Never use the cutting-torch air hose as a “purge.”

Worked purge picture. A 50-foot 7/8-inch suction plus 3/8-inch liquid is a small volume. Crack a nitrogen regulator through a purge-gas flowmeter (not a 200-psi delivery regulator dumped into the tube). Flow until you have five volume changes, drop to a 1–3 psi bleed, heat the joint, and shut nitrogen down only after the last joint is quenched. Brazing in open air and “blowing it out with refrigerant later” is how a new TXV dies in a week.

Silver brazing copper-to-copper and copper-to-steel

BCuP alloys (copper-phosphorus, often sold as Sil-Fos with 5, 6, or 15 percent silver) are self-fluxing on copper. Phosphorus scavenges oxygen on the copper surface. They are the default copper-to-copper ACR braze.

Phosphorus and steel do not mix. BCuP on a ferrous fitting makes a brittle intermetallic joint that cracks in vibration. The CILB item silver brazing copper-to-steel means a BAg (silver) alloy with flux on the steel side: compressor service valves, steel driers, steel receivers, threaded steel adapters. Clean both metals, flux the steel, nitrogen-purge, and braze with AWS A5.8 BAg, not leftover BCuP.

Heat the tube and cup, not the rod. The filler follows the heat into the capillary. A blob of alloy sitting on a cold fitting is not a filled joint. Quench only after the fillet has solidified; water on a still-liquid fillet can star-crack it.

Soft soldering and swaged joints

Soft solder (tin-lead 50/50, tin-antimony 95/5, and similar ASTM B32 solders) melts in the 360–430°F neighborhood. Plumbing drains and some hydronic copper still use it. FBC Mechanical 1108.3.3: soldered joints shall use ASTM B813 flux and ASTM B32 solder, and shall be limited to Group A1 refrigerants at a pressure less than or equal to 200 psi. R-410A high-side design pressure is far above 200 psi (nameplates commonly in the 600 psi region). R-22 high-side design is also above 200 psi on most air-conditioning units. Soft solder is generally not for refrigerant lines on the Florida exam. Braze them.

The CILB still lists soft soldering and soldering swaged joints because the code carves a narrow A1 / ≤200 psi window (some older low-pressure chillers, some R-134a circuits). If the question states R-410A, R-32, or a 400-plus-psi nameplate, the answer is braze, not 95/5.

Swaging expands the end of one tube so the mating tube slips in, making a field cup instead of a wrought fitting. FBC Mechanical 1107.5.1 / Table 1107.5.1 sets minimum and maximum cup depth. Examples: 3/8-inch fitting size, cup 0.20–0.30 inch; 7/8-inch / 1-inch row 0.28–0.42 inch; 2-inch 0.40–0.60 inch. Too shallow and there is no capillary; too deep and you starve the flow or hide an unfilled gap. After the cup is in spec, braze the lap (solder only if 1108.3.3 actually applies). A hammered “close enough” swage that you 50/50-solder on an R-410A suction is two failures in one fitting.

Flaring and welding with filler rods

FBC Mechanical 1108.3.2.1: flared fittings follow the manufacturer; the flare is made with a tool designed for that operation, on the tube material the fitting specifies. Refrigeration flares are 45° SAE. Use annealed (soft) copper. Cut square, ream the ID so a burr does not seed a leak or a restriction, put the nut on before you flare, and form the cone in the block. A double flare folds the tube so the sealing face is two walls — better under vibration on small lines and some automotive-style fittings; HVAC field flares on 3/8-inch liquid are often single when the listing says so. Over-tightening splits the flare. Under-tightening leaks. Dope and PTFE tape do not belong on a flare face (Chapter 18 already parked thread sealant on NPT, not on flares).

Press-connect joints (1108.3.2.2) are allowed only when listed for the refrigerant and installed with the manufacturer’s jaws. They are not an excuse to skip nitrogen on the brazed joints in the same circuit.

Welding with filler rods is the steel-pipe item: clean per 1108.3.5, weld with an approved filler, typically oxy-acetylene or process-welded grooved joints on large mains (Section 25.3). Copper ACR is still brazed. Welding a steel compressor discharge stub to a steel main is filler-rod work; landing copper on that stub is BAg, not a steel rod on copper.

Florida HVAC scenario

A Class A shop in Broward silver-brazes a 1-3/8-inch copper suction onto a steel service valve on a 20-ton (Class A) condensing unit. The apprentice grabs BCuP-5 because it flowed beautifully on the copper elbows, skips flux, and brazes in open air. The steel joint looks shiny and then cracks on start-up vibration; the copper elbows are full of black oxide. Correct method: BAg plus flux on the copper-to-steel valve, BCuP on copper-to-copper, 1108.3.1 nitrogen pre-purge and 1–3 psi hold, then the Section 25.4 pressure test. A Class B helper on a 3-ton change-out in the same county who soft-solders the 3/8-inch liquid with 95/5 “because 1108.7 lists soldered joints” still fails 1108.3.3 — R-410A is not a ≤200 psi A1 solder job.

Traps: (1) Brazing without nitrogen. (2) BCuP on steel. (3) 50/50 or 95/5 on R-410A. (4) Flaring hard-drawn tube, or taping a flare. (5) Swaging past Table 1107.5.1 and calling the extra depth “more bond.”