Went out on a no heat call on a boiler last week and found the most amazing thing on the return line from the basement circuit.
Yes that is a Pressure Reducing valve on a curcuit return line. And that wasn't the only problem, the bladder in the extrol tank was shot and full of water. The only way to feed water to the boiler was to do it manually. And all the piping where the extrol tank was done with some sort of thread locking compound which was almost impossible to break. Ended up having to cut some of the pipe to get the old tank out, and then repipe it for the new tank.
Couple of the finished pics