cold hot water radiators(oxymoron)

damon booz

i went to the junk yard and picked up a bunch of radiators for hot water. the boiler was origially for baseboard---anyway, i converted it and put in about 12 radiators. they work fine when i have a couple of them shut off but when i run them all at the same time one or two always seem kinda cold. my water pressure gauge reads 20 psi most of the time and my incoming water supply is a factory set 12 psi. should i raise it?