Main air vents - how many, where, what

emk2badknees

I have a 1920s home with a one pipe steam heat system. The system has two legs of 2” pipe, each about 35’ long, one going down each side of the house feeding radiators on both the first and second floor. Both legs loop back and tee into a return line that is sloped back towards the boiler. The only main air vent in basement is at the end of the return just before the line drops back down to the boiler.

Every time the steam system comes on, there is a loud hissing from the radiators. I assumed that this was because there was not enough main air venting. Upon inspection, I found that the lone main air vent I could locate was doing nothing, so I replaced it with a Gorton #2. I hoped that this would solve the issue, but the hissing did not go away. Upon further inspection, I found two spots in the main line that have plugs, one located near the end of each of the main runs going to the side out the house opposite the boiler. They are both angled at about 45 degrees from the main.

My questions are these:
1. Is it likely that these originally had main vents too?
2. Can main vents be set up at a 45 degree angle or do they need to be vertical?
3. If I should install vents in these spots, what size vent would one recommend? It doesn't look like a gorton #2 will fit. One is about 4” from the floor joist, the other has maybe 7” clearance (which may be just enough to get the G2 in).

Jamie Hall

The hissing is at least as likely to be due to excess pressure as it is to be due to inadequate main venting. However, additional main vents won't hurt. If your piping is as you describe, however, the additional main vents can be where the single one now is -- build yourself a nice antler and plop a couple more #2s on it. That is assuming that the mains really do come around and come back together dry to that main vent location! If there's anywhere in there that water can be trapped, that won't work -- it has to be a continuous slope (or dripped to a wet return at all low points -- that works, too).

Jamie Hall

I might add -- what pressure is your control set at? And does the boiler turn off on pressure rather quickly? That's a sure sign you need more main venting... It should be set to cut out at 1.5 psi, and back in at 0.5 if your control will go that low.

emk2badknees

Thanks for your help.

The system pressure is as low as I can set it (~0.5 psi cut-in with a 1 psi differential).

I checked the slope with a level and from what I could tell, the first leg of the main after leaving the boiler is sloped back to the boiler, but the other main lines all slope continuously to the return side. I should note that we don't get any water hammer, which suggests that water trapping in the lines is minimal. If it helps anyone, I created a schematic of the system.

I like the idea of putting multiple vents in at one location. Right now, the main vent is boxed in on all sides by floor joists, an i-beam and gas lines (in adjacent space between joists and on the far side of the vent, which can't be seen from this view). Would it still be possible /advisable to put an antler up or would I be better off going to one of the other locations that I mentioned earlier that are currently plugged/capped (see schematic and attached photos)?

If I'm better of doing the latter, is there a way to find out if there is an issue with putting an air vent in at a 45 degree angle?

hvacfreak2

It would probably be best to get a 45 and take off to a vent tree from there ( or a 90 where yours is currently ). I have my vent trees starting with 90 degree ells off of the mains. I wish they could be higher in elevation but there is no room to be had. I had entertained the thought of piping over to the basement stairwell and setting the vent trees higher but I have not had any problem with vents being wrecked from steam or hammer so I have not pursued it further.

Fred

You can actually do either. If you put main vents at all three locations, you can use 45's to get them vertical. You can actually take the current Gorton #2 and move it to one of the other sites. There is no need to vent that long dry return. Using the other two sites will allow you to use fewer vents (rule of thumb is the equalivant of one Gorton #2 for every 20 feet of 2" main). If you put all the vents at the one location, where your current G orton #2 is, you probably need at least one addition vent (over and above what it would take by placing them at the other two locations) because you have all that air in that long return that will vent to that one location and close those vents before steam will start to distribute out to your radiator runs.

Dave in QCA

Fred, I'm just jumping in here with a question. I was quite active on here in the past and have not been so active for the last year or so. Coming back on recently, I have been seeing reference to this "rule of thumb" of one Gorton #2 for every 20' of 2" main. Now... going back to the charts in Greening Steam, and the other information there from Gill and Pajek, if we are working at 1oz, which is half of the 2oz. that was usually used, a Gorton #2 would be good for about 95' of 2" main. Now, I'm all for proper venting of the mains, it is an absolute necessity to achieve even steam distribution. But, can you give me any idea where this new "rule of thumb" came from, and/or the rationale? I'm just trying to catch up here.

Jamie Hall

@Fred , you started an interesting train of thought on that rule of thumb business (which, by the way, seems to me based on my experience to be major overkill -- but that's another story). What needs to be vented is the volume of the system up to, and including, the emitters. The volume beyond the emitters does not need to be included in the volume to be vented, although in the case of a dry return it does have to be vented.

It can be a little confusing.

First, consider a two pipe system. Might -- or might not -- be vapour/low pressure. Now in order to get heat out of the radiators, the steam has to be able to get through the mains and then through the runouts and into the radiators. In order to do that, the air which is in the mains, runouts, and radiators has to go somewhere. The air in the mains can either be released to the atmosphere through main vents at the ends of the steam mains, or it can be released into the dry returns through crossover traps. The air in the runouts and radiators goes into the dry returns. Now then, if the main vents are placed at the ends of the steam mains, the only volume they need to clear is the volume of the mains themselves. The vents on the dry returns -- usually at the boiler -- need to vent the volume of the radiators and the runouts. If crossover traps are used, then the vents on the dry returns -- again, usually at the boiler -- need to vent both the mains and the runouts and the radiators. In no case is it necessary to include the volume of the dry returns in the calculation.

Now a one pipe system is different. In that case, the vents at the ends of the mains vent the volume of the mains only, and the vents on the radiators vent the volume of the runouts and the radiators themselves. Again, there is no need to vent the volume of any returns.

Note that a two pipe system always needs vents on the dry returns. A one pipe one doesn't, if it even has any.

Clear as mud. Sorry...

BobC

On my small system I have about 12 ft of 2" main, it circles the chimney and the radiator runouts (12 to 16 ft long in the cellar) radiate from it to all the radiators. There are 2ea maid o mist #1's on that main (total of 0.66 CFM). I have a 0-3 PSI gauge on the boiler and my main is full of steam long before the gauge begins to twitch. That gauge registers 12oz when all the radiator vents finally close.

I think the key is you want the main to fill with steam at very low pressure so you vent them fast. If you can save a minute or two on each cycle it adds up over the thousand plus heating cycles in a heating season.

just my opinion,

Bob

JUGHNE

So my question is it seems to be implied that no steam will go to the runout/risers until the air vents close. I have a 1P with 100' of 3" and 100' of 2" dry return. (times 2) My vents were added at the end of the dry returns in the boiler room . Piping issues at the end of the 3" and the need to remove the bullhead tee at the end of dry returns, so that is the main reason for vent location.

So there is a time lag for the steam to get thru the 100' of 2" dry return to air vents; however the steam is at the end of the main to the last takeoff before this and as the steam compresses the air out of a smaller 2' pipe wouldn't pressure build up in the system starting to send steam up the runout/risers. I have not watched the pressure closely when the vents closed, vaporstat runs in ounces.

In this case the returns are well insulated and any heat from the returns is not wasted in the conditioned basement.

Fred

@Jamie Hall , I agree with what you said. In this case, the OP has a one pipe system so, I felt he would be good by venting at the two locations on his diagram where he doesn't have vents now and that he could get much more value out of the one Gorton #2 he has at the end of the dry return by moving it to one of the other locations and adding to it as needed. (he does have one radiator just past one of his potential locations but I felt it was so close that it probably was a miniscual amount of air that would easily be handled by the radiator vent.
@Dave in QCA The 1 Gorton #2 per 20 feet of 2" pipe has been the topic of debate for a long time. Some feel it it over kill, others feel it works out well when one measures the time it takes to vent an "open" 2" main compared to the number of Gorton #2's necessary to vent that same main in an equal amount of time, realizing that the Gortons start to close at somewhere around 120/140 or so degrees. I haven't done that testing myself but others have. I guess the fool proof way to vent any main, as fast as possible and similarly to an open pipe (give or take a few seconds/minute), is to run through that exercise before investing in main vents but for those who don't want to take that time, the rule of thumb has been a relatively reasonable one. I happen to use Hoffman 75's on my mains and i have to say, in my case, the number of 75's comes very close to that rule of thumb when I consider that it takes 2 Hoffman 75's to equal one Gorton #2. If that means anything.

BobC

My basement sits at about 47 in the winter and all of the steam pipes are insulated with 1" rigid pipe insulation.

The real test would be to test the vented main timing against an open vent pipe. That will let you decide if more venting is warranted.

Bob

JUGHNE

Fred, that has got to be a lot of 75's in your basement. Is there a reason you prefer the 75 over the Gorton 2? For me they are about the same price ($2-$4 difference).

Fred

@JUGHNE , Since steam will take that path of least resistance, I am of the opinion that open, larger main vents will be that path, until those vents close, followed by the smaller radiator vents. If the main vents are at the end of the main, after the last radiator, steam will meet some resistence from the compressed air in the dry return and begin to make its way up the risers, to the radiators, that are vented. If the main vents are at the end of a dry return (and I'm speaking of a 1 pipe system), the path of least resistance will continue to be that dry return, until the vent closes, and then steam will begin to make its way up the risers.

JUGHNE

I'll try to measure the time difference for this sometime this winter. Also I will offer this hypotheses; the 100' of 2" offers more resistance than the steam run outs which are maybe 3-4' of 2" right off the main, I believe there has to be a pressure build up in the 3" at it hits the 2". Maybe I can find the time in the future. If the last run out gets hot before the vents close then what would that mean?

Anyway about those 75's, are they quieter and do they open quicker at the end of the cycle avoiding a vacuum which I hear on this system sucking thru rad vents, minor annoyance for a church and possibly bending the needle on my 32 oz gauge.

Dave in QCA

Another advantage of the Hoffman is they are either 100% open or 100% closed. They will not close unless steam is present at the vent. They reopen fairly quickly Once the boiler stops.

Fred

@JUGHNE , it could mean that you have a reasonably fast vent on that radiator or it could be that there actually is more resistance in that 3" main where it transitions to 2". I'd be interested in what you learn, if you get a chance to test it.

It is quite a few of them. I don't have the headroom for the Gorton #2s, about an inch shy. I do like them. A matter of personal preference too. Cost wise, during the summer months, I can get them new, on ebay for about half the cost of the Gorton 2's which I never see a deal on. Anway, buying them when I do, and on ebay, the cost of two of them, (which is equal to the capacity of a Gorton 2) is about the same.
You do hear them when they close. A light "clunk" and they do open fairly quickly at the end of a cycle but the radiator vents open faster. On a zero or below day, I hear air being sucked in a couple of my radiators, those that are furthest from the boiler.

JUGHNE

I may put a 75 on the antler to see if that breaks the vacuum, but it is 200 pipe feet away from the boiler gauge. And also being in the boiler room above the vent stack may influence the temp for opening.

nicholas bonham-carter

Remember that you are just not trying to get steam to the ends of the mains in x minutes, but rather you are attempting to have the air escape first from the mains with such little resistance (back pressure), that no riser will begin to fill with steam before any other.
This is why I like the Hoffman 40, with its solid construction, and it's slightly higher resistance.
Afterwards, it may be necessary to make some changes in the radiator venting on the upper floors, if steam is arriving late.
The one pipe system has parallel venting-mains, and radiator which must be balanced in the way described above in order for all radiators to see steam simultaneously.
The two-pipe system is a serial venting system in that all venting is done through the returns, and all the radiators get steam at the same time no matter whether the system venting is generous or very slow.
Most two-pipe systems with functioning crossover traps may not need any conventional main vents, as long as the pressure is very low.--NBC

Dave in QCA

@Fred Thanks for clarifying. It is nice to know that the "rule of thumb" is a result of actual performance to the Gorton #2 in real world conditions. My observations are similar. That is to say, while the G2 performs amazingly well on the test bench as well as a cold start up, its performance on a system with insulated mains running at 1 cycle per hour is far less than the 1.1 cfm @ 1 oz. rating. The rule of thumb suggests that in practice, the performance is in the area of 0.23-0.46 cfm @ 1 oz. In my application, the performance was closer to zero. It all depends on the temperature of the mains (and the air inside of them) when the boiler begins its cycle. But, this is probably a subject for another thread.

Dave in QCA

@nicholas bonham-carter Nicholas, I know from your posts through the years that you have a large 1-pipe system that is running at absolute perfection. There were so many times in the first years of my project had I wished for a 1-pipe system too. There is beauty in simplicity. Two pipe systems, all with their endless variety of propriatary setups greatly increase the opportunity for a knuckleheader to do his thing. They greatly increase the confusion that qualified techs experience when walking into a building or boiler room and thinking, "I've never seen anything like this before!" I'm glad we have our 2-pipe system, its working great, but gee, there was a LOT of effort getting it to this point.

btw, I TOO prefer Hoffman #40 rad vents on 1-pipe steam.

Mark N

If you look at the main vent charts, the lower the pressure the less air vented. A Gorton #2 vents half the amount of air at 1oz. that it vents at 3oz. So the lower your pressure the more vents you need. How many are needed at 0.5oz, 0.25oz, or 0.10oz? And as just mentioned by Dave in QCA the temp of the mains at the beginning of the cycle will definitely affect how long in takes to completely vent them. The colder the pipes the more steam it takes to heat them.

Dave in QCA

@Mark N Mark points out that cold mains may vent slower, or rather, may take longer for the steam to get to the end because more BTUs go into heating up the pipe than if they were warm. But, my point was that if the main is still warm from the last cycle, because of good insulation, moderate basement temperatures, and off times of 55 minutes or less, that that temperature of the main and the air inside it may likely be higher than 130F, and therefore, the Gorton#2, if has even opened, will quickly close from the temperature of the air in the main as the steam begins to leave the boiler header, long before the mains have been vented. This effectively leaves the radiators vents to do all of the venting.

nicholas bonham-carter

I am tempted to start some experiments using traps as main vents, as I am sure they will open and close at temperatures closer to steam temperature. I have had a sixth of my Gorton #2's fail closed.
I often wish I had a two-pipe system, like yours!--NBC

hvacfreak2

nicholas bonham-carter said:

I am tempted to start some experiments using traps as main vents, as I am sure they will open and close at temperatures closer to steam temperature. I have had a sixth of my Gorton #2's fail closed.
I often wish I had a two-pipe system, like yours!--NBC

I know right. And also a boiler return trap or a big iron pot of mercury , some F&T's , a few swing checks , the fun stuff. I have a boiler and some vents , lol.

emk2badknees

This forum and you guys are awesome! Thanks for the great wisdom. Allow me to summarize my understanding, just to make sure I have it right before I start altering my system.

1. Where main vents should be located:

Jamie Hall said:

What needs to be vented is the volume of the system up to, and including, the emitters. The volume beyond the emitters does not need to be included in the volume to be vented, although in the case of a dry return it does have to be vented.

This makes complete sense to me that one would not need to vent the dry return. As many of you said, the goal of main vents is to provide sufficient exists so that one can purge air between the boiler and the radiator both quickly and exclusively (i.e. with almost no air being pushed through the radiators air vents).

I'll move the existing vents to one of the locations just before the dry return - probably just after the dining room radiator runout (top right of my schematic - see above). I'll also put one in at the end of the main line running up to the Master BR (bottom right). Fortunately, upon further inspection based on the comments from the crowd, I realized that there is another dead end pipe just after the Master BR radiator runout (it looks like it used to be a radiator runout that was moved or abandoned during a renovation). I'll try using that location for a 2nd vent.


2. Orientation of main vents:

hvacfreak2 said:

It would probably be best to get a 45 and take off to a vent tree from there ( or a 90 where yours is currently ).

I'll get a 45 to get all the vents going vertical.


3. How many main vents:

Fred said:

rule of thumb is the equalivant of one Gorton #2 for every 20 feet of 2" main.

I have about 40, maybe 50' of length from the boiler to the locations where the main vents will be located. Based on that, I should have at least the equivalent of two Gorton #2s on each main. Does that sounds right?


4. What vents are best:
Along this same line of thought, there have been several main vents mentioned in this discussion

• Gorton #2
  
• Hoffman 75
  
• Hoffman 40
  
• Maid O Mist #1
  

How many of each would be equivalent to two Gorton #2s? What else should I consider when choosing a vent? What are the pros and cons between all of these types of vents? Is it a good idea to use all the same brand/type of vent or is there a benefit to mixing it up when you have multiple vents in the same location?

Fred

If you have at least 7" of clearance, you can use the Gorton #2's . With a 40 to 50 ft run, I'd use two of them at the end of the Mains. It takes two Hoffman 75's to equal aa Gorton #2. As has been said, the Hoffman's open faster at the end of a heating cycle, especially if your mains are insulated. Opening faster reduces the noise you sometimes hear at a radiator when it sucks air into the system to break a vacuum. However, most people are happy with the Gortons. I personally use Hoffman 75's. The Hoffman 40 and the Maid-Mist are radiator vents, not suitable for Main venting.
You can use a combination of Gorton 2's and Hoffman 75's that may help with end of cycle noises by opening the mains a little earlier than the Gortons open but that too is a personal choice. They are both very good vents.

BobC

The Maid O Mist #1 is the same as a Gorton #1, they vent at 0.33CFM cuft. They are a lot smaller than the Gorton #2 but it takes just over 3 #1's to equal a #2.

Bob

nicholas bonham-carter

In theory, the vents can be mounted after the takeoff just beyond the last radiator on the main, but in practice, it is much more convenient to have them all on the ends of the dry returns near the boiler. As time goes by, checking their operation on a frequent basis becomes more difficult if they are located in separate places. Verifying that they all close at the same time is easier, as well.
Make sure that as you put them in, that in the main venting phase, the pressure can be kept to a couple of ounces of back pressure.--NBC

emk2badknees

Good idea to watch the pressure to make sure it stays low during start up. The pressure gauge on my boiler is suspect (it reads 2.5 psi when the system is off) and it looks pretty difficult to replace because it is under the sheet metal cover to the boiler unit. I think I'll need to install a new gauge somewhere in the system to get a good reading. Is it reasonable to install a new pressure gauge among the vents in the vent tree or does it need to be nearer to the boiler? If the vent tree is okay, what type of gauge is recommended? The boiler gauge is 0-15 psi. Is that the range I'm looking for or do I want one that is more sensitive than that (i.e. 0-5 psi)?

Jamie Hall

What I meant by not needing to vent the dry returns is that their volume doesn't need to be included in the calculations to figure out how many vents of what size you need. The dry returns on a two pipe system must be vented as they handle the air from the radiators! As NBC says, the best place for the vents -- if you have dry returns -- is at the boiler, where you can get at them. Ends of the mains works well, too -- provided the dry returns have vents for the radiator air.

As to the pressure gauge -- a 0 to 30 or 0 to 15 psi gauge (in most case 0 to 30 -- twice the pressure relief valve setting) is required by code. A 0 to 3 or 0 to 5 is for you benefit to see what the system is doing. The most normal place for it is on the pigtail for the pressuretrol or vapourstat; a better place if you have a drop header is on the drop header.

BobC

A low pressure gauge could be mounted on the mains but most people mount it with the pressuretrol at the top of the pigtail. Most of us use the 0-3 PSI low pressure gauge because there is seldom any need to go over 1.5 PSI on a steam system.

Bob

emk2badknees

I added a 0-3psi gauge (installed just before the pressuretrol) to checked what happens on system pressure during start up and I saw two pressure spikes: one up to 0.5 psi and the other a little over 1 psi. The spikes coincided with hissing from radiator vents.

Next, I created a branch off of the spot where my main air vent is located (at the end of the dry run) where I have vent tree with 4 additional main air vents now. In total, the system has 4 Gorton #2 vents and one Hoffmann #75. After that, I checked the system pressure upon start up confident that the pressure would be much lower. To my surprise, the system pressure went up twice, the first time it went up to around 0.5 psi and the next time it shot up to 1 psi. The duration of the spikes may be a little shorter and perhaps the hissing is less that it was before, but the change was pretty subtle for the content and cost.

When the system fires up, how high should the pressure spike up to? Should I hear any hissing? What am I missing? I was sure this would work.

Eric

hvacfreak2

I read through this thread again and it almost seems like you have some main vents staying closed . I'll bet on a warm start up firing cycle that 75 will be hot and the others not so much. But something else to consider is how much boiler capacity ( power ) you have versus system capacity. Other considerations may be the gauge location and the gauge internals. I always question mechanical gauge readings ( and digital for that matter ).

emk2badknees

The hissing really mainly happens on a cold cycle - after turning up the heat in the morning when the system has been off all night. Is it worthwhile running an experiment where I temporarily take out a vent, not plugging the hole and see if allowing free flow out of the system prevents hissing and keeps the system pressure down?

Can you help me understand more about boiler capacity vs. system capacity? Can the boiler size create this issue? If so, what is the mechanism? Is there any way to address it or am I just stuck with it?

Finally, the pressure gauge is a 0-3 psi gauge bought from Grainger.

https://www.grainger.com/product/GRAINGER-APPROVED-Pressure-Gauge-18C773

It was not easy to find a gauge. Most places seem to carry only 0-30 or 0-15 gauges. Are there certain gauge internals that I should use? Doesn't the fact that the hissing coincides with the spike in pressure suggests that it is real?

hvacfreak2

emk2badknees said:

Doesn't the fact that the hissing coincides with the spike in pressure suggests that it is real?

It really does point in that direction for sure. I would think that the G2 vents would be wide open , they should be. You can try pulling one or 2 out and see what happens , you may have some that have failed closed.

As far as the boiler over firing that is a stretch , just mentioned as a possibility. Is the waterline stable in the glass ? If the water is dirty it could cause steam to release in a way that could cause the pressure to spike. But yeah if you make a lot of steam in relation to the pipe size the result will be a pressure increase. And 1 psi isn't much so firing rate could cause a low pressure gauge to jump.

Fred

You say this happens in the morning, after the boiler has been off all night. If the house temps have dropped several degrees and the boiler has to run for an extended period of time, and, if the boiler is over-sized, it will build some pressure during that long cycle. That a look at the spec plate on the side of the boiler and see what the Net Sq. Ft. of steam rating is for the boiler. Then, you will need to do a Radiator survey and determine the total EDR rating of all the radiators. That number should be close to the boiler rating. If it is considerably less, the boiler is over-sized. Does the boiler seem to short cycle a number of times during that morning heat cycle? Here is a quick way to get a rough measurement of radiator EDR. It is fairly accurate.

emk2badknees

That is a very useful guide for radiator EDR estimation. Thanks. I measured all 11 radiators and, if I did the calculation correctly, the total EDR rating for my radiators is about 100,000 BTU/hour. I've attached the spreadsheet for reference. What I looked at the boiler, it states that the boiler rating is 225,000 BTU/hour! That's a pretty big safety margin. What symptoms will that cause? Could that be the root cause of the pressure spike and hissing on start up?

Interestingly, I don't find that the system short cycles during the morning. It usually has to heat the place by a good 4-6 degrees in that first cycle, so it is on for about an hour or more. It is when it is maintaining the temperature at the target value that the cycles seem short.

By the way, I tried starting the system when it was cold with one of the Gorton vents removed from the vent tree (attached is a picture of the vent tree in case anyone finds it useful). The pressure still spiked up to 1 psi. I took a video, but it is too large to post. While things are starting up, the liquid in the sight glass moves up and down a lot. After a short while (1-2 minutes of fluctuation), things settle down and the pressure stays steady at 0.5 psi.

Fred

Part of your problem is likely the fact that you have four Gorton #2's and a Hoffman #75 all on a 1/2" tapping out of your Main. A 1/2" tapping/pipe has the capacity to vent the equalivant of about 2 Gorton #2's at 1 ounce of pressure so over half of your venting capacity is wasted. Do you have another tapping anywhere in that area that you can move half of youyr vents to? Can you increase to 3/4" or better yet 1"?
The boiler should have a SQ. FT. rating on the rating plate. What is that? If it is not there, then you need to verify that the 225K BTUH is net output, not Input and there is a "Pick-up" factor built into the net BTUH, usually 33%. It still sounds like your boiler is over sized but not as much as you think, based on the numbers above. See if you can verify the numbers on the rating plate.

ChrisJ

Wow,
Interesting discusssion.

I can say with little doubt that steam does start to travel out to the radiators before the main vents close. I suspect, it does this on pretty much any system.

How much, is another story.

Very easy way to find out.
Start your boiler and go down and hold a run out a foot or two away from the main. Tell me if it gets hot before your main vents get steam.

Try one close to the boiler.

@Fred ?

Fred

@ChrisJ , I to believe steam will start to head out into the radiator run-outs before the mains are completely evacuated. I think the real issue is that any run-out(s) near the end of the mains won't because steam hasn't gotten there yet. Also, I would say, even the run-outs, near the beginning of the main may be somewhat of an issue, depending on how much resistence there is, relative to the larger vents at the end of the mains.
After all, some systems have run for many decades with no main venting, just slower getting steam to the rads.