Main vent sizing thoughts

ChrisJ

I often see people using the size of the mains and how long they are to determine main vent sizing and quantity.

I feel this is incorrect and the only real factor is the size of the boiler. The boiler is what determines how much volume is available to push air out.

My reasoning is the only thing driving the air out of the mains is the boiler and this stays fairly constant regardless of the piping size and length. If you take an EG-40 and install it in 100 different houses with all different piping configurations, it's going to produce the same amount of steam regardless. If the piping is cool, the vents could be much, much smaller and obviously larger piping will take longer to heat, but this shouldn't be considered when sizing.

On very cold nights, where the system runs a lot there may be times the off time is very short and the system needs to still vent freely. The piping it self becomes almost invisible because it doesn't cool and the steam doesn't need to heat much, it just shoots down the pipe. The other night, my system from the time the thermostat called for heat, the time the damper took to open and the burner to light, got steam to the end of the 29' main in 65 seconds. If I had less venting, that would have taken longer, produced more noise from radiator vents, and possibly threw the system balance off a bit for that cycle.

I feel main venting should be sized 100% based on boiler size. If a small system has a grossly oversized boiler this becomes even more important because the last thing you want to do is keep the steam from getting to the radiators with an oversized boiler. The bigger the head start the system gets to cope with the excess steam production, the better.

On an EG-40 I currently have 5 Gorton #1's on a 29' 2" main and a single Gorton 1 on a short 11' 2" main. They're different because I needed to delay the short main quite a bit to keep things timed the same. So, in my case 6 Gorton #1's for an EG-40. Some would say this is too much, but I feel it's maybe 1 too many at best.

Not sure if it works this way yet, but that gives me roughly 17,000 btu/h per Gorton #1 going by the boiler's gross output rating of 104,000. I feel a number of 17,000-21,000 btu/h per G1 is probably a really good bet.

A boiler with a gross output of 150,000 would need 8 to 9 Gorton #1's divided in whatever way is needed to balance the mains. If a main tends to hog the steam early on, it needs to get less venting and the extra can be shifted to the slower side(s) of the system.

Just wanted to share that. Maybe it'll help. I only used Gorton #1's because it's what I've worked with, I have nothing against other brands.

Jamie Hall

Right on, @ChrisJ ! And thank you for the arithmetic. I'd never bothered to figure it out… although I have a feeling that the practical capacity for venting may be higher than you suggest. Have to think about that — probably from heating the pipes.

The whole question of how much and where to vent is really fascinating — and while we have interesting and well-tried rules of thumb (and I've repeated more than one of them), it needs more thinking cap type work. It's more complicated than it looks, seems to me.

Particularly for control ono one pipe steam. Two pipe is pretty straightforward, but one pipe? Ah… not so much. Stay tuned!

dabrakeman

I'm sure you would see some correlation between the pipe volume method and the boiler size method. Good point that a grossly oversized boiler could alter the optimal level of venting needed. There are some systems though that have due to building configuration and boiler room location very long mains.

Guess you could create a table based upon cuft/min of steam generated vs total main venting and start doing some comparisons.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1885970#Comment_1885970

Even with really long mains, the boiler is still the deciding factor on how fast air will be pushed out, no?

The boiler is the pump that's removing the air.

dabrakeman

True but it still could influence how much venting you wanted if you are striving for a time to closure.

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1885979#Comment_1885979

But every boiler, and I mean EVERY boiler from the very smallest is way way beyond able to create enough steam to nearly instantly fill a warm or hot main. That's why I wouldn't consider the size of the boiler to be a variable in the equation of how much venting to use.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1885985#Comment_1885985

I've never seen mine fill instantly, so there's something limiting it.

What is it? I'd have to think it's the device producing the steam, I can't imagine anything else?

Obviously, even with 1" thick insulation the pipe gives off some heat, even in say 5 minutes after shutting down it will cool so amount. Then, that heat needs to be replaced, again, by the boiler.

If we continue down that line of thinking, it seems like the longer and larger the main, the less venting you need because the piping is slowing it down more and more, no? The boiler is creating speed, the piping is reducing it?

The boiler's over there thinking….

ethicalpaul

Here are the things that can limit it to my knowledge:

1. How many BTUs it takes to heat the main
2. The main venting
3. How many BTUs the boiler can deliver

Those are the variables. Which of those variables are you willing to change in order to make your main fill up with steam faster?

1. You can insulate the main
2. You can increase the main venting
3. Is anyone actually going to upsize their boiler in order to fill the main faster?

Yes in normal operation #1 slows the steam down a lot. This is why I believe that the vent capacity chart is not very useful. I prefer timing the main vent (at several temperatures) to find out how much venting you "need" vs how much it costs.

This is also why I'm constantly recommending Gorton #1 when others are recommending Gorton #2. We are on the same side of this argument but for different reasons I think, which is of course fine.

PS: note that I said "nearly instantly fill a warm or hot main". I chose those words carefully. This condition only occurs during very cold weather, or possibly when my CycleGard kicks in LOL (but I believe even that doesn't let much or any air back in the system before it lets the boiler fire back up)

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1885990#Comment_1885990

1. Is anyone actually going to upsize their boiler in order to fill the main faster?

No,

But I think it means for those with oversized boilers, we should be recommending more than typical main venting.

ethicalpaul

It can't hurt (within reason). And since approximately 99% of boilers are oversized and since nearly everyone recommends main venting that is much faster than the main can be heated, I'd say we're already there.

Captain Who

https://forum.heatinghelp.com/discussion/comment/1885992#Comment_1885992

No matter the size of the boiler, the volume of air that has to be pushed out will be a factor, and there is no way that isn't dependent upon the length and diameter of the piping between the boiler and the vent.

I think the way to determine where the law of diminishing returns comes in is determined experimentally by timing the steam to the open nipple at the vent location. At some point, if it costs twice as much to get a very small amount closer to that time, it isn't worth it, just for example.

Edit to add - Just as a thought experiment, even if the main was somehow externally maintained at 212 F or so, the larger and longer main is still going to take longer for the boiler to push that air out of the open nipple min. time test situation with the existing volume it takes up and the existing pipe friction effects on both the air and the steam. Larger diameter does reduce friction (and longer length increases it) but the volume out weighs that factor in a really long main imo.

guzzinerd

Does the calculation include vents on the returns? (2-pipe)

My 430k btu boiler would need 25 gorton #1s according to those calculations

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1886002#Comment_1886002

That's @Jamie Hall territory as I've never touched a 2 pipe system.

But with a 430,000 btu/h boiler that sounds like an awfully big system? What's your EDR 1300-1400sqft?

25 Gorton 1s is similar to I think 8 Gorton 2's? That doesn't sound unreasonable for a large system

guzzinerd

Never calculated the edr. It's in a 6 unit apartment building built in 31', mostly original except for the the 1950s boiler. I've got two #2s on each end of the mains with a #2,#1 and #4 on the returns in addition to the crossovers.