Main Vent Size

bondo · April 13

I have recently had my steam boiler replaced because the (old) boiler split between the sections and was leaking steam internally. Based on reading information here, I think my main vent is undersized, and would like some opinions.

Heating system information:

New boiler: Weil-McLain SGO-3
One pipe system
Low pressure system
Low water cutoff: probe type CycleGard Model CG450-1560
Tankless water heater

There are two main loops. They are sketched in the attached image (layout.png). The black rectangle is the boiler location. There is a single vertical pipe out of the furnace with a horizontal header leading to the equalizer (these are not shown in the sketch). The main vents start with vertical pipes up from the header. The horizontal pipes in the loops are shown in the sketch, with red for the 1.5 inch pipe and blue for the 1 inch pipe. The approximate locations of the take-offs for the radiators are indicated with black squares. The two return lines join together just before they reach the boiler. The location of the main vent is indicated with a green square. It is mounted off a tee at the top of the vertical pipe that drops down to return the condensate.

Combined, the two loops have just under 92 feet of 1.5 inch pipe and just under 40 feet of 1 inch pipe. I calculate the volume to be just over 1.5 cubit feet for these pipes. The current main vent is a Vent Rite 35. I am not sure of the condition of this vent. I do not think it was replaced with the new boiler installation. Based on the 1oz values from "Balancing Steam Systems" by Gill and Pajek, the Vent Rite 35 has a rate of 0.110 cubic feet per minute, which gives a vent time of about 14 minutes. This seems longer than desirable.

I also have connected some sensors to my primary control and pipes and was able to generate the attached graph (pipe_response.png). It shows the temperature measured on the outside of the header pipe (placed directly against the iron) and on the outside of the return pipe just before the main vent location. The graph also shows when the oil valve is open (effectively when the burner is firing) as green shading. The horizontal time axis is labeled with the local time and date with HH:MM, so it starts at 23:30 on 4/9. The gap in the shading is from the CycleGard scheme for checking the water level. I think the drop in the header temperature is just a sensor hiccup.

From this graph, I can see that the main header starts heating after about 2 minutes after the burner starts and very quickly reaches full temperature. This is starting from a "warm" state, since the boiler also provides hot water and does not get fully cold unless I turn it off for a while. The vent end of the pipe does not see a temperature increase until after about 7ish minutes and takes about 12 or 13 minutes to reach maximum temperature, or about 20 minutes after the burner starts. So this seems reasonably close to my calculated vent time of about 14 minutes.

Based on all of that, and thanks for sticking with me if you got this far, I think I should increase the vent to something like a Gorton #1. Does that seem reasonable?

Steamhead · April 13

You say the steam mains are 92 feet in total. I would use at least one Gorton #2. But………..

Do the dry (overhead) return lines join before they drop below the waterline? If so, that needs to be changed. The return lines should drop below the boiler's waterline so steam cannot jump from one to the other, and connect to each other after that.

The best place for main vents is at the end of the steam mains, just before they connect to the dry returns. No reason to fill the dry returns with steam. But if that is impossible, putting them at the ends of the dry returns just before they drop is a second choice. How long is each steam main before it connects to its dry return?

EBEBRATT-Ed · April 13

Each main should have its own vent and the returns should be vented up high. Each return shouls drop below the boiler water line before they tie together.

bondo · April 13

The two loops are about the same size, so each one has about 45 feet of 1.5 inch pipe before the dry return and then the dry return on each one is about 20 feet of 1 inch pipe. The dry returns do indeed join together before dropping below the boiler water line. The vent is up high, right before the vertical drop.

It would be difficult to add vents before the dry returns on the mains. Probably the best I can do is separate the ends of the dry returns and add a vent to each just before it drops.

bondo · April 13

As a side note, probably the first little bump in the temperature of the pipe near the vent is because of the two dry returns joining together. That little bump is not a fluke and happens in basically every heating cycle.

Jamie Hall · April 13

https://forum.heatinghelp.com/discussion/comment/1852140#Comment_1852140

And that is the best thing to do. From the standpoint of even, fast heating it doesn't matter where the main vents are, so long as they are after the last radiator runout. But it is essential in a system piped as yours is that the two "dry returns" do NOT join before they drop below the water line.

hat little bump is, most likely, the steam from one of the two "dry returns" hitting the vent area — and closing it, preventing the other "dry return" and associated main from venting at all.

Danny Scully · April 13

”That” “wasn’t” “aggressive” “at” “all” @Jamie Hall 🤣😉

Steamhead · April 13

@bondo , repipe the returns so they drop separately. Add a Gorton #2 before each drop. Watch how much better and faster it works.

bondo · April 13

Thanks so much for your comments. I am going to look into getting these changes made.

Just as a bit of history, this pipe arrangement has been here as long as I have owned the building (10 years), and the recent boiler replacement did not change it. I also do not really have any issues with the heating in my house. I have mostly been using this replacement as an excuse to get to understand my system better.

Is the current configuration likely to cause any long term harm to the boiler or the system in general? Basically, should I make this a high priority to fix?

ethicalpaul · April 13

No damage, it will just not necessarily distribute the steam as well as you'd like.

I'm surprised no one mentioned that you have mains that consist of 1-1/2 and 1" pipe. That is pretty weird.

Steamhead · April 13

First thing to do is change the main vent. Use at least one Gorton #2.

Then, see if the steam fills both mains all the way to the end before the Gorton closes. If it does, you might get away with the current configuration…………………..

bondo · April 14

They did seem kind of small compared to various systems I have read about, but this system is heating a fairly small area (about 1400 square feet). The building is actually a two unit with the upper and lower floors laid out the same. They have separate boilers piping, and the configuration of the piping for the other unit (the second floor) is almost identical.

109A_5 · April 14

If you are curious, with the present main venting, time from when the boiler starts steaming (header gets hot) to when the end of both (each) of the the 1-1/2" mains gets hot. Then install the better vent and/or split the 1" at the vertical drop to the wet return and install two vents, then re-time the steam propagation.

To me, the less time you are burning fuel to push air out of the system the better.

Are your mains insulated ? The less you need to reheat the mains with every cycle the better too, unless you want the heat in the area where the mains run through.

JUGHNE · April 15

Paul, I believe that the mains are 1 1/2" and the returns are the 1" he speaks of.

ethicalpaul · April 15

Thanks, that makes sense. I hope someone suggested he move the vents to just past the last radiator runout on each main.

JUGHNE · April 15

He has only about 20' of dry return(AKA steam main extension).

And if he cracks the piping open where the returns tee together then that would be the easiest location for main vents.

One would be hard pressed into calculating the heat loss on that 20' of 1" pipe.

Somehow I can't imagine the steam flow just stopping at the air vent anyway, it has to coast on a little ways, IMO.

Also having the air vents at the boiler allow you to check their operation and any dripping, if it occurred, would be noticed.

Can we see pictures of where the end of mains becomes the returns?

ethicalpaul · April 15

I am not concerned about the heat loss on that 20' of 1" pipe. As some of you may recall, I'm not even that concerned about the heat loss on 40' of 2" pipe.

But it is the waste of time heating it up every call for heat that I am opposed to. I believe @Long Beach Ed convinced me of that. See I listen sometimes!

I do hear you about having the main vents in a central location. I have a friend's boiler with that setup and it's pretty good.

I'm not sure how steam can coast a little ways past unmoving air…it might swirl around a bit with the air right there at the tee…I agree it's immaterial anyway.

bondo · April 15

The mains are insulated (and so are the dry returns). The insulation on these pipes is all old and some has been wrapped for containment, so there is no good view of the transition. From what I can tell, it just seems like there is a reducer going from 1.5 inch pipe to 1 inch pipe, and this happens at some point after the last radiator take off.

So the biggest things about trying to add vents after the last radiator is that it would require messing with the old insulation. The pipes next to the boiler are not insulated, so making changes there is much easier.

Main Vent Size

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