Since 1981One stop for all your HVAC&R needs
GUIDES / BRAZING
How to braze copper pipe for AC and refrigeration.
A brazed joint has to hold refrigerant at high pressure for years. Here is how to make one that does not leak: the right rod for the metals, a clean and close fit, dry nitrogen inside the pipe, even heat, and a pressure test before the vacuum.
By Saurabh Jain, Refrigeration Centre · ·
What is brazing, and why not solder?
Brazing joins metals with a filler that melts above 450 °C but below the melting point of the parts being joined. The American Welding Society uses 450 °C (840 °F) as the line between brazing and soldering. The molten filler is drawn into the gap between the tube and the fitting by capillary action, so a brazed joint is only as good as the fit and the cleaning behind it.
Soft solder melts below 450 °C. It is used on some water plumbing, but not on refrigerant circuits, where joints carry high pressure — especially with R32 and R410A — along with vibration and constant temperature swings. Copper joints in AC and refrigeration systems are brazed.
Which brazing rod should I use?
Choose the rod by the two metals being joined:
| Joint | Brazing rod | Flux |
|---|---|---|
| Copper to copper | Copper-phosphorus (BCuP), 0–15% silver | Not needed on copper |
| Copper to brass (valve bodies, brass fittings) | Copper-phosphorus with a higher silver content, or a silver brazing alloy (BAg) | Needed |
| Copper to steel, and steel parts | Silver brazing alloy (BAg) | Needed |
| Steel, cast iron and mixed-metal repairs | Brass brazing rod | Needed |
On copper, the phosphorus in a copper-phosphorus rod acts as its own flux, so copper-to-copper joints need none. That self-fluxing works on copper only: any joint that involves brass or steel needs flux. Phosphorus-bearing rods must not be used on steel or iron at all, because the phosphorus reacts with the iron and leaves a brittle joint.
More silver lowers the melting point, helps the filler flow and gives a more ductile joint that copes better with vibration. Plain copper-phosphorus rod, with no silver, is the economical choice for joints that will not see much vibration or shock.
Refrigeration Centre stocks copper, brass and silver brazing rods, brazing flux and butane cans for portable torches — see consumables and tapes.
How do I prepare the joint?
Most brazing faults start before the torch is lit. Prepare every joint the same way:
- Cut square with a tube cutter, not a hacksaw, which leaves an uneven end and filings in the pipe.
- Deburr the inside and outside of the cut, holding the open end downwards so the swarf falls out rather than into the pipe.
- Clean the outside of the tube end and the inside of the fitting or socket to bright metal with an abrasive pad or cloth, and keep oil and fingers off the cleaned surfaces.
- Fit the joint into a copper fitting or a socket made with a swaging tool, pushed fully home and square. Capillary action needs a close fit: for copper-phosphorus rod the gap between tube and socket should be about 0.05–0.13 mm, which a correctly sized fitting or swage gives.
- Support the pipe so the joint cannot move while it is hot, and cap any open ends you are not working on.
Where the joint needs flux, brush a thin coat onto the cleaned tube end just before fitting it, and keep flux out of the bore.
Why purge with nitrogen, and how?
Hot copper reacts with the oxygen in air. Inside the pipe this forms a black copper-oxide scale that later flakes off and travels round the system, where it can block filter driers, capillary tubes and expansion valves and contaminate the compressor oil. Flowing dry nitrogen through the pipe while brazing keeps the air out, so the bore stays clean.
- Connect a nitrogen cylinder with a regulator to the pipe, usually through a service port or an open end, and leave an outlet at the far end so the gas can escape.
- Purge the air out with a brisk flow first, then turn it down to a gentle flow before you heat: roughly 1–2.5 litres a minute (2–5 cubic feet an hour), just enough to feel at the outlet. Too much flow cools the joint, and pressure in the pipe can push the filler out.
- Keep the nitrogen flowing while you heat and braze, and until the joint has cooled.
Use only dry, oxygen-free nitrogen. Never use oxygen, which can ignite oil residue in the pipe, or compressed air, which carries moisture into the system.
How do I heat the joint and feed the rod?
Use a flame big enough for the pipe size. A portable butane torch suits small sizes and repair work; larger diameters need more heat, such as an air-acetylene or oxy-fuel torch, so that the whole joint reaches temperature at once.
- Heat the tube first, a short way from the fitting, then move the flame onto the fitting so the heat soaks through the whole joint. Keep the flame moving around the pipe so it heats evenly.
- When the copper reaches brazing temperature — a dull cherry red for copper-phosphorus rod — touch the rod to the edge of the joint on the side away from the flame. The hot joint, not the flame, should melt the rod.
- Capillary action draws the molten filler into the gap and towards the heat. Feed rod until a continuous fillet shows all the way round the mouth of the fitting, then take the heat away. Extra rod does not make a stronger joint; it runs inside the pipe.
- Do not move the joint until the filler has set.
Overheating is as harmful as underheating: it burns off flux, oxidises the surface, thins the tube wall and can leave the joint porous. If the filler balls up and will not flow, the joint is not hot enough or not clean enough.
What do I do after brazing?
Let the joint cool naturally, with the nitrogen still flowing. Where flux was used, wash the residue off with warm water and a brush once the joint has cooled: flux left on the pipe attracts moisture and corrodes it. Then look at the joint. The fillet should be continuous all the way round, with no pinholes, gaps or cracks.
How do I protect valves and nearby parts?
Service valves, ball valves, expansion valves, sensors and anything with a seal or a plastic part can be damaged by heat travelling along the pipe. Wrap them in a wet cloth or use a heat-absorbing paste, point the flame away from them, and follow the maker’s instructions: some valves must have internal parts removed, or must be open, while they are brazed. Shield nearby wiring, insulation, walls and panels from the flame.
How do I braze safely?
- Never braze a pipe that still holds refrigerant. Recover the refrigerant first and purge the pipe with nitrogen. R32 is mildly flammable (class A2L), hydrocarbons such as R290 and R600a are flammable, and many refrigerants break down into toxic, corrosive gases in a flame.
- Work with good ventilation and keep your face out of the fumes.
- Wear shaded brazing goggles and gloves, and avoid synthetic clothing.
- Keep a fire extinguisher at hand, clear away anything that can burn, and check the area again after you finish.
- Choose cadmium-free silver rods; cadmium fumes are highly toxic.
- Close the torch and the nitrogen cylinder when you finish, and store gas cans away from heat.
Flare joint or brazed joint?
A flare joint is mechanical: the tube end is flared and clamped with a brass flare nut, so it can be undone. Split ACs use flare joints at the service valves of the indoor and outdoor units. A brazed joint is permanent, stronger and less likely to leak, so it is used to join pipe runs, fittings and VRF branches.
For R32 and R410A, make flares with a flaring tool and flare nuts suited to those refrigerants, and tighten them to the torque in the installation manual. Flares that are under- or over-tightened are a common source of leaks.
How do I pressure test and evacuate the piping?
- Pressurise the piping with dry nitrogen to the test pressure in the outdoor unit’s installation manual, and no higher.
- Check every joint with leak-detection fluid or soap solution, and leave the system under pressure for the time the manual gives to confirm the pressure holds. Allow for temperature: the pressure drops as the pipe cools.
- Release the nitrogen and evacuate the piping with a vacuum pump and gauges to the level the manual specifies, commonly 500 microns (about 67 Pa) or lower.
- Isolate the pump and check that the vacuum holds before you open the service valves.
What are the common brazing faults?
| What you see | Likely cause | What to do |
|---|---|---|
| Filler balls up and will not flow | Joint not hot enough, or surfaces not clean | Clean to bright metal and heat the whole joint, not the rod |
| Filler runs inside the pipe or drips off | Uneven heat, too much rod, or too wide a gap | Heat evenly, stop feeding once the fillet is complete, check the fit |
| Black scale inside the pipe | No nitrogen purge, or the flow stopped too soon | Purge with nitrogen and keep it flowing until the joint cools |
| Pinholes or a porous fillet | Overheating, dirt, or nitrogen pressure in the pipe | Less heat, a gentler nitrogen flow and better cleaning |
| Leak in the pressure test | A gap in the fillet, or the joint moved while setting | Release the pressure and remake the joint |
| Cracked joint on steel | Copper-phosphorus rod used on steel | Remake it with silver brazing alloy and flux |
What do I need to braze copper pipe?
- A tube cutter and deburrer, and a swaging tool for tube-to-tube joints
- Copper fittings to suit the job
- Brazing rods to suit the metals, and flux for brass or steel joints
- A torch and gas, such as butane cans for a portable torch
- Dry nitrogen with a regulator
- A wet cloth or heat paste, a heat shield and a fire extinguisher
- A manifold gauge set and a vacuum pump for testing and evacuation
Refrigeration Centre stocks tube cutters, swaging and flaring tools, torches, manifold gauge sets and vacuum pumps (see tools), copper fittings, and brazing rods, flux and butane cans (see consumables). Send a part list and we will confirm stock and the trade price on the call.
Sources: brazing and soldering definitions from the American Welding Society’s Brazing and Soldering Manufacturers Committee; nitrogen purge and brazing flow from HVAC School; copper-phosphorus rod use, flux and joint clearance from brazing-alloy makers’ product data (Lucas-Milhaupt, Harris). Test pressures, evacuation levels and flare torques come from the equipment maker’s installation manual.