Heat exchanger leaking after 4 months, thi...

I could be wrong, but you do have a chemical problem. Be it PH or chlor. After 35 years in the business I have seen one heat exchanger that came from the factory bad, and none that wore out with out chemical help. If you don't have a PH problem then you have a chlor problem. This is most often at an inline chlorinator that is mounted to close to the heater and lets chlor gas back up into the heater when off or in front of the heater so all the feed is going straight to the heater. This is the more lodgical of the two as fast as your wearing the exchangers out. Green plaster is a big simpton of this as the copper from the heater has to go somewhere. This can be fixed as easy as a chloriene floater or replumb to move chlorinator. Good luck