Two pipe air venting

Isacc

Newby here.  Please help me understand how the air is vented out of a two pipe steam system.  Radiators have a thermostatic valve and a steam trap. (No air vent)
I noticed it has a main air vent in the main steam line.  If the air goes thru the steam trap,  it ends up on the condensate return line and then how does it get out of the system?

Jamie Hall

There is supposed to be a main vent on the return line. A big one. Usually located near the boiler.

pedmec

Are you in a home or a apartment building? I ask because if your using a thermostatic radiator valve then you have no control over the boiler and is a common set up in apartment buildings. So you most likely might have a boiler feed tank or condensate pump depending on size of the boiler. If that's the case then the vent for the system would be on the tanks and would be just an open pipe coming off the side of the tank and pipe overhead.

Pumpguy

If the return lines on your 2 pipe system discharge into a condensate pump, there will be an air vent pipe on the pump's receiver tank.

That's where the air gets discharged to atmosphere.

If steam comes out of this air vent pipe, that means steam is getting in from a leaking steam trap in the system. To fix this, you need to find and fix or replace the offending trap.

Isacc

This is an office building.  Condensate pumps send water to the feeder tank.  

Pumpguy

Isacc said:

This is an office building.  Condensate pumps send water to the feeder tank.  

Are there any vacuum pumps on this condensate pump's receiver tank?

pedmec

What do you mean condensate pump sends water to the feed tank? they have condensate pumps acting as transfer pumps? why are they doing that? If that is what they are doing then you need to make sure that the returns to the condensate pump are not trapping condensate. the tank vent is the vent for that part of the system. Someone adding a main vent sound like they had an air problem.

You should have an f&t trap at the end of your main supply. on the outlet of the f&t trap you should start heading back the the boiler feed tank or condensate pumps. all the thermostatic trap outlet should connect to that return. The return is now vented by the the tank vents. the vent should not have a plug in the tank.

Pumpguy

pedmec said:

What do you mean condensate pump sends water to the feed tank? they have condensate pumps acting as transfer pumps? why are they doing that? If that is what they are doing then you need to make sure that the returns to the condensate pump are not trapping condensate. the tank vent is the vent for that part of the system. Someone adding a main vent sound like they had an air problem.

You should have an f&t trap at the end of your main supply. on the outlet of the f&t trap you should start heading back the the boiler feed tank or condensate pumps. all the thermostatic trap outlet should connect to that return. The return is now vented by the the tank vents. the vent should not have a plug in the tank.

This arrangement is very common on large systems, especially when the return line is near the floor.

Another reason is if the boiler doesn't have enough water storage capacity for the boiler to fill the whole system with steam before condensate returns and it shuts down on low water.

Condensate flows by gravity to the lowest place in the system where the condensate pump's receiver tank is located.

The condensate pump operates start - stop, controlled by the float switch, sending condensate over to the boiler feed pump's storage tank.

Boiler feed pump(s) send condensate from this storage tank to the boiler(s) as called for by the boilers' water level controllers.

The boiler feed pump storage tank will have a low water make up control, and is sized to provide the boiler with feed water for a given amount of steaming time; frequently 10 and sometimes 20 minutes.

An arrangement like this prevents a starve - then - flood condition at the boiler. Also, the boiler is not taking on, and later dumping condensate; a condition which is sure to cause corrosion to the boiler and all steel and iron piping and the boiler itself.

pedmec

Most of our large commercial buildings are gravity drain right into boiler feed tank. The only time we use condensate pumps as transfers are for steam coils as steam coils are everywhere in the building. Generally you try and get everything to gravity drain into the boiler feed tank. It mostly depends on your piping configuration.

I even have an old Nash vacuum system in one of the building i work on. With the liquid ring seal.