Safety valve

After having installed a boiler, a safety valve of 1.5 bar pressure has to be screwed into the clutch which is on the back part of the boiler at the top . This pressure safety valve which is mounted in the upper flow pipe near the boiler, let easily off steam without allowing exceed the pressure and allowing to the boiler to replenish with the water from the system through the return pipe and to cool. The boiler can crack up because of overpressure if the installed pressure safety valve has a higher pressure, is located in the wrong place or is clogged