Heater Failures

[PoolDoc]

Do you have a bypass on the heaters? If not, the failures may well be due to high velocity erosive flow from your 3HP motor. I'd have to see the piping layout (photos!!) to confirm that.

(Why do you have a 3HP pump on a 33,000 gallon pool?)

Also, make sure your acid feed is downstream of the heater. However, since you only added it last year, it is probably not the cause.

Shock with plain 6% household bleach -- it has the fewest side effects. If you have pool bleach AKA liquid chlorine in your area, it may require less handling. However, the pool bleach stocked in this area and wholesaled by SCP is stored for long periods at high temps, so it's probably degraded to household bleach by the time users get it. Store bleach is much more likely to be at the stated %.