My experience with PVC enclosures is that an RHP alone is sufficient. I started off with both an RHP and a UTH, both regulated by a Herpstat 2, but eventually found that the UTH never went on. The RHP was doing all the work and created a perfect hot spot on the surface of the substrate.
I live in South Eastern Mass, so the ambient temp in my house can get down as low as 65 F in winter. In fact, just did a few days ago. The PVC enclosures had no issue. There was a slight drop in temp, but it seems to have been temporary because when I looked everything was normal.
Those RHPs from Pro-Panel are excellent. You're going to love it.
I wish I could do tubs with just a UTH, but like I said, my house gets too cold.
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