One of our showers has plagued us with lukewarm water since we moved in. At first I didn't really notice since it was still fall and really didn't feel like I had to turn the heat up to 11, but once November rolled around, we realized we were in a bind.
The good news is that my Dad, whose a Master Plumber, is only a phone call away. The bad news is he lives about 1,300 miles south of me…so we either have to call a plumber, and then relay messages to my Dad, or give it a go on our own.
The tricky thing with this is: most plumbers won't do this for you, so you might be on your own (even if you don't want to be!). The reason is that they might turn it up too much for you, you get scalded, they get a lawsuit. So rather than playing the “how litigious do these folks look?” game, many plumbers simply opt out of it altogether.
So here's how I did it, but make sure you adjust it to your own preferences (and, obviously, don't sue me if it gets too hot. Test it before you jump in!)