Where does the air come from??

phjamo

May be a crazy question, but I can't get my head around it...I have done a bit of reading about one pipe systems in the last couple of days; I understand the purpose of air vents in the system, and as the steam pushes it's way through the system, the air is released by the air vents...but where does the air come from? Aren't one pipe systems closed? OK some comes out of the water, but enough to continuously create enough air to hold steam out of a rad or many rads?



I think if I had a bag of steam and it condensed, the bag would contract and I would be left with a shriveled bag of condensate (I haven't tried it but theoretically I suppose this would happen)...so if steam is introduced into a cast iron radiator and it condenses does it not create a vacuum, not air? Does air get re- introduced through the air vent?



I encountered my first one pipe system, thought they were all gone...now I need to present a case to restore it to my client, if he asks me "where does the air come from?" and I don't have an answer I would look like a ****.

Jerry Withers

air is everywhere...

Steam systems are not closed systems. they need to breath. If they were totally closed then the steam would not be able to push the air out, so they are open to the atmosphere.

ChrisJ

Air vents

The air vents allow air both in and out.  Once the steam condenses and goes away the air vent cools and re-opens to allow air to come back in.



They breath, so to speak.

Chris_83

Now I'm curious too!

The OP asked about venting a single-pipe system, but what about a two-pipe steam system? 



Do the main vents open under pressure or from the rise in temperature? 



Then, once the thermostat is satisfied the boiler is no longer making steam.  As the system cools down, how does air get back into the pipes to replace the volume of the steam? 

Jamie Hall

There are two flavours

of main vents for two pipe steam.  Almost all of the main vents recommended and installed are the sort which close against steam or water, and open to air.  These, if not overpressured (typically less than 3 psi) will reopen without trouble if they cool or if there is no water, allowing additional air out (if any!).  They should all open as the pressure drops, allowing air back into the system when the system pressure is slightly below atmospheric (fractions of an ounce).



There are a few vents which will not reopen but will hold a vacuum.  These were much more common back in the day and on vapour systems; if the systems were really tight they could maintain a vacuum of several psi.  However, it is uncommon to see these in use today.

Chris_83

Venting 2-pipe systems

So, the vents are "open" and will pass air in both directions unless there is steam or water in the pipe where they are located?

gerry gill

yes,

correct.

crash2009

Forget about the air for now

The air part comes much later in the process.  Lets talk about how the steam gets there, and the air part will fall into place. 

What is supposed to happen is boiler makes some steam, steam heads to the main vents first, trys to escape through the vents but the vents close when the steam heats them up.  So now all the air has been expelled from the main and is replaced by steam.  Next steam goes up all the risers all at the same time, If all of the risers have been balanced with proper venting, the steam will go up the tallest risers faster.  The trick is to have the steam arrive at the entrance to all the radiators, at the same time.  So now the air in the risers has been replaced by steam.  Next the radiators begin to fill, all of them should fill equally, at the same time.  Large radiators are vented faster than small radiators so they fill proportionately.  Most of the time the radiator vents do not even feel the steam, so they do not close.  There is still some air at the vent end of the radiator.  But, let's assume that it's a very cold day, and the boiler continues to run.  Eventually, the radiator will fill completely.  Steam will hit the radiator vent, the vent will close.  Perfect balance is difficult to achieve.  Normally the radiators fill completely, within a couple minutes of each other.  Now all the air is out, all the vents are closed, the thermostat is still not satisfyed, the boiler continues to run, some pressure is beginning to build.  eventually the thermostat is satisfyed, the boiler is shut off, the pressure holds for a bit, then the vents start to open one by one, and allow the air to come back into the system. 



That's where the air comes from.

Chris_83

Vents and more vents

I have looked around this site for a complete diagram of a typical 2-pipe system that would include the vent locations, but can't seem to find anything that would help me out. 



Are vents installed at the end of the mains and the end of the dry return? 



I'm a homeowner who has been "following" this site for several years (it has been extremely helpful - thank you!) and want to finally figure out exactly what's installed in my house and how it is supposed to work.  There are return vents installed in the basement, but now I am wondering if there are (or should be) other vents in the system too.

crash2009

Well Chris,

 we would be glad to help you out with venting your 2-pipe system.  There are a few different kinds of 2-pipe.  Could you start a new thread, and we will look at your system specifically