Turn off the heater NOW.......
Sounds like the heater is bypassed and empty..
The valves are behind the heater Look on the outside of the rig and see where the heater panel is then go inside and and look for access inside.
You should see three valves, two in the blue and red lines, turn them on them one should be on a line that connects the blue and red lines, turn that off
Fill the heater by opening a faucet. THEN turn the heater on
__________________
|