Bypass is part of the water system.
Cold water inlet valve on bottom of tank......S/B OPEN to allow water into water heater tank
Bypass valve connects cold water line with WH tank hot outlet line....S/B CLOSED so cold water doesn't flow into hot water plumbing lines
Hot water outlet valve (IF you have one...lots don't just have check valve) S/B OPEN
Electric element turned on w/o water in tank......burnt element will need replaced (they are immersed in water which keeps them from burning out)
Propane.....fired up then shut down immediately. Wasn't because of no water in WH tank.
Water heater tank would need to reach 130*F/140*F before normal t-stat would shut down propane....that would take several minutes with tank empty
Immediately shutting down is usually due to 'flame proof' failure.
Flame needs to engulf spark electrode when lit....it then sends signal to circuit board that flame is lit and keeps gas valve open until temp set point is reached then t-stat opens and gas valve closes.
Some of usual issues with flame failure
Electrode out of position
Bad ground...loose/dirty ground screw
Loose/dirty spark wire connections
Cracked ceramic on electrode