Single pipe main vent sizing, benefit to excess venting?

steve_173

Assuming I've done the the math correctly, I'm trying to size single pipe main vents for one main being 1.68 cu ft and another 1.77 cu ft. I didn't count any of the branches that go to radiators. I could possibly add a little to each for the volume in the drop down to the wet return that is above the water line? And maybe a tad more for some of the Hartford loop? These are not the large numbers some seem to have. Do I just match this to the vent rating at 1oz? I was leaning toward a Big Mouth, but perhaps just a Vari-Vent is enough? Or do I go for excess venting with one Big Mouth and one Vari-Vent per main and just use the variable one to balance? Is balance much of an issue with these similar mains?

Or do I need to time the steam to the vent location without a vent and then add vents until the timing is close?The mains are not insulated at the moment so that process might be futile.

Both mains end in a tee that drops to wet returns. Currently the vents are directly in the side of the tee. Do I make an antler up at least 6" and back at least 12"? If space doesn't allow this to go back, can it go perpendicular to the side? I think the tee is tapped 3/4" with either a 1/2" or 3/8" a reducer in one of them. Should I stick with 3/4" pipe to have the full 9.5 cfm available rather than the 3.4 cfm with 1/2"?

mikeg2015

IMO, you almost can't have too much main venting. Ideally you want 100% of the boiler steam generation to go to the main, then the risers. Then the radiator venting can be minimized to control the steam.

With 300k output boiler, I'm running 3 Varivents, 3 Gorton D's scattered in 4 locations along the header, then a 1 Gorton 2, 2 Gorton 1, and 1 Big Mouth on a 3/4" tapping. That's what I had lying around. I have 16 Radiators of varying size for about 700EDR. With a 3" main header, the system runs at <0.5oz pressure.

Bottom line, is you need as much main venting as it takes to get the system to heat up evenly while venting radiators primarily based on their size, not relative location.

Although ideally, the dead men ran the header starting at the North or northwest corner of the house and circling clockwise from there so the coldest shaded side of the house get steam first.

Fred

Put a big mouth on each main and be done with it. Each main is close enough to the amount of air to be expelled that you don't need to try to balance with additional vents. Also, the goal is to vent the main(s). If this is a one pipe system, you don't need to worry about the air in the returns and the Hartford loop is filled with water so there is no air to expel there. Also, the radiator vents will push air out of the radiator run-outs. Get the air out of the mains.

Fred

@mikeg2015 , you must mean you have vents along the Main? If they are actually at the header, they will close immediately when the boiler starts to make steam. I'm also not sure that I'd put vents along the main. Each one will close as steam get to it, reducing the amount of available venting by the time you get to the end of the main. I guess it's ok. just seems like poking a lot more holes in the main to accomplish what you could do by putting all the vents on a larger tapping at the end of the main.

Jamie Hall

It is, as has been said, almost impossible to over vent a main (or, on some two pipe systems, the dry returns). However, beyond a certain amount of venting on a main, the additional venting -- while it will do not harm will also be of no benefit. At which point economics gets into the picture.

In the case of @steve_173 , with uninsulated mains, this is even more true. The whole objective of the exercise is to allow the air in the mains to escape at a minimum pressure, to allow the steam to enter and travel along the main as rapidly as it possibly can. The various sizing suggestions which one finds in manuals and all over the Wall here are based on the principle that one wants the air to leave the main in less than a certain time -- so one calculates the volume of the main and picks a vent or vent combination which will do that at some desired minimum pressure.

Fine, so far as it goes.

The velocity of the steam front in the main, however, is only partly determined by the ease with which air can escape. It is also limited to a surprising degree by the need to heat the inside wall of the pipe to steam temperature, or near steam temperature, as the front progresses; The steam will not progress beyond pipe which is cool enough to condense it -- simple as that. If the pipe is not insulated, this will take longer -- perhaps many times longer -- than if it is. Thus one insulates the pipe.

Then it begins to make sense to increase the venting to decrease the time it takes for the steam to get to the end of the main, because then one may indeed be limited by the vents.

If one has a low pressure gauge on one's system, there is a really simple way to find out if one is adequately vented: the pressure should rise, when the boiler fires, up to a plateau in the neighbourhood of a few ounces per square inch. That slow rise is accounted for by heating the pipes as noted above. If the pressure rises to a higher plateau, then it is quite likely that additional venting will help. If it doesn't ever plateau -- just keeps rising -- additional venting is really almost mandatory to get any sort of balance at all. One can also find by experiment that beyond a certain point (which varies with the system) that additional venting beyond the point where the plateau is at a few ounces per square inch does nothing to lower the pressure further. This is because at that point what one is measuring is the friction loss of the steam traveling through the pipe, and has nothing to do with how fast the air can escape -- thus that plateau region will be higher with smaller or longer pipes, or lower with shorter or bigger pipes (note that the plateau pressure can be calculated; doing so is a pain in the neck and not worth the effort).