So Michael, if I understand your post, you are demonstrating that the expansion tank should be installed vertically and at least 18" from the cold water inlet. Both of which this install complies with, however, this install does not provide adequate support for the tank. According to the install instructions the tank should have been spliced into the horizontal line going to the water heater so that it would be supported on both sides.

Are you in congruence that the install is incorrect/insufficiently supported as shown in my picture?

Thanks for the .pdf

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