Service valves, king valves, reversing valves, and expansion valves all have components that can be damaged if excessive heat is applied when brazing is performed. Teflon components and rubber gaskets are commonly damaged. There are several things that can be done to help reduce the chance of harming components when brazing.
Photos are of a reversing valve slide that was damaged due to excessive heat during installation. Reversing valve would not shift.
Safety First! Use proper PPE. You will be working with sharp objects, hot metal, and open flame.
Before brazing, inspect joints and copper to confirm they are clean. Clean metal is essential for a strong braze joint. Brazing alloy will not adhere to oil, oxide, or dirty metal. Before brazing, a final cleaning should be done with a brush, sanding cloth, or a cleaning abrasive pad. Trying to braze with dirty metal will be very difficult. Many times, a quality joint is not possible no matter how much alloy one uses.
Make sure that the joint and fittings are not too loose. Additional alloy and heat will be needed to fill in any gaps in loose fittings. Filling in gaps can be very difficult. The potential of leaks and damage to the valve is much higher than normal.
When brazing any joint within the refrigerant system, never use soft solder or flux. Soft solder will melt at a lower temperature and will not require as much heat to make the joint but is not designed to withstand the pressures seen with a 410a refrigerant system. Soldering is a joining process in which the filler metal melts completely below 450C (840F), whereas brazing is a joining process in which the filler metal melts completely at temperatures above 450°C (840°F).
5% silver is the minimum when soldering copper to copper and 15% silver when copper to brass. When brazing aluminum to aluminum or aluminum to copper, a low temperature alloy solder with a noncorrosive flux is recommended and are designed especially for aluminum. There are many different brands to choose from.
Wrapping the valve body of any component with a wet rag, thermal barrier gel, or thermal compound before applying any heat is one of the best and easiest ways to prevent damage. If able, remove Schrader cores and caps. When these are damaged, they are very difficult to remove. Take extra care not to get any water or gel in the valve openings. Wrap valve body and solder one joint at a time. When brazing is complete, replace Schrader cores and caps.
The most common joint in HVAC is copper to copper. Start by heating the tube that goes into the joint. Heating the tube first will conduct heat into the fitting making brazing easier. Alternately working the heat to the tube and joint until both reach brazing temperature. Apply brazing alloy drawing it into and around the joint. Brazing alloy will flow or be drawn to the hottest part of the metal. If the joint is not hot enough, brazing alloy can clump. Keep alternating heat between tube and joint unit, maintaining temperature and adding alloy until the entire joint is filled. There is no need to apply extra brazing alloy to a joint once it is filled. Doing so only adds extra heat to a joint and potential damage to components. After brazing, allow solder to solidify before cooling. If cooled before solidifying, the alloy will crack.
Remember to flow dry nitrogen through the refrigerant system when brazing to prevent copper oxide from forming inside refrigerant system.
There are many different types of torches used in the HVAC industry. Each have their advantages and disadvantages. Practicing with each is the only way to tell which torch is the best for you.
Take the extra steps and protect the vital components when brazing. Practice joints and get comfortable with the torch. It will save you time and money.