If you are getting luke-warm water then maybe the bypass is not your problem.
If you have an exterior shower, check to make sure both the hot and cold water valve are turned off and the shower head is not the valve that is turned off. Same for the interior shower.
Common problem is that people turn the shower off by using the valve on the shower head and then forget to turn off the hand valves. This allows hot and cold water to mix in the shwer valve which dilutes the temperature of the water.