How can i fix my boiler powermax 155x?

I had no hot water so i checked my boiler and the pressure
had dropped to 0,so i turned the pressure up to 1 thinking that would fix it.The burner kicked in immediately but the pressure started to rise as well.Once it reached 3.5 i turned it off and the pressure began to drop slowly but when i turned it on again it did the same thing.If i turn the hot water setting off the pressure stops rising