I have a similar problem with the hot water. The hot water starts out hot and with plenty of presure. During a shower, you constantly have to turn the single knob toward the hot because the water turns cold. By the end of the shower, the knob is turned all the way to hot and the water is luc warm. The shower duration is not very long. As far as a bath, the hot water runs out before you fill the tub. The water heater is around 6-7 years old and has a 40 gallon capacity. The shower valve has a single valve. I appreciate any advice you can give. Thanks.