Just in the past week my husband and I started noticing a decline in the hot water in our shower/bath. It needs to be turned almost all the way up by the time the shower is over, but it starts hot. We get scalding hot water everywhere else in our house, with no decline in the hot water there. Our dishwasher also continues to run at a very high temperature. Can someone PLEASE tell me what could be causing this? Do we need to replace our water heater since it was something that just started recently? Thank you in advance for any help!

Renee

A broken or deteriorated dip tub in the water heater is my first thought. The dip tube is a plastic tube at the cold water input connection that takes incoming cold water to the bottom of the tank. Hot water is taken from the top of the tank. If the dip tube is broken the incoming cold water simply moves across the top of the tank and goes out the hot water side.
You have probably turned up the temperature settings in an effort to get more hot water, so you get extremely hot water when you initially open a faucet. As soon as the cold water starts coming it mixes with the hot and you get warm water. The really hot water staying in the bottom of the tank.

Dip tubes are easily replaced but you may want to engage a plumber..

When is last time you flushed you water heater?

My wife and I had a similar issue recently, and sure enough, it WAS the "dip-tube". We replaced the water heater and all was well. .

Be prepared to clear out the faucet aerator (screens) if you replace the water heater.

---From answers on here and some research, I found that Dip-Tubes go bad if the Water Heater isn't taken care of properly (you need to flush the once in a while). Our water heater was from 1995, which supposedly had a "shotty" dip-tube. It crumbled away into small pieces and clogged out faucets.