sizing main vents

dobro23

a few quick questions about venting. 1st, how do i size the main vent on a 2 PIPE system? is the length of dry return figured into this or just the length of supply main to the last radiator? 2nd, on a 1 pipe system with dry returns back to the boiler, should the main vent be 15" back from the drop to the boiler or should it just be after the last radiator even if that is 30 feet away from the return drop? Think thats it for now. thanks guys!

JUGHNE

For the 2 pipe is the end of the steam main dripped into a wet return or an F&T trap?

For the 1 pipe after the last rad connection is thought best.
However it is nice to have them in the boiler room in sight.
If there you are just bring the steam closer to the drop.
More time to get steam to the main vent but IMO the same time required to get steam to the last rad run out, regardless of where it is.

Jamie Hall

On the two pipe -- size it for the steam main. Don't worry about the dry return -- it doesn't (or shouldn't!) ever get steam.

That said -- the dry return on a two pipe system must have it's own main vent, usually placed near the boiler where it turns down to become wet. That vent must be at least as big as the steam main main vent.

Further, some two pipe systems don't have steam main main vents at all -- they have crossover traps instead, which vent the steam main into the dry return -- which is, in turn, vented as noted just above. If you have crossover traps, do not put a main vent on the steam main! At best it is a redundant waste of money; at worst it can defeat some of the ways in which the system is intended to operate.

Fizz

Yet some 2 pipes have no steam main vent or crossover traps, though they are rare, and are vented at the end of each dry return, or at one vent above boiler where 2 dry returns meet. One such system is the Richardson, which is in our house, and is 101 yrs old.

dobro23

so, as the original poster to this thread i should tell you what prompted it. i installed a boiler 2 winters ago in a house which is a little strange for my area. I consider myself to be pretty good with steam and didn't really see this house as a challenge. I knew we would be making the system work 1000 times better than it has in years. here are the technical parts. we ripped out an old smith mills with 2 mechanical zone valves and check valves on the wet return. replaced with burnham megasteam 513 and abandoned 1 of the mains and wet returns. steam in that section of the house hasn't worked in years and was replaced with hw baseboard many years ago. so now it's only 1 zone. i believe that the new load on the house was around 490 if i remember correctly. there are now 8 radiators total. heres where it gets weird. 5 of them are 2 pipe with 1/2" sarco traps. the other 3 are one pipe with hoffman 1a vents. the main serving all of these is 3" as it leaves the boiler room. it remains 3" all the way to the end where it drops into an F&T and returns to the boiler as 1-1/4. right at the drop is where the main vent is. BUT on the main just before the F&T is a tee that ultimately is the take off for those 3 1 pipe radiators. there are 2 take offs on this branch that probably had radiators at one time but have been removed and replaced with hoffman #75 vents. the main vent back in the boiler room at the drop is also a hoffman #75. when we replaced the boiler we rebuilt all the traps, replaced the entire F&T and replaced the main vent, the radiator vents on the 1 pipe radiators and the main vents on the end of the 1 pipe supply take off. we also replaced the entire wet return. every thing was fine till last month. read on in the next post.

dobro23

got a call completely unrelated to the steam system last week. i wasn't the tech to go but he told me that the steam feeder was feeding a gallon or so every hour and the counter on the VXT was over 300! i know that from dec 2016 to oct 2017 we only fed 24 gallons and now were over 300 in 3 months! he also said that the main vent in the boiler room was pouring out water like a faucet! not sure what he did from there but scheduled us to go back and investigate which happened yesterday. I went out there yesterday to find everything pretty much working normally and then out of nowhere the vent started pouring water and the feeder turned on. the homeowner said it's been feeding about once an hour for at least 2 weeks. i checked the obvious stuff first and although i can't remember exactly i want to say the level in the boiler was normal but was occasionally dipping below the lwco. there was no condensation in the glass and the water wasn't running down from above. i closed the service valves, pressurized the boiler and blew out the mudleg, i replaces the main vent with a big mouth and piped the discharge to drop into a 5 gallon bucket for curiousity. i verified the feeder settings to LWCO and 2 minutes. the pressuretrol is the newer clean plastic front kind. 404F i think. it was set to 2 with 1.5 subtractive differential. I watched it for about an hour, did a few more things, hoped for the best and left planning on coming back today to see what happened over night. read on....

dobro23

so today i showed up to find that the feeder fed 22 gallons in 24 hours and my 5 gallon bucket under the main vent was overflowing. the brand new bigmouth vent is probably shot but anyway, the boiler waterline was normal level and stable. no white smoke up the chimney either. the vent wasnt leaking when i got there but obviously yesterday did not cure anything. every radiator still heats quickly and evenly etc. no banging etc. so today i turned the pressuretrol down to 1 psi and .5 subtractive differential, installed a wye strainer on the main vent in the boiler room, set a 6 minute feed delay as opposed to the 2 minutes previously, watched it run for over and hour and verified everything again. i assume the traps are all working based on what i felt with my hand. i didnt actually put a thermometer on the trap discharge or any other method. dont really have alot of access to any piping after the boiler room. the feeder didn't kick in at all in about 3 hours today or yesterday. after steaming for about an hour the boiler finally started cycling on pressure. at this time the water level was a little lower that i would've liked. but still acceptable. gonna check on it again tomorrow with the changes i made and see if anything is better but not sure what to do if its not! anything you guys can tell me is helpful! i have pics of the old and the new but i dont know how helpful they will be.

BobC

The bigmouth has no float in it so it will not stop water. How far above the NWL is the vent that is pouring out water?

If somethings on the crawly edge lower pressure would help and make sure there isn't a flap of crud intermittently blocking the ptrol in the boiler or at that tiny hole at the base of that ptrol.

A skim session might be worth it while your looking around.

Bob.

dobro23

update as of this morning. feeder only fed 1 gallon since about 2pm yesterday and the bucket i have under the vent only has about a quart of water in it. wondering if it would have been more if i didn't install the wye strainer in the vent yesterday? at least it seems like i am on the right track. just strange that everything worked fine for a year or more and then this all of a sudden.

DanHolohan

You have F&T traps at the ends of the steam main. The condensate from the traps drops into a wet return. You can't do that. The only time you use F&T traps at the ends of steam mains is when the drip from the trap goes into a dry return, to which the returns from two-pipe radiators are attached, or if there's a condensate- or boiler-feed pump on the mob. There's no pressure beyond the traps (especially now that you've rebuilt them). The only pressure available for condensate to enter the boiler is the static weight of the water between the F&T's discharge and the boiler water line. That's clearly not enough so the condensate is backing up and overflowing from the main vents, causing the feeder to feed.

You need to remove the F&T traps. They never should have been there.

And it's not uncommon to have a mix of two-pipe and one-pipe radiators on the same system.