main vent reopening temperature

dabrakeman

My system is one pipe but with separate condensate return lines below each radiator. Two mains about 35' each with the first 10 ' of each being 3" pipe and the rest of the mains 2". 2 story home. Weil-McLain EG65. I have main vents back at the ends of the returns on risers by the boiler. I have a Gorton #2 and a Ventrite 35 antlered on each riser. Generally the system runs fairly well but when temperatures are very cold or on initial morning cycles at times the radiator vents whistle hard, either releasing air or just as often sucking in air. Excessive air release from the radiators does not occur on the first cycle but seems only to occur when it is extremely cold out or temperature is being raised the 4 or 5 degrees typical in the morning. The inrush of air seems to occur quite often. I guess the best way to sum up the performance is, "inconsistent".

My thought is that what might be happening is that when the Gortons are open, things work well. Steam gets out through the mains and then the Gortons close. All is good. But, if another cycle of heat is needed I don't think the Gortons are opening up again. I have stood by the boiler for 10-15 minutes after boiler started firing again, there is nothing coming out of the Gortons and the radiators are whistling and pressure rises sometimes shutting down the boiler at ~2.5psi (note, pressure generally doesn't lift the needle off the stop on first cycles and when things seem to be working good). I also suspect that when I am hearing the loud inrush of air into the radiators it is again because the system is cooling and the mains are still closed.

My question is whether there are differences between main vents in terms of at what temperature they open and close. It would seem to me that maybe I would be better off if I added something that reopened at a hotter temperature.

Looking at Supplyhouse at the Gorton #2's and the Hoffman 75's it seems both are listed the same as operating temperatures 140F-240F. Is it correct to assume the 140F is the theoretical closing and opening temperature? Is there data somewhere of comparable opening and closing temperatures for different model valves?

BobC

Maybe putting some uninsulated pipe between the mains / return port and the vent would let them open earlier.

Bob

Fred

They do close around 140 degrees. It doesn't seem like you have enough Main venting. The Gorton #2 will vent about 1.1PSI at 1 ounce of pressure while the Ventrite 35 will only vent .11 ounce at 1 ounce of pressure. With the length of your Mains, and the fact that a 10 ft. section is 3", you need a minimum of 2 Gorton#2's, 3 would be even better.
As for the radiator vents, during short cycles, the rapid condensing of that steam in the system and radiators when the boiler shuts down will create a small vacuum tht can cause the raditor vents to be sucked open and suck air in. That air cools the vents enough that they stay open until the steam from the next cycle closes them, hence thee sucking of air and the expelling of air on second and consecutive cycles. That is fairly normal when the boiler short cycles. In your case, that is probably compounded by the need for more main venting. I think most of the vents all close around 140 degrees. I'm not sure you are going to get around that, shy of an open pipe, which obviously won't work unles you want a sauna.
Sommeone on here may have ideas about putting a vacuum breaker somewhere on the Mains???? don't know.

BTW: If you have a supply pipe and a drian pipe on your radiators, you have a 2 pipe system.

dabrakeman

No drain pipe on the radiators. Condensate exits same pipe as steam coming in. Under the floor separate condensate returns drop off from the supply in a tee.

I had insulated most of my returns as well as supply lines so I wonder if that is keeping the vents hot longer.

Anyway, sounds like I could add a couple more Gorton #2's and then reevaluate. Wish those things would go on sale from time to time...

dabrakeman

Just copied and pasted below a section from a 2007 thread authored by Gerry Gill: (comparing Hoffman 75's to Gorton #2's)

"...they have totaly different construction.

the hoffman uses a volitile liquid capsule and the gorton uses a bi-metal strip..both vent close to the same..here is the difference..the hoffman will stay open in very high ambiant temperature..the gorton will start to close about 110 degrees and be closed at about 130 degrees ambiant temperature..so, the question would be..where are you putting these vents..i use both brands depending on ambiant heat..if the vent is above the boiler in the ceiling, next to a steam line..well, i'd choose the hoffman..if its just about anywhere else, i'd choose the gortons."


Based upon the above if there is any other concurrence out there maybe I should add a Hoffman 75 to each line. Right now I have antlers with three fittings each (one sized to fit the Hoffman or Vent-Rite and the other two sized for the Gorton fittings).

nicholas bonham-carter

More main venting is needed if you can hear the radiator vents. Look upon the cost of the extra vents as an investment, instead of an expense. Fuel cost for forcing air out of small vents is an expense.
Put another couple of Gortons on the lines in question, using a longer antler to cool them down, and then see how things are working. While you have your pipe wrench out, put a low pressure gauge on next to the pressuretrol (better yet a vaporstat).
When the steam collapses, and the vacuum is forming, you have a considerable difference in pressure moving air back into the system, and this has to be accommodated with adequate venting.--NBC

Fred

dabrakeman said:

Just copied and pasted below a section from a 2007 thread authored by Gerry Gill: (comparing Hoffman 75's to Gorton #2's)

"...they have totaly different construction.

the hoffman uses a volitile liquid capsule and the gorton uses a bi-metal strip..both vent close to the same..here is the difference..the hoffman will stay open in very high ambiant temperature..the gorton will start to close about 110 degrees and be closed at about 130 degrees ambiant temperature..so, the question would be..where are you putting these vents..i use both brands depending on ambiant heat..if the vent is above the boiler in the ceiling, next to a steam line..well, i'd choose the hoffman..if its just about anywhere else, i'd choose the gortons."


Based upon the above if there is any other concurrence out there maybe I should add a Hoffman 75 to each line. Right now I have antlers with three fittings each (one sized to fit the Hoffman or Vent-Rite and the other two sized for the Gorton fittings).

I actually use all hoffman 75's because I like them better and they never fail but it is a matter of personal preference. The thing I will say is that it takes 2 hoffman 75's to expel the same amount of air as a Gorton #2 and since the price of eacch vent is about the same, it costs twice as much to sue Hoffman's as Gortons.

Fred

nicholas bonham-carter said:

When the steam collapses, and the vacuum is forming, you have a considerable difference in pressure moving air back into the system, and this has to be accommodated with adequate venting.--NBC

The real issue he has (and one that I experience occassionally) is actually during a situation where the boiler short cycles on pressure as a result of a set back or , in my case, the outside temp and wind is minus 10 or lower.
During that short cycling, there is a small vacuum created at the radiator,when the boiler shuts down that is enough to pull those radiator vents open but not enough to pull the Main vents open and because the Mains are so hot, they vents really don't have a chance to breath during that 2 - 3 minutes before the boiler kicks back on and makes steam again which leaves any air that was sucked in to have to be expelled by the radiator vents that were pulled open and are now cool as a result of sucking that room temperature air in for a couple minutes.
He does need more main venting, no question but that won't fix this issue.

Jamie Hall

There's something to be said for crossover traps and vents only on the dry returns (which don't get hot), isn't there? Chortle...

Fred

Jamie Hall said:

There's something to be said for crossover traps and vents only on the dry returns (which don't get hot), isn't there? Chortle...

Yes there is, Jamie

dabrakeman

Fred, your description makes perfect sense. I believe that is exactly what is occurring here.

However, Jamie, my main vents are on the dry returns. Nonetheless, they still get very hot. This summer I insulated all my piping after the new boiler install last winter. I decided to insulate my dry return lines as well as the supply lines. I could have made unknowingly made the issue a bit worse by increasing the average temperature of my condensate return and thus the temperature of my main vents at the end of the returns? I guess I will at least peel off a few sections of the pipe insulation over the return lines closest to the mains.

In the 20's yesterday and today and no issues. -2F tonight so we will see...

dabrakeman

Sorry Jamie, I think the "Chortle" had flown right past me...

Fred

dabrakeman said:

Sorry Jamie, I think the "Chortle" had flown right past me...

It means to smirk or chuckle. I had to look it up myself

dabrakeman

I guess I knew what a chortle was, but, I obviously get way too serious when talking steam and just missed the facetious humor.

I do plan on getting more venting I just still wonder about the Gorton #2's vs the Hoffman 75's relative to this situation.

Fred

dabrakeman said:

I guess I knew what a chortle was, but, I obviously get way too serious when talking steam and just missed the facetious humor.

I do plan on getting more venting I just still wonder about the Gorton #2's vs the Hoffman 75's relative to this situation.

I think either will be fine. I believe the Gortons do start to close a little earlier than the Hoffman 75's but you need 2 hoffmans for every Gorton 2 so cost comees into play. In reality if you mix the gortons with the hoffmaans, on each main, you'll probably get the best of both worlds.
When it comes to what happens on short cycles, neither will change things much. Having said that, having more venting on the mains may reduce initial presure enough that you reduce the number of short cycles.

dabrakeman

Was there a calculation worksheet somewhere for determining venting recommendations based upon main pipe volume?

Fred

Yes, you can buy an electronic copy of Balancing a Steam System using a Vent Capacity Chart, by Gerry Gill and Steve Pajek, at the store on this site. It costs $10.00 and that money is donated to a charity. it tells you the venting rates of a variety of vents and the volume of air per linear foot of pipe, by diameter.

Fred

Another way to do it is to take the vents that are on there off, using the open pipe the vent was on, time how long it takes to get steam to the end of the main, after the Header is hot and put enough vents on the main to math that as closely as possible.
I used the chart that I referenced above and it worked well for me though. Could I get even faster? Maybe but there is a point of deminishing return.

Fred

Point well taken. I guess the old rule of thumb, 1 Gorton #2 per 20 feet of 2" maybe 2.5" pipe is reasonable close.

dabrakeman

OK. Getting a bit confused on the actual venting recommendations.

I have a stupid question. How exactly do I time how long it takes steam to get to the end of my mains and what really constitutes the "end" when my main vents are at the ends of the dry returns? Do I need to start from a totally cold system, wait until the header is hot (of course I have this all insulated) and then wait down at the end of the main and try to tell if it is getting warm (again, pretty well insulated although I can probably work a finger in at and elbow). First sensation of heat? Both of the steps so far sound very subjective.

dabrakeman

One other thought. With the main vents at the end of the dry returns it seems likely that steam could find a short path toward the near radiators and back through their dry return piping and rather quickly heat up the main vents. I will check again next time the system starts off from cold but I seem to recall earlier finding those short loop return pipes red hot well before the ends of my mains are hot or any radiators getting warm.

dabrakeman

And your "start" time is when you can't keep you hand on the header?

dabrakeman

This will be a bit long.

I have attempted to quantify a full cycle with a 2 degree upswing in temperature (64-66F by stat on 36F day) demand starting from a luke warm system. Recall the system has 2 mains (N and S). Total EDR on North main is 312sqft (however, 130sqft of this is shutoff at the radiator valves including the radiator in the north parlor where the thermostat resides and 2 of the 3 upstairs bedroom radiators). There is 212sqft feeding off the South main.

TIMELINE: (these are absolute times not incremental):

0m - boiler kicks on
1m45s - header becomes too hot to touch
4m30s - steam to end of S main (so 2m45s as commonly referred)
4m30s or less - steam also to end of N main (already hot by time I got over there)
5m20s - steam to end of N returns (Gorton #2 closes)
9m0s - steam to end of S returns (Gorton #2 closes)
17m-19m - Far radiators on the South line are 50% hot
20m-29m - Far radiators on the North line are 50% hot
(note these differentials are in part by choice with the quicker heating occurring in the main living areas)
24m-29m - All South line radiators are 100% hot
35m-38m - All North line radiators are 100% hot
38m - System pressure finally starts to rise off 0 per the stock EG65 gage
40m - 1psi (thermostat already reading 66F)
41m - 1.5psi
43.5m - 2psi
46m - 2.5psi
48m - 3psi and system shutdown
51m - 1psi system starts up again
52.5m - 1.5psi
54m - 2psi
56m - 2.5psi
57.5m - 3psi and system shutdown. Therostat still reading 66F.
59.5m - 1psi system starts up again
1h4m - 2psi
1h7m - 2.75psi system shutdown.
1h10m - thermostat still says 66F. Most of rooms with alternate thermometer more like 68F-69F.
1h10m - loud vacuum inrush of air going into all radiators. Lasts a couple of minutes.
3h30m - restarted (outside temp 39F, thermostat still showing 66F)

According to an earlier post getting steam to the ends of the mains in 5 min or less is a high performing system. Mine appeared to do it in less than 3min if really just going by ends of mains. 3 1/2min to 7 1/4min if to the shutoff of the Gortons at the return line ends. I have two new additional Gorton #2's that arrived in the mail yesterday. Should I put them on or save them for replacements?

I know my boiler is a bit oversized. EG55 would have been about right at 520sqft whereas my EF65 is rated for 654sqft. However, this boiler was already a full size smaller than the one it replaced last year and we kind of had to figure other possible owners might want to turn all the radiators on. We also have an add-on with 720sqft of floor space that we might want to expand the steam into (another subject).

The bigger questions I have from the observations above are:
1) Why does the North branch of the system close its main so fast relative to the South branch? Yet, likely due in part to radiator vent settings, the South radiators heat up more quickly (everything is pretty much balanced as desired based upon usage of the rooms).
2) Other than inefficiencies of which I am ignorant because I have no basis of comparison my only "issue" is with the whistling when the rads are sucking air back in. Is there really any reason to believe more main venting would help this? They would still be hot and closed 3 minutes after the system shut down (and likely so wherever they were placed on the antlers). Almost seems like some kind of vacuum relief valve (which one could theoretically put anywhere on the system) would solve this.
3) Is the fact that the thermostat is in a room with the radiator shutoff (been this way for the 11 years we have been in the house) in essence making the system work longer creating a larger swing in most of the rest of the house and on the system than intended? Should I turn this radiator on? There is some fear that a valve not touched in decades could become a mid-winter emergency...it already gets some blow-by since it heats up ~25% in the latter stages of the above long cycles).

Appreciate you just reading all this...

dabrakeman

Maybe if I just turn that parlor room radiator on it might be the initial best course of action. This would shorten the necessary cycle time by raising temp in the stat room quicker as well as increasing the effective EDR on the system. As you say in my case most of the lost efficiency is in the final 30 minutes where the system needs to cycle on and off a couple times on pressure. Also the wear and tear on the radiator valves.

Do you think it worth putting another #2 on the South line?

Any thoughts on any remedies for the vacuum issue or will this likely go away if I can get rid of the pressure issue?

Fred

#1 You have to get the pressure to no more than 1.5PSI Max. There is no reason to run it higher and higher pressures are less effecient.
#2 The thermostat needs to be in a heated room. That plan and simple. It is a temperature control device and you are hiding the room temperatures from it. The other rooms, technically aren't over shooting the set temp, the boiler is trying to heat that unheated room.
#3 Adding more vents, as I indicated in an earlier email won't fix the vacuum issue that causes the radiator vents to suck air. The Main vents are closed at that point and will stay closed until they cool enough to open, no matter how many you put on there. By the time they open the vacuum has broken. You can try a vacuum breaker some where on the system but it really isn't an issue. It only occurs when the radiators are 100% hot and virtually all the radiator vent is completely closed.
#4 Closing off all those radiators is compounding the issues with an over sized boiler. Each radiator you close off just make the boiler that much more over sized.
EDIT: Open those other radiators, either move the Thermostat or heat that room, lower the Pressuretrol to cut-in @.5PSI and Cut-off at 1.5 PSI and there is a very good chance that 99% of the time there won't be a vacuum to suck air.

dabrakeman

Just to clarify I allowed the temperature to drop a degree further than normal by manually adjusting the stat just so I started my test with a cooler system. I manually set the stat 2 degrees warmer than room temp to start the boiler and start the test. The normal stat swing is 1 degree. That said, I do have a 5 degree setback at night and a 3 degree setback midday.

Opening the two bedroom radiators isn't going to happen. Those were sized for sleeping with the window open...

The parlor radiator that is off is 56sqft so not insignificant and turning it on should help in alot of different areas here. I just hope the valve doesn't leak.

dabrakeman

It cycled on pressure twice with a 2 degree setback so I am pretty sure a 3 degree setback might only cycle a few more times. But, I will repeat the test with the radiator open in the thermostat room (it is a pretty wide open room by the way with two sets of open double doors and another single door all to other heated rooms).

Fred

OK, so the 2 bedrooms aren't going to have heat, heating the room that has the thermostat will probably go a long way towards making the temp in those bedroom more palatable. Putting smaller vents, like Hoffman #40's will probably finish the job.
The best way to optimize your steam system is to normalize the environment as much as possible, open all radiators, set the pressurettol like it should be, make sure the mains are properly vented (you have done that), eliminate the set-backs on the thermostat, fix anything that you know or suspect is broken.
Then balance the system and watch how it runs. It may take an additional tweek here and there, especially with the radiator vents but when you're done, the system should be running as it was designed.
At that point, you can change the environment, one step at a time and note how that change impacts the overall operation of the system. then you can decide if thaat is acceptable to you BUT you will also understand that it is not a failure in the system but rather a choice made by the homeowner.
I'm really concerned that you are adament about leaving those bedroom radiators shut off, when we know that affects the boiler sizing.
There is no way that boiler should run for over an hour per cycle so you have to know it is essential to let the thermostat sense the room temp in the heated rooms.
I know you've seen enough comments on this site about setbacks greater that 2 degrees and you have two of those everyday. Is it saving you money? I bet not when the boiler runs for over an hour to accomplish a 2 degree climb.

dabrakeman

In these conditions letting the boiler start in a normal manner (still with the parlor radiator off) the system comes on for about 30 minutes getting most of the rads to 100% and shuts off without any pressure registering on the meter.

Just turned the parlor radiator on after the cycle had finished...

Captain Who

Another option is to install a remote sensor if your thermostat allows it. I did that with my White Rodgers but it required that the stat receive constant 24V power in order to use it.

dabrakeman

Good news the valve was OK. Had one a few years ago that let go on first manipulation and no tightening of the packing nut or even rewinding would close that thing. Had to live with it until Spring before anyone would touch it. Good thing that room had tile floor...

The suction issue was only ever after recovering from a setback or sometimes when it was well below zero out. Part of the purpose of having the rad off in the room with the stat was that the room is not highly used. With the current arrangement the rooms in use are two or three degrees warmer than the stat. There have been some thoughts also that generally longer cycles are better than shorter ones (which is the reason my heating guy said that rad was off). If I find the system short cycling on stat temp then I will look into a different stat.

I'm curious, do you really just leave your heat at the same temperature day and night?? Is there a general consensus that keeping the temperature 5 degrees warmer in a house than needed for a full 1/3 of a day in a poorly insulated old house is fiscally beneficial compared to recovering from a morning setback. This may be getting off topic and I know there are many other threads on this topic. I have read some and have not really found myself convinced one way or the other.

BobC

The problem with degree days is it does not take wind or infiltration into account. Living in a 95 year old house I can tell you they have a big impact on fuel use.

Bob

Captain Who

It would be better if they had "feels like" days or wind chill factor days. I'm not sure what the difference between the two is. Solar is so hard to factor in but needs to be. Somebody needs to make a sensor that could be the input for a heating controller that takes all that into account. Sort of the way a mass air flow sensor works in a car maybe.

dabrakeman

I really appreciate everyone's comments and help throughout this thread.

I agree it is a difficult thing to nail down from a data standpoint. Because of the noise factors such as wind, solar heating and temperature swings you need much longer term data, i.e. 3 years one strategy vs 3 years another alternating years in a random order. Most of us of course end up having our own opinions or hunches and don't want to put that much time (and possibly money) into a strategy counter to our opinion(: Trying not to change anything else in the system over such a period might also be difficult.

In the mean time the general learnings here to apply will be to keep the rad on in the stat room and reduce my swings to only a couple degrees or so (will also need to raise the overall temp a degree or two since the main living quarters now don't get quite as warm).

There still seems to be some mystery to the optimal main venting as well though. Given some of the comments many still would believe more venting was needed. Subsequently I got two more Gorton #2's. Is there any potential negative of putting those on?

dabrakeman

Yep, its 3/4". I am going to rerun my 2 degree recovery test again with the parlor radiator on first, then add the additional #2 on the south line.

dabrakeman

Checked the Honeywell Pressuretrol (PA404A 1009) and it appears to already be set for 0.5psi cut-in on the external scale and 1.5psi cut-out with the internal dial at 1 (both adjustments are at their lowest setting). Can these pressuretols be off by up to 1.5psi on cutout or more likely I am just getting a bad reading from the pressure gage at those low pressures? I have heard talk of vaporstats and 5psi gages. Saw a Winters Instruments PLP305 on Supplyhouse. would this be an appropriate gage to use on a steam boiler? Vaporstats seem pretty pricey so might be first worth seeing what the real pressures are.

Fred

Most of us use a 3PSI gauge from WIKI. The supply link is below. I would first check the Pigtail (looped pipe) the Pressuretrol is mounted to. They often get clogged andd the Pressuretrol can't sense the actual presure. Take it off and clean it out good. If that doesn't bring the Pressuretrol into a reasonable range, following is the procedure to recalibrate it:
Inside the Pressuretrol, right below the micro switch, there is a pivot arm. At the end of that arm you will see a screw pin that is activated by the diaphragm at the bottom of the Pressuretrol. If you look very carefully at that screw pin, you will see it actually has a tiny (I mean tiny) hex head on it. It takes a .050 hex wrench and you can turn it clockwise (Towards the bottom of the Pressuretrol to decrease the Cut-out pressure or counter clockwise to increase the cut-out pressure (which none of us want to do but who knows, your Pressuretrol may be really screwed up!). Turn the power to the unit off first. You may find the first attempt to turn that screw a little bit stubborn (relatively speaking) because it has some Locktite on it but it does turn. Don't turn too much, a fraction of a turn goes a long way towards getting it adjusted where you want it (maybe 1/32 inch turn to start with). You may need to play with it to get it exactly where you want cut out to be.

Link for 3PSI gauge: http://www.valworx.com/product/low-pressure-gauge-25-0-3-psi

dabrakeman

We checked the pigtail about a month ago. I will try adjusting the Pressuretrol.

Reran the test with the parlor radiator on (but throttled down a bit w/ the vent). Outside temp =34F. Cool system to start. Stat at 64F but the distant rooms at 62-63F.

0m0s - header hot
2m40s - end of N main
3m15s - end of S main
<3m15s - end of N returns at main vent (this is what I find very puzzling)
8m45s - end of S return at main vent
~17m-18m - S radiators ~50% full plus close hallway radiator off N line
~22m-23m - furthest large radiator on North line and upstairs radiator on North line 50% full
~30m - stat reading up to 66F (only 4 of 28 columns hot in parlor radiator)
34m - other large radiator on North line 50% full and most of S line radiators 100% full.
43m - pressure at boiler barely start to move needle off stop
45m - 0.5psi
47m - parlor (room with stat) radiator 50% full. All other radiators in the house 100% full
48m - 1.5psi
56m - 2psi. Stat room radiator 100% full.
58m - 2.5psi
1h30s - 3psi and boiler shutoff
Boiler did not recycle
Thermostat reading 66F but other rooms 67-68F and thermometer next to stat reads 67. This was a 2 degree swing at the thermostat but peripheral rooms swing was more like 5 to 6 degrees. Eventually there was a full 2 degree overshoot at the stat.

Most notable difference here from the first test was that no recycling was needed but pressure still getting too high (if it is right). Still got vacuum escape noise afterwards. Most perplexing to me is the rapidity in which the north main and returns close off the main vent on that circuit. It is like it is being short circuited somehow because then the north rads all heat up much slower than the south.

I'll try some more rad vent balancing including freeing up the stat room radiator some more.

Fred

Tell us again how many vents and what type you have on each main and how long eaach main is as well as its diameter. It sounds like (and your data suggests) you may have one main vented faster than the other and those vents close faster but steam isn't moving up the radiator runs because the boiler is still running through the other main to its end. May need to add another vent on the slower main or shift one from the faster main.
Not sure what puzzles you about the returns. They really don't need to be vented if you have vents at the end of the mains.

Over-shooting the thermostat by a couple degrees and the boiler cutting out on pressure still suggests some control isn't quite right yet. What type thermostat do you have?

dabrakeman

Actually measured vs estimating this time (overestimated the North before). Two mains about 35' on South and 23' on North with the first 8 ' of the South and 6' of the North being 3" pipe and the rest of the mains 2". Weil-McLain EG65. I have main vents back at the ends of the returns on risers by the boiler. I have a Gorton #2 and a Ventrite 35 antlered on each riser. There are no vents at the ends of the mains, just the ends of the returns back at the boiler.

The south main is a long straight classic main with 4 rather evenly spaced branches. The north is a little more contorted and also feeds the upstairs. It doesn't make sense to me on the surface why steam is closing the main vents on the North circuit so much faster than it does on the South circuit yet the South radiators significantly outpace the north radiators regardless it seems of my radiator venting. I have the dining room radiator with a Hoffman 1A set near 6 yet it is ice cold while most of the South radiators are at 50%.

I can put another Gorton #2 on the South and see what happens. I can't see any logic in adding another to the North line.

Fred

Do those lengths include the returns as well? If not, is the South return considerable longer that the North Return?
Do you have 1 or 2 risers out of your boiler to the header? If only 1, is it closer to the faster Main?
Are the Mains and returns pitched correctly? Are the returns pitched correctly? Are there any sags in any of slower pipes that might allow water to set /pool in any of them that might cause the steam to condense before it can move down the Mains or radiator runs? Are the radiators all pitched towards the inlet pipe that feeds them?
The south Main has about 50% more air to vent than the North, so it makes sense that the North vents faster and closes the venting. Put another Gorton #2 on the South Return.
Steam is going to take the path of least resistance, which is the open vent(s). When they are all closed, then it will seek out the next least resistant path.
If you can more closely equalize the time each main vent closes, it should start to get steam to all the radiators, unless there is water setting somewhere that is causing the steam to condense before it gets to the radiator(s).