Apparently this is going to be an increasingly sensitive fail point as our rigs age.
Maintaining complicated systems with aging or non-existent parts requires some reengineering challenges
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I am familiar with a couple of rebuild techniques to maintain the heat exchanger (search the forum for Aqua Hot Hacks) to avoid an $8K replacement.
This seems like a good opportunity for someone to design a solid state controller for this unit.
Certainly we should be able to do better than $900 for old parts, and have a unit with increased reliability and maybe improved functionality.
Given the current state of electronic fixes available, from programmable logic chips, raspberry pi computers and solid state relays, I would think that a sufficient number of presales for a redesigned unit might encourage and inspire folks with the expertise to design and align themselves with a builder and knock out a few copies of an Aqua Hot controller.
There are a lot of Aqua Hot units installed out there, the customer base should be substantial enough to make it worthwhile.
I would sign up for one.