How many PSI should my oil hot water furnace been when hot and cold?

I have a Peerless hot water boiler, it's about 8 years old. It has a max psi rating of 30 lbs. PSI. When the boiler would kick on, the water pressure would rise and water would gush out of the relief valve on the back. I adjusted the water pressure regulator (Looks like a bell) to where when hot, it's at 30 lbs. but when cold it's at 0 PSI.PSi Is that right. Now, when it kicks on, the water pressure gets to 30 and that's it. Could the expansion tank be bad. Should it be 30 PSI when cold? HELP!