Hi, APL:

Your assurances of "balance" raise some questions for me.
If the numbers you are citing now are the same ones you had when you were using your SWCG, then that may be one of the sources of your problems.
Ben's knowledge of pump HP and flow rates is far beyond mine so I'm just going to look at the chemistry. It may well be you are simply blasting too much water through your heaters and that is the cause of failure. I don't know.

Back to the chems.
FC: 1.5
CYA: 20
TA: 85

I'm not the SWCG expert but I do know that every manufacturer recommends a relatively CYA level of something like 70-90ppm
At those levels, FC should be no less than 5% (so CYA=80, FC >= 4ppm).
Also, a rising pH can mean your TA is too high. For years we were taught TA should be 90-120, with some scales going from 80-125. But we've been finding out experientially that this is not necessarily true. For example: All this season my TA has been a startlingly low level of 60-70ppm. WELL below "standard". Yet my pH has been MORE stable this season than it ever has been. What seems to be the REAL case is that pools with rising pH tend to be "happier" with lower TA levels. In fact, for SWCG pools, our resident experts ALWAYS recommend lower TA levels. Still, also it's well-known that many SWCG owners face continual rising pH.

What also concerns me (and I may WELL be off-track here) is that your continual acid feed may not dilute properly by the time it reaches the heater...if so, the problem is pretty straight forward. Have you ever considered testing water (somehow) just before it enters the heater to see if its pH is low?

So, I'm guessing you have a number of potential causes of your regular heater failure.

These are just some thoughts.

Carl