How can I find (and test) my steam system main vent?

Finished basements are the worst. Your questions are good ones…lots of times the vents are closed off behind drywall.

Can you see any parts of your main(s)? (The mains are the big steam pipe(s) that run horizonally around your basement at the ceiling)

Your video is better than most we see from homeowners, however could you post still pictures of the boiler from all sides. Also show us the typical radiator that shows both ends of it.

Just an old guy here who likes to looks at still photos.

It looks to me like that main starts in the boiler room, loops around the basement and re-enters the boiler room where it drops into the return. The pipes look to be the same size as they leave and re-enter the boiler room.

Check the last riser off the main before it drops to the return. That looks like 3/4" pipe and I'll bet there is a main vent screwed into it, just above ceiling level.

@CoachBoilermaker , it's the last tee with a pipe going up into the ceiling before the main drops to the floor. You might be able to shine a light up in there, or you might have to make the opening a bit bigger.

First tee is closest to the boiler steam supply — it's the radiator supply that is reached first by the steam. Last is at the other side of the main (the end of it). It is reached last by the steam.

The main drops to the floor via a vertical pipe at some point to let the water return to the boiler, which presumably is near the floor. The picture you posted isn't really applicable.

From :17 to :21 in your second video you are looking from bottom to top of the vertical "drop" that marks the end of one of your mains.

Your video isn't that helpful, it's probably not your fault because so much is behind walls and ceilings. But the majority of your video seems to be aimed at sewage and water lines.

You might have a vent on this little riser, seen at :24 in your second video:

Here at :28 I believe is your last riser to a radiator, at least on this main. I can't tell if you have more than one main.

The end of the main would drop and connect into the bottom of the boiler.

It may drop to the floor and come up to a Tee called the Hartford Loop……or might just go into the lower tap on the boiler.

This is why still pictures are better than a video.

Don't worry about 0:28, he was just saying that's the last tee that actually serves a radiator.

Now, @CoachBoilermaker , next step is to measure the length of the main as it runs around the basement. I think the pipe size is 2-inch, the outside diameter would be a bit bigger than that.

You can buy Inspection canmera's for $20 or so that have long 3/8" flexible probes and a screen that are designed to inspect inside of wall cavities and even piping. They have LED lighting so you can see what's inside dark places.

One of these would save a lot of work compared to tearing a ceiling apart.

Bob

There could just be long runouts from the main to the radiators in the addition too depending on how everything is arranged.

Yes it looks like a Gorton #1. Yes it might pre-date the sheetrock.

You can tell if it's properly letting air pass by seeing if the pipe leading to it gets steam hot shortly after the main below it gets steam hot

Then you might be able tell if it's properly stopping steam by carefully reaching up there and seeing if it is letting hot steam out.

Alternately, you could put a pipe wrench on the 3/4" riser pipe and remove it. Then blow into the pipe with the vent upright. It should pass air. Then flip it upside down (vent at the bottom of the pipe) and blow into the pipe. The vent should block air.

The only way to properly test any vent is to blow cold air through it in its operational position, then have steam go through at under 2 psi to see if it closes.

Steam reaches C, then goes up B, main vent closes. We are all good to this point.

Does B get full? Yes, B gets full of steam.

Won't more steam from C now reach A since B is full of steam (and vent closed)?

No because there is currently air in A all the way to the wet return/boiler where it ties in. The air won't let the steam go by. And the air will stay in there because there is nothing, no vent, to let it out.

The reason the air gets to C and then B is because the vent at the top of B is letting the air escape (that is the vent's whole reason to exist in fact).

Steam does flow kind of like water, but more like air since they are both gases. Steam flows from high pressure to low pressure, and at B there is high pressure so the steam won't flow there.