It doesn't matter how old the UTH is, you still need a tstat monitoring it. A lot of times it's not so much about how hot they are while they're working as it is how hot they can get when they quit working. A lot of UTHs will fail-on, so they just keep getting hotter when something goes wrong. Just use the search function on here to see what can happen if you don't have a tstat regulating a UTH.