I have two showers with the same trouble. After a few minutes the water gradually gets colder so you have to keep turning left to get hotter water. I changed both stems but still have the same trouble.

Do you have electric or gas water heater?

John

Gas with 40 gal tank about 10 years old.

See the image below and go look at your gas valve and tell me where is it set at, Thanks.

John

My has setting "normal" and A B C. I've been on the c setting for the last 5 years. I think it is 140 F

How is the weather, and after how long you have to move the handle to left to give you hot water?

Look at chart below, Thanks.

John

TEMPERATURE SETTINGS
HOT - Is a thermostat setting of approximately 120° F,
Which will supply hot water at the most?
Economical temperatures..
A - Is a thermostat setting of approximately 130° F.
B - Is a thermostat setting of approximately 140° F.
C - Is a thermostat setting of approximately 150° F.
VERY HOT - Is a thermostat setting of approximately 160° F.
It is recommended that the dial be set lower
When ever possible.

I think you should also take a look at your dip tube located directly below the cold inlet. If the tube is broken or deteriorated the cold water gets mixed wuith the hot as you draw off hot water.

I have been thinking that the dip tube could be the problem but I can't imagine what could happen to a copper tube inside the tank. Is this a common problem? Plus when I turn to the hot side it stays hot for a few minutes then cools down. The cold water is very cold at this time of year.