No Main Vents in Two Pipe Condensate Pump System

sreja

Hi all,



I haven't posted in a long time, not since 2008 when I got a lot of great help from the wall crowd on some major work we did on a 5 story condo steam heat system (replaced all traps, installed trvs, new boiler, insulated, etc.). And i've also gone through all of Dan's books and loved every page.



So here's the story: The heating system has been working great. No complaints at all. Seems to heat pretty fast too.



But I have a confession to make -- the issue of main vents for our system was one i never fully grasped. And the fact is, we have no main vents in our system.



I've started to get an itch to do some more optimization on our system to try to lower our pressure (we are currently running at about 2-5 psi without pressuretrols to measure low psi). So my plan is to get good low pressure pressuretrols and try to reduce pressure.



BUT BUT before I start all that, I want to understand if we need to have main vents installed.



SO.. here is what we have now: It's a two-pipe system with a condensate return pump.



The condensate return pump has it's own vent, and then right before the return line goes into the pump, a pipe tee's off up 8 feet or so and is just open to the air. (it's actually a very funky situation at the end of that pipe, where it has about 3 openings that used to be closeable, and then a spigot). See photo attached.



I wonder about the history of that strange open air "vent" pipe.. Here's what I can tell you about it, before I got to work fixing our heating system, so many of the traps were failed open, that tons of steam was coming back into the boiler room through the return line, and escaping out of that pipe, filling the room with steam. And i could see people had tried to close off that pipe in an attempt to "keep the steam from escaping". BUT after we fixed all of the building traps, etc. I am *extremely* proud to say that not a drop of steam is coming back through that return line, so it's just air.



There are several F+T traps around the ground floor if that's of any interest, but like i said no "Main Vents" other than what I've described at the condensate pump area.



I assume that that one pipe at the return line "IS" the way air is getting out of our system.



So I guess my questions are all about air venting and main vents:

Is that open pipe enough to vent the air even if i try to cut the pressure down to 1psi, etc? Why are normal main vents mechanical "trap"-like things, and yet we have an open pipe here that seems to be serving same purpose? does it have to do with the fact that its at the "wet return" level? Would we see much benefit from having "main vents" installed?



ps. in the photo it may look like the open pipe thing is connected to another big pipe at the very upper right but that's just an illusion.  the pipe just rises up from the return line and makes a 90 degree right hand turn and is just purely open to the air all the way across despite all of the funky fixtures on it (what were they thinking!?)

Jamie Hall

In theory...

on many (not all) low pressure and vapour systems an open pipe will do just fine.  In fact, a good many vapour systems were piped just that way, with the open pipe going into the chimney.  The trick to it is that first, the top of the pipe has to be high enough so that water won't back out of it -- that's a pressure thing.  And second, the traps on the radiators have to be good.  If they are, the only thing that should ever get to the vent -- never mind get out of it -- should be air, when you think about it.



Hoffman Equipped and a few other vapour systems had contraptions of one sort or another to let steam under boiler pressure into the dry returns, and relied on the main vent(s) to close against the steam.  That was to allow the pressure in the return to get the water back into the boiler if the boiler pressure got too high (which they meant to be about 7 ounces per square inch!).  In all vapour vacuum systems (including those above) the vents closed against vacuum as well, holding the a vacuum in the system as it cooled.  So they needed vents, rather than an open pipe.



The main purpose of main vents, as I see it, is that if steam somehow gets into the dry returns the vents will close and not let it into the boiler room or basement.  In a system which is really running right, they should never close.



And an open pipe makes an excellent vent -- provided everything else is working as it should be!

Jamie Hall

Let me hasten to add...

that the above comments on main vents applies only -- ONLY-- to main vents located on dry returns.  Systems which use main vents on the steam mains, instead of crossover traps to dry returns have to have vents which will close against steam -- you can't use an open pipe on those, and vents on those will close as soon as the mains are emptied of air.

sreja

re: No Main Vents in Two Pipe Condensate Pump System

See if I have this right:



Basically the setup we have now, *ALL* air leaves the system by flowing through the radiator traps and then out through the condensate return line and then out through that open air pipe right at the end of the system.



And "Main Vents" are mechanical vents that one puts on the main steam supply lines that open to air and close to steam, and can evacuate air earlier (and faster) in the process, giving air a shorter way out of the system than having to go through the radiators and out the condensate return line.



The part that still confuses me a bit is how in Dan's books and constantly on the forum here people are always saying how super important it is to have good big main vents to evacuate the air efficiently, and how if you have trouble running at low pressure it could be insufficient main venting, etc.



That's where I get confused -- I don't understand if our system doesnt really need those because we use a condensate pump and can use a simple open pipe at the end of the return lines.. Or if we really DO need the fast-air-venting benefits of serious main vents.



I guess a side question I have is about when the Main Vents really do the helpful work.. I can understand completely why rapidly expelling air would let you get the initial steam out to the radiators faster when you are first turning on the boiler.  But if the boiler is running basically 24hrs a day, do the main vents server any real purpose once the system is constantly generating steam?



As you can see, venting is the one area of steam heating that I've never really "gotten a handle on"...

Jamie Hall

It can be confusing!

But the thing to keep in mind is what one is trying to do: get the steam to the radiation as fast, and as evenly, as possible.



From the standpoint of venting, there are three common system arrangements (like French verbs, there are lots of irregular ones...). 



First is the simple plain vanilla one pipe system.  Here, the air has nowhere to go unless there are vents.  Further, there are no returns as such -- any condensate comes back in the steam mains.  In a setup like this, there have to be vents on the radiators themselves, otherwise the air couldn't get out of the radiator at all, and the steam couldn't get in, and you'd have no heat (which is a handy way to turn a radiator off, by the way).  Now in order to get heat to all the radiators as evenly as possible, one wants to have steam throughout the length of the mains as rapidly as possible -- and the radiator vents can't do that.  So... one puts main vents at the ends of the mains, which are sized to let the air out of the mains just about as fast as the steam can get in.



Second is what I would call the ordinary two pipe system.  Unlike the one pipe system, this system uses condensate (and air) outlets on the radiators.  Therefore, there is no need for vents on the radiators.  However, one still wants to get the steam to the ends of the mains as rapidly as possible, and since the only other place air can go is through the radiators, one again puts main vents at the ends of the mains, sized like one pipe system vents.  But where does the air in the risers and radiators go?  Into the returns, through the radiator traps.  Therefore, if you have dry return(s), they need vents as well.  If you have only wet returns, then the returns themselves each have to be vented near the bottom of the return -- above the boiler water line and B dimension, but below any radiators.



Third is the majority of vapour systems, and more advanced "ordinary" two pipe systems.  These also have a condensate/air return on each radiator, with a trap or some other arrangement to keep steam from reaching the return but let air and condensate out (the contraption varies with the system).  Unlike "ordinary" two pipe systems, though, the problem of getting air out of the steam mains themselves is solved by using "crossover" traps at the ends of the steam mains.  These are just like ordinary radiator traps, allowing air and condensate to pass but not steam, although they may be a size larger than the radiator traps.  They are arranged above the steam main, and drain into the dry return, and they allow air to cross into the dry return freely (any condensate is handled, in most cases, by a drip to the wet return at the same location).  On systems of this type, the only "main" vent(s) is (are) located where the dry return(s) drop into the wet return and back to the boiler.  If the system is running properly, particularly if it is running all the time, the chances are that these vents will never see steam, and will never close.  However, if the pressure gets too great and steam gets past the radiator traps, or a radiator trap fails, they will see steam and they need to close to prevent steam getting loose into the boiler room.  In addition, in some systems there are arrangements to deliberately let steam into the dry return if the pressure gets too great, to force condensate back into the boiler, and in those systems these vents must close for the system to work right.  However, in a great many systems of this type an open pipe will work just as well as a vent, provided everything else is working right (with the bonus that if there is an open pipe, and a radiator trap fails, you know about it by the steam in the boiler room!).



So there is no "one size fits all" here.  Although the majority of systems will benefit from nice big vents on the ends of the steam mains, not all will -- as noted, some vapour systems won't even work properly with them.



This help any?  If not, keep coming with the questions!

sreja

re: No Main Vents in Two Pipe Condensate Pump System

Hi Jamie,



Thanks for talking about this with me -- it is helping and I love that I am learning this one area that has always stumped me.  I hope I am not trying your patience.



Ok so a couple of follow ups:



I think I understand the 1pipe scenario pretty clearly -- there's a case where one clearly needs main vents, no way around it.



The second two scenarios..



You say "if you have dry return(s), they need vents as well.  If you have only wet returns, then..."



But I thought that every system has "dry returns" and that this referred to as the part of the return pipe that is above water line of boiler.



Tell me if I'm wrong here, but i'm thinking it works something like this:



You need to be able to get air out of the system.  In a two pipe system, air will flow through the radiators and into the return line, where it must be vented at some point.  In addition, you could OPTIONALLY have "main vents" that let air out from the SUPPLY lines, using a mechanical vent that acts like a trap, blocking steam from escaping but allowing air out.  And the idea is that these main vents in a two-pipe system help speed up the removal of air, but they aren't strictly needed because air does have another way out, through the return lines (which are vented).



And furthermore, that in a system that is working well, the return lines should not contain steam (unless a trap somewhere is failing), so a tall open pipe at the end of the return line should be fine in this scenario.



Assuming i'm correct in that understanding above, I'm still not sure of the answer to my lingering questions, which are:



1. I can understand completely why rapidly expelling air would let you get

the initial steam out to the radiators faster when you are first turning

on the boiler.  But if the boiler is running basically 24hrs a day, do

the main vents server any real purpose once the system is constantly

generating steam?



2. We only have the open pipe that lets air out at the end of the return, and we have no main vents.  If we want to run the boiler at low pressure (under 1psi), is the lack of main vents going to make that harder?



3. Would adding large main vents save us enough money (or any?) to make it a worthwhile investment? I realize that this kinds of question is hard to answer, but I guess I'm still just struggling with how much benefit main vents get you other than the very initial removal of air from the system, which only happens a few times per heating season (or am i wrong about that?).

Jamie Hall

A more specific answer...

To question 1 -- no, not really.  In principle, this would apply to all systems -- if one is constantly generating steam, even at absurdly low pressures or even a vacuum, once you've gotten the air out it's out, assuming the system is sealed.  This is one reason why with coal fired boilers, back in the day, and two pipe dry return systems there was, typically, only one vent on the dry return, and it was small.



To question 2 -- again, no.  You might be a little slow in initial distribution, but as you say -- if the thing is running at some level virtually all the time, who cares?



To question 3 -- probably not (do I hear cries of "heresy"!!! in the background?).



The ringer in the deck is that most residential systems don't run all the time any more, and when the burner shuts down and the boiler cools off air will come back into the system -- it is a rare system indeed, vents or no vents, that is perfectly sealed against a vacuum.  In most residential systems this happens so fast that one typically ignores it -- it's a matter of minutes, if that.



To go up to the comment about dry returns etc., most two pipe systems do have dry returns -- as you note, the parts of the returns which are well above the water line in the boiler.  However, not all systems do, and I wished to be complete...  Otherwise, you are correct in your understanding, so far as I can see.  And again, for the sake of completeness (just in case others are reading all this!) do note that some vapour systems must NOT have vents on the steam mains!

sreja

re: A more specific answer...

Awesome.



Thank you so much for taking the time to answer my questions Jamie!  Once again you guys have made working with steam heat a pleasure and an education.

sreja

re: No Main Vents in Two Pipe Condensate Pump System

ps,



Dan on the off chance you are reading this:  I have loved all of your steam heating books and read them cover-to-cover, highlighting and made notes in the margins (my favorite is the big one, the Lost Art of Steam Heating).



Main vents are the one thing in the books that I found consistently confusing. Confusing because most of the time you are remarkably consistent about saying without qualification that Main Vents are so incredibly super important -- but then occasionally giving mixed signals when talking about different kinds of steam systems (for example on p.136 you say "the vented condensate receiver is usually the air vent for the entire system").



You might consider this issue as one worth revisiting in later editions.

Steamhead

On Vapor systems

very often you will find radiator or F&T traps venting the air from the steam mains and routing the air to the dry returns. In this case you would only have one or more centrally located dry return vents, such as the one on a Hoffman Differential Loop.



Check the ends of your steam mains. Post pics of what you find there.