One of the worst things that can happen when you are taking a shower is when someone flushes a toilet or kicks on the dishwasher and the shower water turn scalding or ice cold. It can lead to second and third degree burns and with children and the elderly it could mean a trip to the hospital. So, you have two options; 1. Redo your entire plumbing and put in pex lines that are not connected like traditional plumbing but allows you to have dedicated lines to each item that needs water. Or 2. You can install a thermostatic shower valve. This valve is hands down the most reliable way to obtain constant water temperature.