Help Please! Old 2-pipe System

Hot_n_Cold · February 3

Replacing old boiler that burned up on failed LWCO. 2 pipe system, has an old Dunham 220 Air Eliminator, check valve, and some other stuff I'm not familiar with.
I've attached pictures.
New boiler bottom is much higher than old, I want to save as much near piping as possible. Cleaned it out.
I'm concerned that the old return piping is too low compared to the new boiler. Also concerned the air eliminator & equalizer tube setup may not be functioning properly.
Also, Dan recommends 24" from top of boiler. However, manufacturer recommends 24" from normal water line (for bottom of steam header).
What are some things I should check for on this old system? I had it hooked up but solid water was coming out of the supply header union. Gonna re-do it and raise it up. Please see pics attached

Hot_n_Cold · February 3

Also when it was running for a few mins, air eliminator was whistling pretty good, wet return got hot, yet supply wasn't really warming up. Maybe im impatient...

Steamhead · February 3

This sounds like one of the Dunham Vapor systems described in @DanHolohan 's book "The Lost Art of Steam Heating". The wet return probably got hot from the condensate coming down from the steam main. Did the radiators start to heat?

Jamie Hall · February 3

Is this already in? If not... try to match the old water line as closely as you can with the new one. So long as the dry returns and steam mains are at least 24 inches above the boiler block and the wet returns are solidly below the water line, that should be fine.

Vapourstat control, cutout not more than OUNCES per square inch.

EBEBRATT-Ed · February 3

24" from the water line is a minimum measurement it can be as high as you want.

Hot_n_Cold · February 6

System running. Air eliminator whistling very erratic. Seems like return water takes awhile to get back to boiler. Also, one of the Radiator Valves will literally start slowly steadily closing itself to about 25% closed. This happens when I fully open it while system is running. It's literally like watching a ghost slowly creepily close it.
Also, when I try to drain the boiler, it gurgles and doesn't drain at all until I open pressure relief.
Observed it for several cycles, it seems to be heating the house sufficiently. However, I want to learn more about those concerns I brought up.
Thanks in advance!

mattmia2 · February 6

Steamhead said:
This sounds like one of the Dunham Vapor systems described in @DanHolohan 's book "The Lost Art of Steam Heating". The wet return probably got hot from the condensate coming down from the steam main. Did the radiators start to heat?

if water was gushing out of the header as I think they described it was probably the hot water being thrown out of the boiler heating the returns. skimming and proper riser and header size and order and such would be the first to check as well as both headers connected if required by the manual.

I assume by the vapor valve that it is a vapor system that needs to be run at a few ounces pressure but you didn't even get that far

Hot_n_Cold · February 6

Steamhead said:

This sounds like one of the Dunham Vapor systems described in @DanHolohan 's book "The Lost Art of Steam Heating". The wet return probably got hot from the condensate coming down from the steam main. Did the radiators start to heat?

Updated photos . Dunham 220. Yes. Radiators hot. Now one is automatically closing itself off when I open it while running.

Hot_n_Cold · February 9

Yes water was gushing from supply union, header was too low & union was leaking. Fixed both problems.
All radiators heating now.
However, a day or two after leaving. Customer complains of a single loud bang upon startup. Says it sounds like it's near the unit in basement.
Also, the Radiator closest to the boiler has a steam supply valve that keeps closing itself back to 25% shut after I open it fully. Customer said it always did that.

mattmia2 · February 9

The valve that is moving on its own probably just needs to be rebuilt with new/a little more packing or "no packing".

Jamie Hall · February 9

A single loud bang upon startup -- especially if it is heard before significant steam starts out on the mains -- is probably something in the boiler or very near it expanding. However... if it's more or a boom than a bang, it may be a problem with the burner. Really kind of have to be there to get a feel for it,, never mind diagnose and fix it!

mattmia2 · February 9

Looks like it might be a bellows type packless valve, see around page 7:
https://heatinghelp.com/assets/documents/dunham.pdf

Hot_n_Cold · February 12

Update : A few mins after startup at normal water level, the air-eliminator whistles erratically, supply pipe heats up, then water level bounces up and down slightly (As I've seen in normal operation).
THEN, the air-elim (Dunham 220) whistles like crazy. The water level keeps steadily dropping until sight glass is empty. Lots of banging in pipes. At this point I cut the power to save it from damage.
When power is cut, the water returns rather quickly.
It seems to me like there is some type of Vacuum issue.
I looked through that old manual, mattmia2.
I found some things that were not installed properly.
I will list them in this post and provide the images in following comment.

1. Return Checkvalve from main return should be above boiler bottom water level, yet below hartford loop.
However, it is actually slightly lower that the boiler bottom return pipe.

2. The diagram shows a 1/2" line with a check valve & "stop-cock" connecting the top of the Supply Pipe with the top of the main-return where the air eliminator hooks into a T.
Old system didn't have that installed.
ALSO WORTH MENTIONING : Old boiler burnt up due to low-water.

3. Air Eliminator top is slightly lower than where it should be
(according to manual, the top plate should be level with center of main return pipe. It is about 1.5" lower than. Is this enough to cause a problem?)

4. Of course it needs a Vapor Stat & Gauge. However, I tried dialing the Pressuretrol down to ounces (based slightly upon guesstimation.) and it doesn't seem to make a difference.

5. How much lower can the "Scale Pocket" be vs. the Boiler Bottom or Hartford Loop? Currently 11" Below Boiler Bottom.

All help is greatly appreciated!!! Thank you for your replies & advice.

mattmia2 said:
Looks like it might be a bellows type packless valve, see around page 7:
https://heatinghelp.com/assets/documents/dunham.pdf

Jamie Hall said:
A single loud bang upon startup -- especially if it is heard before significant steam starts out on the mains -- is probably something in the boiler or very near it expanding. However... if it's more or a boom than a bang, it may be a problem with the burner. Really kind of have to be there to get a feel for it,, never mind diagnose and fix it!

EBEBRATT-Ed said:
24" from the water line is a minimum measurement it can be as high as you want.

Jamie Hall said:
Is this already in? If not... try to match the old water line as closely as you can with the new one. So long as the dry returns and steam mains are at least 24 inches above the boiler block and the wet returns are solidly below the water line, that should be fine.

Vapourstat control, cutout not more than OUNCES per square inch.

Steamhead said:
This sounds like one of the Dunham Vapor systems described in @DanHolohan 's book "The Lost Art of Steam Heating". The wet return probably got hot from the condensate coming down from the steam main. Did the radiators start to heat?

Hot_n_Cold · February 12

It shows bottom pipe from air eliminator should be 6" from normal water line. Is this a minimum or an absolute #?
Also, the check valve from supply line to return main should go which way?

KC_Jones · February 13

Your description of the water leaving the boiler and banging sounds like classic surging to me. Can you post a picture of all the near boiler piping? Has the boiler been skimmed to remove all the oils? How did you size the boiler to ensure good low pressure operation?

The Hartford loop connection is too long which could cause some banging, but I doubt it's the primary source of the issue.

Sylvain · February 13

outside of the problem:
The pipe coming down from the safety valve is threaded at the bottom.
Cut it, and preferably at 45°, to be sure nobody will put a cap on it.

mattmia2 · February 13

Are the crossover traps open so the mains can vent? If it is surging because it hasn't been skimmed properly that will throw the water out of the boiler.

If the mains can't vent then that might build enough pressure in the boiler to push the water out depending on how the check valves are arranged, the height of the returns, and how much steam it is making vs what the system can condense. If the system can't vent the air can't get out so the steam can get to the emitters and condense.

ChicagoCooperator · February 13

As a total aside and totally off the thread topic, just realized that I walk by the HQ of Dunham when I walk to my office (it was in the Fisher Building).

reggi · February 13

@Hot_n_Cold I think this might be a good thread to read, you might find some very useful information on what you're looking for....
https://forum.heatinghelp.com/discussion/139813/another-look-at-vapor-vacuum/p1

Hot_n_Cold · February 13

KC_Jones said:

Your description of the water leaving the boiler and banging sounds like classic surging to me. Can you post a picture of all the near boiler piping? Has the boiler been skimmed to remove all the oils? How did you size the boiler to ensure good low pressure operation? The Hartford loop connection is too long which could cause some banging, but I doubt it's the primary source of the issue.

Will attach photo of all near boiler piping. Single supply tap going into a header about 30" from normal water line.
Skimmed boiler, purged sediment after 1 week operation.
Customer said they monitored water level, would go down, then flood sight glass when turned off.
Measured & counted all radiators, Salesman calculated what size is necessary. Old boiler was much bigger. I think 120k btu. This one is 80k if I remember correctly.

mattmia2 said:

Are the crossover traps open so the mains can vent? If it is surging because it hasn't been skimmed properly that will throw the water out of the boiler. If the mains can't vent then that might build enough pressure in the boiler to push the water out depending on how the check valves are arranged, the height of the returns, and how much steam it is making vs what the system can condense. If the system can't vent the air can't get out so the steam can get to the emitters and condense.

I believe they are open. However, i noticed the original piping is missing g that crossover on top in the diagram. The check valve & stop cock that connect top of supply main to a T joined with air eliminator on Return Main.
I'm considering adding that in to see if that fixes it, although I don't entirely understand it's function. Can someone explain?
Also, what is a "stopcock" and what would it's purpose be?

Seems like mains are venting. Air eliminator whistles like crazy, water levels drops to nothing. Seems to be getting stuck in some type of vacuum.
I read that the air eliminator will purge all air out, and create a vacuum to store a column of water in the return line.
The A dimension is plenty high enough from what I can tell though.
I'm suspecting that missing connection between the supply main & return main has been the culprit the whole time.

Hot_n_Cold · February 13

KC_Jones said:

Your description of the water leaving the boiler and banging sounds like classic surging to me. Can you post a picture of all the near boiler piping? Has the boiler been skimmed to remove all the oils? How did you size the boiler to ensure good low pressure operation? The Hartford loop connection is too long which could cause some banging, but I doubt it's the primary source of the issue.

mattmia2 · February 13

Where does the pipe to the "inlet" of the air eliminator go?

I don't know specifically with the Durham system, but usually that sort of connection is to the steam main to inject pressure in to the returns to let the water return to the boiler if the pressure differential is too high. If you keep the pressure under 10 oz it shouldn't be a problem. Do you have a low pressure gauge that can accurately measure the pressure on the main?

A stopcock is just a plug type of valve, it opens and closes with 1/4 turn.

Hot_n_Cold · February 13

I'm considering converting this to a simple 2-pipe system without the air-eliminator & vacuum vapor operation.
Basically just bypassing the air-eliminator, adding a properly sized main-vent, and leaving all the radiator supply valves fully opened.
Does anyone see any flaws in that plan?
Need to get heat the simplest way possible.
I don't want to risk messing with the system too much.

Hot_n_Cold · February 13

mattmia2 said:
Where does the pipe to the "inlet" of the air eliminator go?

I don't know specifically with the Durham system, but usually that sort of connection is to the steam main to inject pressure in to the returns to let the water return to the boiler if the pressure differential is too high. If you keep the pressure under 10 oz it shouldn't be a problem. Do you have a low pressure gauge that can accurately measure the pressure on the main?

A stopcock is just a plug type of valve, it opens and closes with 1/4 turn.

The Inlet is connected to the top of the Return Main, which seemed peculiar to me.
However, in the original manual, you'll see it's supposed to connect onto a T on top of the Return Main which also connects over to the Supply above the Header (with the check valve & stop cock in line).
That is curiously absent.

Brilliant ! That makes sense now. However, why does it show the check valve as only swinging open towards the steam main? And what's with the Stop Cock?

I don't have a low pressure gauge on there. Been trying to find one. All local parts stores can't find one.
Any suggestions?

Thank you for your help, btw.

mattmia2 · February 13

Supplyhouse.com or dwyer for the low pressure gauge. I strongly suspect that the Dunham vent is closing because the pressure is too high. Is the water line stable when it is firing? How much of a bounce do you see? Usually a new boiler requires skimming a couple times to get all of the oil out of it.

Are the pipes in the header at least what is specified in the manual, especially the riser out of the boiler and the header itself?

Jamie Hall · February 13

I'm going out on a limb a bit, and maybe you've already taken care of this -- but the Dunham residential systems were and remain exceedingly sensitive to excess steam pressure. In theory the air elinator trap float should close the air vent -- which, incidentally, must be the ONLY air vent on the system -- when the pressure in the boiler gets too high to allow condensate to flow back to the boiler. This will allow pressure from the boiler to build in the dry return, and hold the water level where it belongs. If that float doesn't close properly -- or if it does, but there is another vent ANYWHERE on the system, ... repeat: ANYWHERE -- even on a radiator! ... that won't happen, and the boiler water will back out into and flood the dry return and, very likely, the returning steam main.

This sounds to me very like what you are experiencing.

There are three things to do. First, the boiler must be controlled by a vapourstat, not a pressuretrol, and set to cut out at not more than 10 ounces. This must be verified by an accurate low pressure gauge. Second, go out in the system and, if you find a vent anywhere, remove and plug it. Third, ensure that the float in the air eliminator is free and that the vent opening on the eliminator is really closed when the float is up

Come to think of it, there's a fourth: if you find a steam main somewhere out in the wilderness which terminates, there must be a crossover trap there to the adjacent dry return (and a drip from both the man and dry return to a wet return at floor level) -- and that crossover trap, and the crossover shown in the diagrams near the boiler (which I think you said was missing?), must be operating properly.

If the Dunham air check is in place on the air eliminator, then the system may be able to produce a vacuum of considerable depth -- in which case the check valve on the return must really work.

Hot_n_Cold · February 13

mattmia2 said:
Supplyhouse.com or dwyer for the low pressure gauge. I strongly suspect that the Dunham vent is closing because the pressure is too high. Is the water line stable when it is firing? How much of a bounce do you see? Usually a new boiler requires skimming a couple times to get all of the oil out of it.

I set the pressuretrol as low as possible. Shouldn't be more than half a psig.
Water level bounces quite a bit. An inch at a time. Starts off slow, then bounces 1-2 inch fluctuations, each bounce ending lower until finally the glass is empty.

I skimmed about 3 full buckets out of it. I could try doing it again. However, the old system was having problems too.
The closest radiator to the boiler has a supply valve that would close itself to 25% shut when I open it fully. That's been an ongoing thing for awhile, even with old boiler.

Jamie Hall · February 13

Hot_n_Cold said:

mattmia2 said:
Supplyhouse.com or dwyer for the low pressure gauge. I strongly suspect that the Dunham vent is closing because the pressure is too high. Is the water line stable when it is firing? How much of a bounce do you see? Usually a new boiler requires skimming a couple times to get all of the oil out of it.
I set the pressuretrol as low as possible. Shouldn't be more than half a psig.
Water level bounces quite a bit. An inch at a time. Starts off slow, then bounces 1-2 inch fluctuations, each bounce ending lower until finally the glass is empty.

I skimmed about 3 full buckets out of it. I could try doing it again. However, the old system was having problems too.
The closest radiator to the boiler has a supply valve that would close itself to 25% shut when I open it fully. That's been an ongoing thing for awhile, even with old boiler.

Pressuretrols cannot be set low enough. You want the cutout at not more than 10 ounces, cut in at not more than 4 ounces.

Hot_n_Cold · February 13

Jamie Hall said:
I'm going out on a limb a bit, and maybe you've already taken care of this -- but the Dunham residential systems were and remain exceedingly sensitive to excess steam pressure. In theory the air elinator trap float should close the air vent -- which, incidentally, must be the ONLY air vent on the system -- when the pressure in the boiler gets too high to allow condensate to flow back to the boiler. This will allow pressure from the boiler to build in the dry return, and hold the water level where it belongs. If that float doesn't close properly -- or if it does, but there is another vent ANYWHERE on the system, ... repeat: ANYWHERE -- even on a radiator! ... that won't happen, and the boiler water will back out into and flood the dry return and, very likely, the returning steam main.

This sounds to me very like what you are experiencing.

There are three things to do. First, the boiler must be controlled by a vapourstat, not a pressuretrol, and set to cut out at not more than 10 ounces. This must be verified by an accurate low pressure gauge. Second, go out in the system and, if you find a vent anywhere, remove and plug it. Third, ensure that the float in the air eliminator is free and that the vent opening on the eliminator is really closed when the float is up

Come to think of it, there's a fourth: if you find a steam main somewhere out in the wilderness which terminates, there must be a crossover trap there to the adjacent dry return (and a drip from both the man and dry return to a wet return at floor level) -- and that crossover trap, and the crossover shown in the diagrams near the boiler (which I think you said was missing?), must be operating properly.

If the Dunham air check is in place on the air eliminator, then the system may be able to produce a vacuum of considerable depth -- in which case the check valve on the return must really work.

Is it possible to remove that check valve and just replace with a normal main vent?
I feel like the simplest thing would to just convert it to normal 2-pipe gravity steam.
If that's not practical, then I'll work on the Vacuum operation.
Vaporstat is ordered, still haven't found a low pressure gauge.
Didn't see any vents anywhere.
Also, when this happens, only the end of the Steam main gets warm, not the return main.
Also, the entire wet return gets warm too.
Dry Return, Air Eliminator, and Cross-over all stay cold.
Actually, come to think of it... The reason it didn't back out into the main return is because of the swing-check valve.
But I still didn't see an air-vent anywhere...

mattmia2 · February 13

https://www.supplyhouse.com/Winters-Instruments-PLP305-2-1-2-PLP-Steel-Low-Pressure-Gauge-1-4-Bottom-NPT-w-Brass-Internals-0-5-PSI

Hot_n_Cold · February 13

mattmia2 said:
https://www.supplyhouse.com/Winters-Instruments-PLP305-2-1-2-PLP-Steel-Low-Pressure-Gauge-1-4-Bottom-NPT-w-Brass-Internals-0-5-PSI

I skipped over some of those because they didn't say they're specifically okay to use on Water Boilers.
Is this one okay?

Jamie Hall · February 14

You can use any good big main vent on that thing. Maybe two. If you want to get fancy and retain some vacuum capability and have the cash you could use a Hoffman 76, but honestly I wouldn't bother.

The pattern of warm (not steam hot, but warm, sometimes quite warm) is just fine.

Hot_n_Cold · February 14

Jamie Hall said:
You can use any good big main vent on that thing. Maybe two. If you want to get fancy and retain some vacuum capability and have the cash you could use a Hoffman 76, but honestly I wouldn't bother.

The pattern of warm (not steam hot, but warm, sometimes quite warm) is just fine.

Honestly I'm just gonna try the main vent.
Basically just remove check valve, and put main vent there?
The Float shouldn't block the main vent right?
Any other mods necessary?

P.S. I'd personally love to tinker with the Vacuum system more if I had more time & money.
However, it's not my choice and there is a tight budget (and timeline is already past due...)

mattmia2 · February 14

Hot_n_Cold said:

mattmia2 said:
https://www.supplyhouse.com/Winters-Instruments-PLP305-2-1-2-PLP-Steel-Low-Pressure-Gauge-1-4-Bottom-NPT-w-Brass-Internals-0-5-PSI

I skipped over some of those because they didn't say they're specifically okay to use on Water Boilers.
Is this one okay?

it would be teed in after the pigtail where the vaporstat is, the pigtail shields it from steam and if it is airtight, water.

Hot_n_Cold · February 14

mattmia2 said:

Hot_n_Cold said:

mattmia2 said:
https://www.supplyhouse.com/Winters-Instruments-PLP305-2-1-2-PLP-Steel-Low-Pressure-Gauge-1-4-Bottom-NPT-w-Brass-Internals-0-5-PSI

I skipped over some of those because they didn't say they're specifically okay to use on Water Boilers.
Is this one okay?
it would be teed in after the pigtail where the vaporstat is, the pigtail shields it from steam and if it is airtight, water.

Awesome, thank you!

Steamhead · February 14

Hot_n_Cold said:
Honestly I'm just gonna try the main vent.
Basically just remove check valve, and put main vent there?
The Float shouldn't block the main vent right?
Any other mods necessary?

P.S. I'd personally love to tinker with the Vacuum system more if I had more time & money.
However, it's not my choice and there is a tight budget (and timeline is already past due...)

Use a Gorton #2.

Hot_n_Cold · February 14

Steamhead said:

Honestly I'm just gonna try the main vent. Basically just remove check valve, and put main vent there? The Float shouldn't block the main vent right? Any other mods necessary? P.S. I'd personally love to tinker with the Vacuum system more if I had more time & money. However, it's not my choice and there is a tight budget (and timeline is already past due...)
Use a Gorton #2.

They only have a 1 in stock.
Also, from what I see on diagrams.
If I want to convert it to 2 pipe steam, I'll need to install an F&T Trap at end of supply main, connecting over to return main, as well as removing the air eliminator check valve and replace with a normal main vent.
Correct?
Called supply house and they don't have any F&T traps.
Can someone recommend one that would work?
Another thing is that in one of Dan's YT videos "piping in steam systems", he talks about the fact that 2 pipe Vac systems were able to run with much smaller piping, radiators, etc. And that "once it's a vacuum system, it stays a vacuum system" . Basically saying that running on normal pressure alone will be inadequate.
So here I am back to trying to fix the vacuum setup.
Still haven't made a clear diagnosis on what is causing this condition of vacuum, empty boiler, water hammer, and water getting stuck at end of supply main into wet return, while dry return stays cold.
I'm kinda stumped here.

The only things I can think of are adding that check valve from return to supply main, checking the traps, getting a low pressure gauge & controller, and testing.

Hot_n_Cold · February 14

Steamhead said:

Hot_n_Cold said:
Honestly I'm just gonna try the main vent.
Basically just remove check valve, and put main vent there?
The Float shouldn't block the main vent right?
Any other mods necessary?

P.S. I'd personally love to tinker with the Vacuum system more if I had more time & money.
However, it's not my choice and there is a tight budget (and timeline is already past due...)
Use a Gorton #2.

Jamie Hall said:
You can use any good big main vent on that thing. Maybe two. If you want to get fancy and retain some vacuum capability and have the cash you could use a Hoffman 76, but honestly I wouldn't bother.

The pattern of warm (not steam hot, but warm, sometimes quite warm) is just fine.

Here's the diagram I was referring to in my last comment regarding the problems of switch to steam instead of vapor vacuum

mattmia2 · February 14

If you try to make it something it isn't you are going to have a bad time. Look at the things @Jamie Hall listed and get the pressure down. The crossover traps have to be working right and the air eliminator has to be working. If you try to make it ordinary 2 pipe you will have to make sure all of the radiator traps are working. If it is still a vapor system and the metering valves are in place on the radiators you just need to keep the pressure down on the boiler and make sure the radiator traps are open. If any of the radiators are missing the metering valves you need to make sure they either have a working steam trap or an orifice plate.

The trap needs to be open so that the radiator can vent and drain condensate. The combination of the low pressure and the metering valves limit the amount of steam that a radiator gets to a bit less than what it can condense so all of the steam that gets in to the radiator condenses, none of the steam can pass out of the return of the radiator and in to the return mains. As long as the trap is open it will function correctly. It does not need to close when it sees steam if the system is properly set up as a vapor system.

As with any boiler replacement, servicing the whole system has to be included in your quote.

Hot_n_Cold · February 14

mattmia2 said:

If you try to make it something it isn't you are going to have a bad time. Look at the things @Jamie Hall listed and get the pressure down. The crossover traps have to be working right and the air eliminator has to be working. If you try to make it ordinary 2 pipe you will have to make sure all of the radiator traps are working. If it is still a vapor system and the metering valves are in place on the radiators you just need to keep the pressure down on the boiler and make sure the radiator traps are open. If any of the radiators are missing the metering valves you need to make sure they either have a working steam trap or an orifice plate. The trap needs to be open so that the radiator can vent and drain condensate. The combination of the low pressure and the metering valves limit the amount of steam that a radiator gets to a bit less than what it can condense so all of the steam that gets in to the radiator condenses, none of the steam can pass out of the return of the radiator and in to the return mains. As long as the trap is open it will function correctly. It does not need to close when it sees steam if the system is properly set up as a vapor system. As with any boiler replacement, servicing the whole system has to be included in your quote.

That makes perfect sense to me. I greatly appreciate your explanation. I have the vaporstat ordered & the low pressure gauge. I suppose I'll try working with the system as-is first. If all else fails, try going to 2 pipe steam without vac.
As far as testing the traps, do I need to remove each one individually and do the method Dan showed (hook up to a modified tea kettle and see if steam passes?) Or can I measure a temp differential while system is running? (Like he also showed).
Also, as far as I can see, the traps and metering inlet devices are all original and untouched. (And hopefully still fully functional)
Been researching & absorbing as much as possible on this system. It's starting to make sense now.
However, I am still confused as to why it seems the hot water is backing up into the end of the steam main & wet return.
I had a hypothesis in a previous comment inspired by the guy who mentioned that the pressure may be too high and pushing the hot water/steam out past the Hartford loop.

mattmia2 · February 14

You might try taking the current vacuum valve out of the air eliminator and leave it open and see what happens, it might be too restrictive for it to vent quickly enough. If everything else is ok there should never be steam in the returns so it should never vent steam, if it does you have other issues you need to find and fix like bad crossover traps, pressure too high, or part of the system that were once below the water line that are no longer below the water line with the newer, smaller boilers.

BTW, this is a slog now, but if you keep at it you can be the person that fixes steam problems that others can't.

I wouldn't rule out some of it being that the boiler is still throwing water up in to the mains because there is oil on top of the water.

Help Please! Old 2-pipe System

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