Are you sure it is a pressure relief valve and not a back flow preventer which is also designed to vent water.
I don't know if your boiler is steam or hot water, but for hot water applications many (all?) codes require a back flow preventer in the incoming water line. I don't know the requirements for steam boilers, but I would expect its similar.
If you don't already have a back flow preventer, you may want to consider adding one instead of a simple check valve.