[Excaliber]

Over the years I've tried a couple of different "special" t-stats. Nothing has worked better than a plain old ordinary run of the mill t-stat. I don't get it, don't have a clue why that is. I just go NAPA or whatever and get one off the shelf, done.

If it doesn't open until 200 and it's rated 180 then clearly it is defective. I'd say your problem is solved.