Fast Main Vent, Slow Rad Vents - How Come?

Chris M_2

All,

Why is it that having as big a main vent as possible, while keeping the radiator vents more restricted is the optimal setup?  In other words, wouldn't it be better to get the steam filling the radiators as quickly as possible, so as to keep the burner's firing time at a minimum?  -I have a Burnham Megasteam 396, Beckett AFG burner.   -Thanks

Mike Kusiak_2

Uniformity of heating

Fast venting of the mains has a lot to do with getting steam to all the radiators at about the same time.



If the radiators have fast venting, steam will tend to fill the closest radiators to the boiler before steam has a chance to get to the furthest radiators at the end of the main. This can result in uneven heating.



Venting the mains first will get steam to the ends of the mains before it has a chance to fill the closest radiators to the boiler, so all the radiators will get steam at approximately the same time. Once the steam reaches the end of the main and the main vent closes, steam can then enter the radiators at a rate determined by their own individual vents, resulting in a more balanced system.



Not only do you want to drive all of the air out of the system as soon as possible, but you also want to do it in a fashion which promotes even and uniform heating of all the radiation. As described above, the best way to accomplish this is with fast main venting and slower venting of the individual radiators.

Chris M_2

Fast Main Vent, Slow Rad Vents: For Even Distribution

Thanks for that clear answer, Mike.  It makes perfect sense. Thanks again.

MarkReynolds1

OK, but...

I get the idea to vent the mains fast (I added several Gorton #2's and my system seems much more responsive to the thermostat). You want to get steam to the radiators quickly

But, why vent the radiators slowly? If you get the air out fast, the steam gets to heat the radiators for a longer time, right? What am I missing here?

Mark

The Steam Whisperer (Formerly Boilerpro)

Take a look here......

This talks about vent sizing.   http://www.heatinghelp.com/article/11/Hot-Tech-Tips/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell

Dave in QCA

You're not seeing the whole picture

Your idea that if you get the air out of the radiators real fast, they will heat longer makes sense at first, but you have to remember that your boiler makes steam at a certain rate.  That rate is not nearly fast enough to fill all of your radiators with steam in a few minutes.  It takes a while.  If you over vent your radiators, your system will end up with severe balance problems.  Here's why.



With a properly vented main, the steam comes rolling down that pipe from the boiler.  You want big vents on the main because you want the path of least resistance to be to the end of the main.  You want the steam to get all the way to the end before it starts to enter the radiators and heat them up.  That way you will get even heat.  On a cycle in moderate weather, your radiators may only heat half way across.  And that is what you want.  It is critical that they all heat about the same amount.  If you have one that gets hot all the way across, and others that only heat about a fourth of the way, or don't heat at all, you have obvious problems.



To achieve even steam distribution, there are two parts.  The first part is fast venting in the mains.  The second of slow venting of the radiators.  If you have TOO fast venting on your radiators, the steam will start to enter some of the radiators before it gets to the end of the main, no matter how big and fast your main vents are.  That is an obvious problem, right?  But that's only the beginning.  To get a clear concept of what goes on in a radiator that gets steam real fast, you have to do a little visualizing. 



Imagine you have a radiator that is connected to a 10 psi steam supply line.  It is up to pressure. There is a very large boiler that has the capacity to maintain 10 psi.  The radiator valve is shut tight, and the radiator is cold.  There is a very ample FAST vent on the radiator, and a pressure / vacuum gauge to let you know what is going on.   OK, now turn on the steam valve.  Open it all the way.  Steam is rushing into this stone cold radiator, and even though it is able to quickly push out most of the air though the vent, the radiator has hundreds of pounds of cold cast iron.  It is beginning to get warm, but no means up to temperature.  Now, shut of the steam valve very quickly and also put a piece of tape of the port on the vent.  Steam condenses and shrinks to water.  The compound gauge is now going to suddenly show that there is a very strong vacuum in the radiator.  And this phenomenon, in very subtle pressure differences, so small that they are almost undetectable, is what will happen in a system where the radiators are vented too fast.  You will almost certainly set up a situation where some of the radiators get steam too fast, and as a result, they start to suck all of the steam that the boiler can produce, leaving nothing for the other radiators in the system.  Sometimes it may be only one radiator left cold.  It may always be the same one, or sometimes the steam is fickle and it will randomly change which radiator is left cold.



Small radiator vents will cause a slight increase in the pressure, perhaps an ounce, maybe less.  Just enough so that the steam is slowed down and kept orderly.  A little bit of steam, going nice and easy into each one of your radiators.  Each one getting its fair share.  The slow vent prevents any one radiator, or little gang of radiators from getting loopy from too much steam and turning into a belligerent HOG that takes over your system and sucks all of the steam that it can get, leaving other radiators cold and hungry.  Slow venting your radiators does not cause steam to be lost or energy to be wasted.  It does however, force your radiators to be considerate and fair, and to share the steam equally with all of the other radiators on the playground.