Remember the old style 2 Handle Showers? You could take a shower, but you did so with the fear the someone would flush a toilet and thus scald you.
Those valves would lose all of the cold water pressure once the toilet was flushed. A pressure balanced valve, which is now code required, eliminates this problem.
The pressure balanced shower valve is designed to compensate for loss of pressure on either the cold side or the hot side. The only drawback to this type of valve is that you do lose water pressure when this balancing occurs.
So, for large showers and custom showers, you would be better served by installing a thermostatic shower valve.
