Mystery: No hot water from single-handle Moen shower valve
I recently started fixing up a house that was remodeled about 8 years ago, and both the tub and shower use the same single-handle Moen mixing valve. You turn the shower or tub faucet on by moving the lever counterclockwise, and the farther you go, the more hot water you get.
The tub's valve works fine, but there's no hot water at all coming into the shower regardless of the lever's position. I've taken them both apart, and they appear to be the same in all respects.
I now have two questions:
1) Is it likely that the shower valve is so misadjusted that it's not letting in any hot water? Is it even possible to screw it up that badly?
2) How can I replace the valve? There are threaded connectors going into the valve itself, but it appears that they were soldered on to copper pipe with no means for unscrewing them short of cutting or desoldering the pipe. Am I overlooking something obvious or is it time to open up the wall and start cutting?
I've tried calling plumbers in on this, but everybody is too busy with heating failures right now.
Thanks in advance for any help you can provide.