I finally got around to bleeding my radiators today. Two had a little bit of air (30-60 seconds to bleed). 6 hours later, I was in the basement and heard the relief valve blow.
Last year, draining half the expansion tank cleared a slow drip from the relief valve. But today, it didn't stop the slow steady stream coming out of the relief valve. So I shut off boiler, the feeder valve, boiler in and out valves and still got water coming out. I assume the only place it could come from is the expansion tank. I opened up the relief valve and it finally drained out the remaining few gallons.
I plan on checking to make sure the expansion tank is empty in the morning and then turning on the system. Is there a particular order to opening the valves? Would you bother seeing if the valve is still leaking first or just replace it?