The confusing aspect is the 13# pressure reading on the gauge. If you have a hot water coil in the boiler, it could have a pinhole leak, but the gauge might be bad, as well.

Not as familiar with this setup. What I feel confident about is that you haven't had two defective PRV valves. With old valves, you sometimes do get crud between the seat and the washer when you open it that allows it to continue to leak a little. Usually popping it a couple of times flushes it out and solves the problem. But if you were getting dripping before and you have it again now, I would strongly suspect that you are near the point of the valve opening because of temp or pressure. The answer is to actually measure these things and not guess. You need a recording pressure gauge (that shows peak pressure) I've seen these reasonably priced at Lowes and Home Depot. Do this first since the problem is usually pressure related. If that doesn't show up what you expect, then you need to get a recording of the peak temps. That will be a little harder to do, but you need to do it. My Fluke mult-meter will record temp and Min-Max readings. Perhaps you could borrow one.

22 pounds is to high. 12 is more like it. They probably increased the system pressure to purge the system and forgot to reset the pressure reducing valve. The relief valve as it is called probably lifted off its seat because you were close to or at its release setting which is usually 30 pounds. Use the drain on the boiler to reduce the pressure to 12 pounds and make sure the reducing valve isn't feeding anymore water into the system after that. If it is the valve has to be adjusted. And no you cannot put anything at all, including another relief valve, anywhere on the first valve, that could be very dangerous. To change the relief valve all you have to do is isolate the boiler from the piping on the supply and return, then drain the boiler down to below the point where the valve is installed and screw in a new one. Hopefully whoever installed the system originally was nice and put valves in for you.

One of my pet peeve's has been people who post on forums seeking assistance, asking for help, but then, once they find the answers necessary to solve their problem, don't bother reporting the solution leaving the string the dangling! It’s as though they don't realize that a public forum is not solely for their use or benefit but also (arguably more so) to serve others that will be searching for the same information about the same issue, possibly, for many years to come. I realize we’re all more motivated to write when pressured by those problems that need solving but, considering the time spent and efforts made by others kind enough to generously offer their assistance, taking a few minutes to post a solution once found is the least one can do. I didn't get back to this as soon as I would've liked to but I’ll not be one of those people so back to my story and its exciting conclusion. They've recently been replacing the main gas lines in our area. I didn't immediately see any connection but it seemed too much of a coincidence that these previously nonexistent leaks and drips should randomly begin to occur very near the same time that construction began. I talked to one of the guys which lead to talking to some of the other guys which eventually lead to speaking with the guy in the water department. What I learned is that the waterline in this area is, more or less, a loop supplied on both ends. In the event that, while excavating to install the gas lines, some sort of damage is done to the water line that necessitates an emergency shut off, they shut off the supply on one in advance so that, if needed, they could immediately stop it by shutting off the other side. This apparently caused an increase in the water pressure of the loop. He (the guy from the water department) opened up the other end and our pressure here dropped from 148 down to 120, apparently still considerably higher than normal or is desirable, but low enough to make a difference. All leaks have stopped. So, to answer my own original questions, higher than what is considered normal water pressure —but not necessarily in excess of its stated psi rating —will or can cause flow from a pressure relief valve. Also (I was previously unaware of this) other fittings, valves etc. throughout the house plumbing are only designed to withstand up to a certain pressure. As for me, I suppose I still need to install a pressure reducing valve but I don’t need to install one today. In the meantime, I may be able to pressure wash the house without a pressure washer!