I have a tub/shower. In it the tub faucet turn on is separate from the shower controls. The tub has plenty of hot water. When using the shower there is water how ever we are lucky to get hot water. Sometimes the water is hot and other times it is not and will not become hot. What is the problem.

To help we will need the brand name of the valve. One handle or three knobs? Back to you. Regards, Tom

I wonder if this is a grohe.