Hi, my name is Mike and I have been in the home improvement and repair field for 20 years. I will be very glad to help you today. When you turn on the hot water the diaphragm for the change over valve could be worn or there is a problem with your diverter. Most likely the diaphragm though. When you turn on the hot water you should see the pin in the middle of the diaphragm assembly fully push out. Does it seem to do that?