Does anyone know at what pressure a steam boiler should be set?
Don't play with steam boilers; get a pro to check out the entire system and write down what you need to know to operate the system. It's cheap insurance.
What does the user's manual from the manufacturer say?
That's where to set it, after you are sure you've done all the maintenance it needs.
I've had two steam systems, and the settings were different.
I cannot find the users manual but have figured out the it should be @ 2 psi