Pipe suddenly won’t supply steam

That bottom picture is with the valve open and steam in the main?

Valve part fell down the tube? Not unprecedented. Maybe stick a shop-vac nozzle into the valve (valve wide open)?

As an iPhone programmer, it pains me to see people take a screenshot of the photos app and share the screenshot instead of sharing the original photo itself!

Isn't this one-pipe ?

Will do, then. See ya.

Thanks @Msteam1 for the explanation. I did also offer some attempted insight which was "Isn't this one-pipe?"

I think I maybe see a trap in your photo roll on that screenshot, but I don't see one on that seemingly one-pipe radiator. Are there traps and also 1-pipe radiators on this system?

If it will supply under vacuum, I'll bet there's a pool of water there. You said it's hot up to the first elbow? Riser next to it is working fine?

Upload the picture looking into the valve.

The runout is that piece of pipe off the main that connects to the radiator. It should have pitc toward the main. The fact that there are shims under the radiator indicates that someone was trying to solve some sort of pitch problem at some point.

The valve should have a washer of some sort on it, frequently leather. That may have rotted and fallen off or the screw or nut came off and it is now resting on the seat. I said try turning off the other radiator because the boiler may not have enough capacity to fill all the radiators or for some reason that section of main doesn't have enough capacity for both(like some trapped condensate is collapsing some of the steam). What if you open the valve after is has run a good long half hour or so cycle, is there steam there then?

it is possible that somehow some debris got in there and is forming a dam that is holding water in the runout if all of it has pitch toward the main, pouring a couple gallons of water through it could potentially dislodge that. if this radiator didn't heat with the others because the trap is bad or the return is holding water or got heated from a bad trap elsewhere i could see where all the other radiators combined could be consuming all of the steam the boiler produces. a better test would be to see what happens with the valve open with a relatively cool system.

the runout could have dropped when you disconnected the radiator too

3 choices take your pick

1 Take the supply valve off the pipe. Get a piece of clear poly tubing at the big box and cut one end on a 45 and stick the other end in your shop vac hose and tape it tight and try to work it all the way back to the main

2 Dump water down the pipe and see how fast it drains. But if you wash debris into the main you may just move the problem to a different location. Or it might spread out the debris so it will not be a problem

3 Cut the pipe off between the 45 and the main and remove or snake or flush out the debris and put it back together with 2 shorter nipples and a union. But you have asbestos to deal with

Open up the trap side and see if you get steam out of the return. you can have a failed F&T allowing steam in the return and trapping air unless the returns drop into a wet return.

aldso have you checked the boiler to make sure its not short cycling?