Question is why the plumber got
a reading of 115. I hope is reading was wrong.
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Frankly, I think the guy is either a flake or a fraud.
Water heater never expanded, to claim they did is ridiculous.
I doubt your water pressure was ever 115psi.

Personally I would not want 90 psi, but then I don't have to pay a plumber
to put in a valve.