There could be several reasons why the technician recommended replacing the outside heat exchange coil instead of just fixing the brazed joint in your central AC system. Here are a few possibilities:
Severity of the leak: If the coolant leak from the brazed joint is extensive or if there are multiple leaks, it may be more practical and cost-effective to replace the coil rather than attempting to fix each individual leak. A compromised coil could potentially develop more leaks in the future, so replacing it ensures a longer-lasting solution.
Coil damage: The technician might have determined that the heat exchange coil itself has sustained damage or corrosion, making it less reliable or inefficient. In such cases, replacing the coil is a more comprehensive solution to restore the optimal functioning of your AC system.
Warranty coverage: Depending on the warranty terms for your AC unit, it may be more financially beneficial to replace the entire coil rather than attempting a repair. If the warranty covers the coil replacement but not repairs, it would make sense for the technician to recommend a full replacement.
System compatibility: The technician may have considered the compatibility and performance of the existing coil with the new compressor. If the components are mismatched or the coil is outdated, it might be more practical to replace the coil to ensure optimal compatibility and efficiency.
It's important to discuss these reasons with the technician who assessed your AC system to gain a clear understanding of their recommendation. They should be able to provide you with specific details about why they believe replacing the coil is the best course of action in your particular situation.