I have good water pressure, but in my shower on the third floor (the only shower in the house) I have start with hot water, but after a few minutes I've got lukewarm water coming out when the tap is on full-on hot. I figured it was my hot water heater, so I called the guy out to look at it. (it's rented) and he says it's my regulator valve, not the hot water heater. Does this sound right? I'm a single mom with limited finances and I want to make sure that I'm repairing the correct thing before I call a plumber.

Also, my furnace runs on hot water, and it hasn't been blowing hot air so much anymore. Could all this be related?

Also... (I know... lol) My water softener resin tank (?) has a crack in it, and it's been off for about a year now, would this effect anything?

Thanks in advance!
:)

Here is the part that he said needed replaced. My dad bypassed it I think, but I've still got the same problem.