[Cajundaddy]

Heat from an older gas heater. When your system shuts down with the heater running, water in the heat exchanger is boiling and migrating back through the plumbing and thermo-forming the pipe. Add a fireman's switch to your timer that will shut the heater off 30 minutes before system shutdown. Always shut down your heater first and allow a 20 minute cool-down period before system shut down.

Modern pool heaters don't retain heat in the exchanger to the same degree so when you upgrade you will no longer have this problem.