When I am showering, if someone else in the house flushes the toilet, I scald, or someone turns on the dishwasher or washing machine, the shower water turns cold. How can I be assured of hot water in the shower even if someone flushes, or uses the sink or whatever??

Sounds like you have a volume problem. How old is your home and what material are your pipes? Copper, plastic or galvanized?
Let me know, Tom

Permco...

How many handles do you have at the shower valve... one, two or three? Let us know... OK? If more than one handle involved you may have an older fashioned valve that is not designed to handle volume fluctuations... much like Tom (speedball) suggested.

Answer his questions and then answer mine.

Thank you...

MARK