The Pressure Relief Valve on a Little Giant Water Heater is a safety measure in place to prevent unexpected and dangerous high pressure from rupturing the expensive copper heater coils. These valves come preset from the manufacturer to bypass water that is pressurized beyond 1000 PSI.
Little Giant Heater Coils on standard pressure models are pressure tested at 900 PSI at the factory. Little Giant does not recommend adjusting the pressure relief valve, they are preset for a reason. It is a good maintenance procedure to check that the set screw in the pressure relief valve is not lose so that the valve will not back out and allow for increased pressure to the system.
The pressure relief valve is essential for safe operation of Little Giant Water Heaters. Replace immediately if pressures exceed 1000 PSI and the relief valve does not open.