water coming out of a steam radiator vent?

Well… it would be tempting to say the vent was bad — they do fail — but there are so many other possible causes.

How's the near boiler piping? Any water getting up into the mains would be a problem, particularly since this is the nearest radiator to the boiler.

Is the boiler surging?

What pressure is this operating at?

Can condensate accumulate in the runout to the radiator?

Is the radiator inlet valve fully open?

For starters…

For starters, check the pressuretrol on the boiler and that the pigtail isn't clogged. The pressure must be set below 2 lbs.

Then check the radiator vent. What is the size of that vent? It should be 4,5,6,C or D. If all else looks okay, replace it with a #4 Gorton or a #40 Hoffman vent.

And look over all the pipes running to that radiator ascertaining that all have a significant pitch toward the boiler, and none have sags or missing hangers that can cause them to collect water.

There are a few inadequacies in that boiler piping which may also contribute to this problem. Is the water level stable?

That is also a fairly new radiator. I assume that it has never done this before?

Do you "set back" the room temperature at night?

Both pigtails should be checked. It's part of regular maintenance. If you have steel ones and they do commonly clog; brass, less so. When they are clogged, the boiler does not shut off on pressure, so on an extended run, the radiators can accumulate condensate with higher-than-ideal pressure holding it up in the radiator.

Step 1 is to remove the bath mat so it does not absorb the water when it leaks. With that done, you might be able to trace the leak back to its origin better.

Condensate trapped somewhere…

If either radiator accumulates condensate, something is wrong with the piping or the trap, if it has one.

Paul and Jamie, I completely agree. Unfortunately problems often only present themselves when a certain threshold is crossed. Sometimes that threshold is an unusually long boiler run, or an exceptionally cold radiator or system.

On a mild or more typical day, an inferior system may putter along merrily. Thank you guys, for your notes.

Copied post from @Fred to me 10 years ago:

Inside the Pressuretrol, right below the micro switch, there is a pivot arm. At the end of that arm you will see a screw pin that is activated by the diaphragm at the bottom of the Pressuretrol. If you look very carefully at that screw pin, you will see it actually has a tiny (I mean tiny) hex head on it. It takes a .050 hex wrench and you can turn it clockwise (Towards the bottom of the Pressuretrol to decrease the Cut-out pressure or counter clockwise to increase the cut-out pressure (which none of us want to do but who knows, your Pressuretrol may be really screwed up!). Turn the power to the unit off first. You may find the first attempt to turn that screw a little bit stubborn (relatively speaking) because it has some Locktite on it but it does turn. Don't turn too much, a fraction of a turn goes a long way towards getting it adjusted where you want it (maybe 1/32 inch turn to start with). You may need to play with it to get it exactly where you want cut out to be.

Pressuretrols are not very accurate. This is a method of fine tuning. MAy still not be possible to get all the way to a 1.5psi cut-out and 0.5 cut-in. Will depend upon your unit. AFter each incremental change you make watch a couple cycles because the cut-in may get too low and prevent the boiler from turning back on in which case you will have to back off your adjustment slightly. This is also pointless if you don't have a 0-3psi or 0-5psi gage. Suggest you get one if you don't have one already.

Or, you can get a vaporstat.

No. It won't come into play at all unless the lower-set one fails. You can put it lower if you like but I assume it has a manual reset button so you don't want it to cut out casually.

I will defer to one of the pros to advise you on the vaporstat 009 vs 017 vs 033.