Rheem Water Heater Pressure Relief Valve "system lock mean" Questions

system - is there a lock on via controls that is set to lock out under 20'F - no matter what if boiler is working properly it should be able to maintain 20-30'F temp difference from inlet to outlet ex

...system but keeps falling over after a few showers. There must be some sort of internal trip that senses the air lock to protect pump - suspect this is faulty - anybody any ideas where this is/what I ...

...system at the proper pressure, too high will lock-up the valve. Step #2 Check proper polarity of "NEUT" and "120" on the ignition control by reversing the wires, if reversed gas valve will not open. ...