5% vs. 15% Silver Solder (Brazing Rod)
First, let's get straight that BRAZING is when you use a filler rod that isn't the exact same material as the base metal but that melts ABOVE 840°F. Soldering is the same but at temperatures below 840°F.
With HVAC rods melting at around 1200°F it confuses me why we usually call it “silver solder” but we will often also call it brazing rod. The best term to call it a “brazing alloy” and I try to remember it but I often find myself calling it silver solder.
The most common rods used for typical HVAC brazing are 0%, 5% and 15% with several other levels mixed in there.
The percentage is the percentage of silver content in the rod. The only real reason to use lower silver levels is the cost and the difference can have many techs and owners wondering what the difference is.
So the big question is –
Is more silver worth the price?
First, let's establish that we are talking about copper to copper applications here because that is the most common use for these rods. In copper to copper none of these phos/silver/copper rods need flux or even benefit from it. The phosphorus allows the rod to self-flux on copper and flux when overused can get in the system and cause more harm than good. Flux is required when joining brass to copper using 15%, just make sure not to use too much flux, a thin layer on the male side of the tubing only is all you need.
The silver increases the “ductility” of the filler and allows it to flow at a slightly lower temperature. This results in a better flow of solder into the joint and a lower odds of cracking with thermal expansion and contraction or with vibration. The increased silver also allows the solder to remain strong when filling slightly larger gaps due to ill-fitting copper.
Have you ever seen a leak in a discharge line fitting that you SWEAR wasn't leaking when the compressor was installed? This can be attributed to poor brazing practices (failing to pull solder into the joint) and often you will find that 0% or 5% rod was used.
The reason we went to all 15% rod is due to the costs of callbacks and refrigerant. With labor prices and refrigerant prices increasing and technician brazing skills on the decline, we want to give techs the best possible chance of making a connection that will stand up to temperature changes and vibration, especially when we are doing an install or making an expensive repair.
If you are going to use the less expensive rod make sure it won't be in a location with a lot of vibration, that you get the fit between the tubing and the fitting REALLY tight and that more heat is used to “draw” the solder into the joint.
The biggest mistake new techs make is just “capping” the edge rather than pulling the solder into the joint for a solid bond.
PS – We are big fans of Solderweld products including the round 15% Sil-Sol rods. You can find out more at productsbypros.com