Boiler pressure troubles
This all started a couple of nights ago when I started hearing some air bubbling type noises from one of my radiators. The pressure gauge on the boiler was reading zero, and on closer inspection I discovered that the PRV had been, and currently was leaking. All this time, I still had good heat in the house. I checked the pressure in the expansion tank, and it also read zero. I isolated the expansion tank, drained it, and pressurized it to 12 psi, and ran the system for a while. The expansion tank held pressure fine. Meanwhile, I was still getting intermittent leaking from the PRV and inconsistent pressure on the gauge. By this point I had a couple of theories as to what was wrong, but the only one that didn't involve multiple failures was simply a PRV leaking at inconsistent (but low) pressures. I purchased and installed a new PRV, which resulted in no leakage (at least initially). The cold system pressure, however, was still very low, though it would build as the water heated. Today when I came home from work I discovered that the PRV had discharged again, and the system pressure was ~25. I shut it down, and drained the expansion tank and removed it. The diaphragm is definitely intact, and it definitely hold air. So I put that back. Next, I isolated the feed line with the pressure reducer and backflow preventer from the system. There is a pressure relief valve directly after the pressure reducer on that line, and when I manually open that valve, nothing comes out, even when opening the purge on the pressure reducer. I then pressurized the system through the boiler drain using a hose, to 12 psi, and I'm currently running it as a closed system. It seems to be stable now at 170 degrees and 24 psi. That pressure seems a little bit on the high side, but like I say so far it is stable and no leakage. So, I'm guessing either the back flow preventer or the pressure reducing valve is plugged up. Are either of these repairable items, or should I just figure out which one is causing the problem and replace? Anything missing in my methodology?
UPDATE: Running as a closed loop it got to 180 degrees and 26 psi and the PRV started to discharge again. Is a 30 psi PRV intended to have a crack pressure lower than 30 psi, and full flow at 30? Or is my pressure gauge reading 6 psi low?