Is tub faucet cartridge failing or HW heater

[AKRBT]

I have a 1.5 year old HW heater, a home warranty replacement, it is set to run pretty hot (not scalding).. Over the last six months we have to turn our tub valve more and more to the hot side just to get comfortable shower temps. Sometimes it starts hot but gets cooler as we shower, and you have to turn it more towards the hot side. The tub valve appears to be an old cartridge style (I have not actually removed the face plate yet) from maybe the mid 1980s. We don't seem to have a problem getting decent hot water from our kitchen sink faucet. It is a small ranch house with copper piping, and no visible leaks in the finished tall crawlspace.
Do you think it is the cartridge or the relatively new hot water heater?
Thank you

[Cass]

If you have a temp / pressure balancing shower valve...that could be the problem not the water heater...you need a plumber to check it out...this time of year the incoming water temp is higher than the winter and if anything you should be adjusting the valve to use less hot water...

[Thatguy]

Measure the temp. of the water coming out of the WH drain. That's as hot as it gets.
Incoming should be 40F to 70F, in the summer closer to 70F.