A higher pressure relief valve may be required when you are pumping to a higher elevation so you can maintain pressure at the high point (pressure will drop 0.43#/foot of elevation gain). You need pressure at the high point to prevent the water from potentially boiling or flashing to steam. You'd have to read the specs on the 30# unit...it might support a higher value, but it's only safe when you actually need it, otherwise, if there were a fault, the water temp could get much hotter. A 30# relief valve works for most typical homes, and if not needed, the boiler with the 50# valve could be changed to a 30# one with no loss in function.
