P1,T1 = P2,T2
With no idea of the safe working pressure of the boiler - AND - no pressure gauge, it would be pure insanity to increase the temperature by any means.
Does the boiler have a pressure relief system? If so what is it rated at. That may add a modicum of safety to this foolish endeavor.
I suggest you talk to the manufacturer or leave it all alone while everyone is still alive and un-scalded.
Watch this... 500-feet into the air.