quote; the local winnelson backed up the plumber's plumbing-
That is like saying you went to Home Depot and the guy in the aisle said it was done correctly. Do you REALLY believe that someone working the Winnelson counter actually worked as a plumber and then "downgraded"?
Licensed residential and commercial plumber
I don't believe it's in the code book that the check valve is "required" on the recirc, but if you leave it out, the cold water goes backwards up the line.
You know that now too. And I'm guessing your plumber is putting that new knowledge on his check list.
We install the check valve, not because it's code, but because it's a better way. I use a spring check, as they make less noise while the pump is running than using a swing check.