Problems with heat exchangers are common, typically for any of three reasons:
1. Failure to maintain overall pool chemistry -- mainly, pH > 7.0 .
2. Adding acids via the skimmer. (Even though the OVERALL chemistry is OK, this can damage heaters)
3. Trichlor in skimmer, or in feeder upstream of heater, or (with pump timer) NEAR the heater, even though downstream.
Trichlor damage to heaters is VERY common, but usually occurs only on pool with pump timers. It is good general practice to set timers to have TWO 'on' periods, no matter what the total hours.
Good luck!
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