Collapsing steam in main radiator slow to heat

now i want to know what swear word or slur was used to describe an intentionally out of square thread to provide an offset of a pipe to a fitting.

in the pictures the main looks like it has downward pitch. i would check that with a level and look for reducers that make a lip that keeps the condensate from draining. if i don't find any of that I would take off that vent tree and put a hose thread adapter there and hook up a hose there and flush that main back to the boiler in case there is some mound of sediment that has somehow ended up in that main.

i also winder if there is a piece of teflon tape stretched over the end of one of those fittings making a dam in the pipe.

It seems you are missing the point…

" It’s something else. "

" I just opened up that valve I added by main vent and dumped out about a quart of water in a bucket which seemed just about all that was in the main and then that radiator heated right up fast and as quiet. "

You have a huge puddle of water at the end of your steam main. Where water is steam is not. Probably a pipe pitch issue. It needs to drain back to the boiler, continuously. There are various ways to correct this issue, but until you do the puddle will just come back.

If your are getting significant water at the Orange arrow when you open the valve (should be almost none), the Main at the Red arrow is flooded (it should not be), preventing normal steam flow. There is a pipe pitch issue.

What @mattmia and @109A_5 said.

If the piping at the boiler was causing the issue you would be getting pounding and banging.

You will always have some water (condensate) in the system pipe when the boiler is on.

When the boiler is off the pipes have to be pitched so every drop of water (within reason) drains back to the boiler and the returns below the water line.

The condensate also has to be able to drain while the boiler is running so it does not accumulate in the pipe.

You system for whatever reason is accumulating water in the pipe at the end of the main.

1. Something is not pitched right. Weather it never was, or something changed a pipe hanger dropped, the piping sagged none of that matters now. It is what it is.
2. You can find the pitch problem with a level or a laser. Or you can pipe a drip to the end of the main

I would not remove any insulation until you find the bad pitch. You can check the pipe size/look for the reducer by taking a piece of wire (coat hanger) and poke the pipe through the insulation. Measure the od of the insulation, not the circumference poke it with the coat hanger till you hit the pipe. That gives you the insulation thickness

Deduct twice the thickness from the od and that measurement will be the approx. pipe OD

1 1/4 is about 1 1/2" od

1 1/2" is about 2" od

2" is 2 3/8" od

You might be able to take the nipple out at the end of the main between the 90 and the 45 and put one in 1/2" shorter will give you some pitch back. You would need about 3/4" between the top of the pipe and the joist to do this.

If the pitch can't be easily fixed then I would get some 3/4" copper tubing and run a return back to the boiler or into another lower return

-- drop header at boiler MUST be pitched towards equalizer

-- main pipes on your system MUST be pitched towards boiler

if the above factors are checked, you might have the issue i had: one of my rada made the loud sounds of steam collapsing into water. the culpirt turned out to be a supply pipe very close to an uninsulated wall and a heavy draft around it. the section of the pipe never had a chance, it was constantly cooled by the outside air. this caused the steam insude to constantly collapse. generous insulation through the inspection hole solved this problem.

Some two pipe systems have vents on the steam mains, @ChrisJ . Some don't — or don't seem to. The ones that don't, however, have another means for venting air: crossover traps. These are just like radiator traps (often are, in fact!) but set above the steam main and related dry return — and allow air to go from the steam main to the dry return (let's not talk about F&Ts… different application and principle) and the dry return takes the air back to the main vents at the boiler.

Now water is another problem. Any main carrying steam will have condensation in it, and that has to drain by gravity somewhere. The problem is that if it can't drain, or can't drain adequately, it will severely slow the passage of steam by the puddle, since until that whole puddle is up to steam temperature steam will just condense and add to the puddle, rather than going by.

(For the mathematically inclined, the condensate volume is somewhere around a half cup of water per square foot of EDR per hour).

Now if you take a nice long main and put an elbow going up at the end of it, and the end of it is even as little as half a pipe diameter low rather than high enough to drain back to the boiler, the condensate is going to make it to that elbow and… sit there. And the amount of condensate on startup is quite enough to block the steam flow (worse, in our OP's case, it also tends to block air flow to those lovely vents, so not only is steam condensing to add to the puddle, it's flow is partly blocked by air…).

if that area has water I say open the unions and see how far back toward the boiler you have trapped water, that will tell you which direction of the union to look in. if the union closest the radiator has no water then that runnout and offset has something going on. if it it all the way at the first section of new pipe then look further toward the boiler.

how do you get from the existing pipe to that new 1.25" pipe?

if you find it is in the asbestos covered section then maybe you put a drip at the connection to the first section of new pipe instead of getting that abated to find out what someone did 60 years ago.

Which union did you open up, Orange or Yellow ? If you had significant water coming out at the valve near the vent and there was no water at the Orange arrow, that seems odd, unless it drains away slowly over time.

So its like this ? Is there a rag or something in the pipe between the Green and Red arrows ?

is there a drip connected to that main anywhere? the water line in that boiler looks really low so there might be something that was below the water line with the old boiler that isn't now.

if it is a capped pipe and it doesn't connect to something like a return, the pipe itself isn't a problem but how is the header arranged, can any condensate that gets in it drain somewhere?