Can you "over vent" your steam main?

tkos115

I have been thinking about adding another big mouth main vent to my main. I currently have 1 which I added this year which has helped out a lot. I was thinking of maybe adding 1 more on to it with a tee. Is it possible to over vent a steam main though?

My main is about 57ft. long off the head pipe to the end where the main vent is, and about 2.5in diameter.

Thanks for your help!

Fred

You really can't over vent the main. You can reach a point, however, where there is little to no additional benefit to adding more vents. One Big Mouth vent on your main is probably enough. A second one probably won't add huge additional benefit.

tkos115

Thats what I figured. It seems pretty good right now. A little under 10min. from when the boiler kicks on until it reaches the end of the main. (And thats with the boiler not firing up for at least a day for heat).

ethicalpaul

You can watch the big mouth during the start of a cycle to see how fast the steam escapes out of it. If it looks like a whistle with air blasting out of it then maybe it's near capacity and you could benefit from another one.

If the air just gently puffs out as the steam advances, you are probably fine.

Fred

ethicalpaul said:

You can watch the big mouth during the start of a cycle to see how fast the steam escapes out of it. If it looks like a whistle with air blasting out of it then maybe it's near capacity and you could benefit from another one.

If the air just gently puffs out as the steam advances, you are probably fine.

@ethicalpaul , I don't think that is a very good gauge for adding more vents. In reality, what you are describing is really a factor of how fast the steam is moving down the pipe to the vent. On a cold start, it will be slower as more steam condenses, heating the pipe along the way. On a warm/hot start, steam will move faster pushing air out faster, creating more velocity at the vent but the volume of air is no greater than the pipe will hold.

ChrisJ

Fred said:

ethicalpaul said:

You can watch the big mouth during the start of a cycle to see how fast the steam escapes out of it. If it looks like a whistle with air blasting out of it then maybe it's near capacity and you could benefit from another one.

If the air just gently puffs out as the steam advances, you are probably fine.

@ethicalpaul , I don't think that is a very good gauge for adding more vents. In reality, what you are describing is really a factor of how fast the steam is moving down the pipe to the vent. On a cold start, it will be slower as more steam condenses, heating the pipe along the way. On a warm/hot start, steam will move faster pushing air out faster, creating more velocity at the vent but the volume of air is no greater than the pipe will hold.

Ok,
So use Paul's way, only do it when the pipes are hot.
Fire the boiler, get steam to the radiators then shut it down for 5 minutes and do it again, this time listening to the main vents.

Fred

ChrisJ said:

Fred said:

ethicalpaul said:

You can watch the big mouth during the start of a cycle to see how fast the steam escapes out of it. If it looks like a whistle with air blasting out of it then maybe it's near capacity and you could benefit from another one.

If the air just gently puffs out as the steam advances, you are probably fine.

@ethicalpaul , I don't think that is a very good gauge for adding more vents. In reality, what you are describing is really a factor of how fast the steam is moving down the pipe to the vent. On a cold start, it will be slower as more steam condenses, heating the pipe along the way. On a warm/hot start, steam will move faster pushing air out faster, creating more velocity at the vent but the volume of air is no greater than the pipe will hold.

Ok,
So use Paul's way, only do it when the pipes are hot.
Fire the boiler, get steam to the radiators then shut it down for 5 minutes and do it again, this time listening to the main vents.

I suppose, if you want to make a project out of it. Depending on the main vent you use, some may not even be open in five minutes. The tried and true way is to calculate the volume of air that needs to be expelled and size the venting accordingly.

ethicalpaul

I'm not sure how that could be more "tried" than actually trying it but OK

I'm looking forward to seeing if my venting goes from "OK" to "insufficient" after I finally insulate my mains this fall after I note some numbers with them still "naked" for comparison.

Another way to "try" it is to see how long steam takes to get to your vent. Then, with the same amount of time passing since the last firing as the first timing (whether it's 5 minutes or 50 minutes), take the vent off and fire it up again, again noting the time. If it's roughly the same time, you have enough venting. Feel free to correct me @Fred

PS: I always want to make a project out of it

tkos115

I may crunch some numbers to see if it will be worth adding another vent. However I do know that when the boiler starts up and the steam starts traveling through the vents that there isn't very much "blow through" by any means. It doesn't seem like its too restricted but hey, if I can improve it some why not?

I also insulated some of my mains and near boiler piping that were missing insulation. I can't say for sure right now if it is going to make a huge difference in terms of money saving but it did seem to help a little bit with the steam reaching the main vent. It seemed to be a bit faster. Not a huge difference but enough that I could notice it.

acwagner

@tkos115 eventually your main venting capacity will be limited by the tapping size that the vents are mounted on. For two big mouths you need 3/4" or bigger. Or multiple taps.

tkos115

I was wondering how many can fit on a 3/4in tap. Currently that's what mine is, just a single 3/4in tap on the main. I can't imagine that I would need more than 2 big mouths. Whereas 1 may be enough anyway.

ChrisJ

Do the test @ethicalpaul recommended and do it with the piping hot.

It'll answer your questions.

acwagner

There was a PDF for sale by Gerry Gill on this site that was a table of venting capacities basically for every radiator and main vent ever made....I can't locate it in the "Store" section anymore.

A 3/4" tapping is maxed out with 2.6 Big Mouths or 4.3 Gorton #2 according to his chart.

tkos115

That would be a nice piece of info to have on hand. Maybe it can be located elsewhere. Probably not a bad piece to have for the average DIYer with steam.

Fred

tkos115 said:

That would be a nice piece of info to have on hand. Maybe it can be located elsewhere. Probably not a bad piece to have for the average DIYer with steam.

That document, by Gerry Gill was in the store and sold for $10.00 which was donated to a charity. It is now free and I think it is in the library, on this site. The document is called: "Balancing Steam Systems"

acwagner

@Fred you are right! Here's the document:

https://heatinghelp.com/assets/documents/Balancing-Steam-Systems-Using-a-Vent-Capacity-Chart-1.pdf

SteamingatMohawk

Last winter I took on "fixing" my steam heat in a converted 2 family near Schenectady, NY. The document mentioned is tremendously valuable.

Among the problems, some of which I created by not using the proper radiator vents, I learned about the true capacities of the different manufacturers' vents. Later this fall I will update my threads, but one really remarkable one is that Vari valves, while I liked them, did not completely shut off venting when shut. I ended up with other valves so I could better control the venting and what a difference.

Another thing that is an eye opener is that a pound of water flashed to steam at atmospheric pressure expands from 0.017 cubic feet/lb(rounded) to 26.8 cubic feet per lb or roughly 1600 times the volume. My system only needs about 3 ounces of water to fill the system with steam. Of course, this does not take into account how long it takes for the steam to remain as steam when heating up the piping and radiators. Bottom line it isn't much at all.

If you are interested, do a quick calculation of how long it takes for your boiler to put enough energy in to make enough steam to fill the piping (again ignoring the heat losses and condensation while the system heats up and the energy to heat up the boiler and water in it). Just look at it simply as a blob of water being heated by the capability of the boiler.

Sorry for the rambling comments.

Jamie Hall

@SteamingatMohawk 's comment implies a very good point: it's worth taking a few moments to figure out just how fast your particular boiler can, in fact, make steam. There's no point in adding venting capacity beyond that to the mains...

mikeg2015

tkos115 said:

I was wondering how many can fit on a 3/4in tap. Currently that's what mine is, just a single 3/4in tap on the main. I can't imagine that I would need more than 2 big mouths. Whereas 1 may be enough anyway.

^^^^ This. On my formerly vapor vacuum, it just has a single 1/2” tap. It’s my limiting factor. Going to “Cheat” and probably just drill and Tap another 1/2” on the 2-1/2” elbow drop to the wet return. Drill it 45 degrees and then place the vent as high as possible.

Jamie Hall

For what it's worth -- if anything -- the system Cedric (very roughly 330,000 BTUh output) powers has 3 3 inch steam mains, each roughly 70 feet long. The whole thing is vented by an old Hoffman 76 and a Gorton#2, and it's quite adequate...

ChrisJ

> @Jamie Hall said:
> For what it's worth -- if anything -- the system Cedric (very roughly 330,000 BTUh output) powers has 3 3 inch steam mains, each roughly 70 feet long. The whole thing is vented by an old Hoffman 76 and a Gorton#2, and it's quite adequate...

I have a 30' 2" main with five Gorton #1s. I also have a modified Big Mouth I've been meaning to add for a few years.

nicholas bonham-carter

This is where the low pressure gauge comes in handy: measure the back-pressure as the air is being pushed out as steam is rising.
If the pressure is above 2 ounces, I would say add more main vents.—NBC