Generally on a boiler you will have a combination pressure reducing valve and pressure relief. Bell and Gossett and Watts are two common manuf. The prv is set in that 15-18# range. It is there to protect the system from house water pressures in the 40-60+ # range. The combi system I am describing is installed on the cold water feed from the house to the boiler. If you can find that line there is likely a valve that is shut off. Open that valve slowly and let some water into the system. Do not exceed the 18# range. Monitor the pressure for a few days to see if it has stabilized. Boilers are not a diy deal in my opinion, so don't be impetuous here! Call the pro if you need him.

thanks for the good advice, I will check it out.