I am at wits end on how to fix this problem that it seems to be very common. I have hot water everywhere in th house except in the bathtub. I looked for shut off valves behind the wall and there isn't any. I have a Moen fixture that seems to be welded to the pipes and I don't know how to replace it without tearing up the bathroom. Am I missing something? Can someone please point me into the right direction before I spend hundreds of dollars for a plumber?

Shut off the main supply line entering the house and open a faucet on the lower level, this stops all water and drains the pipes. Your Moen is soldered, not welded, but it does not need to come out. The Moen has a cartridge which can be removed fairly easily, cartrdiges are free for life from Moen but they usually can just be cleaned/ cleared of the debris and reused. This link should help Installation Help / Animated Tutorials for Moen Faucet click on the blue word [I]animation[I] or just read the steps. If you need more help just click back on this link and ask away, there are several plumbers and handymen here