I need some help. I just bought a townhouse. My shower is at best luke warm. But I can still get very hot water out of the sink faucets in other parts of the home. If I leave the sink running on Cold, the shower will get hot.
Sounds like a Hydro Nick was there cross connecting your hot and cold lines and also took a branch off your H/W return.
What you should try is let the hot water run go down to your H/W tank and feel the discharge line and feel if it is getting hotter, If not then you have a great idea it is off a circulation (return line)
Then make sure the check valve on the recirculation line is functioning. Good luck