2 pipe steam return line seems to be getting pressurized

lostinheating

So I previously posted 2 pipe getting air bound. It actually turns out the return seems to be getting pressure as well. What would cause this?

Steamhead

Bad traps, probably. Or maybe someone removed the guts from one or more traps. Look for steam in the dry (overhead) returns, trace it back and you'll find the problem.

Another possibility is steam short-circuiting through drip lines in the basement that connect above the boiler's water line.

the_donut

Steamhead said:

Bad traps, probably. Or maybe someone removed the guts from one or more traps. Look for steam in the dry (overhead) returns, trace it back and you'll find the problem.

Another possibility is steam short-circuiting through drip lines in the basement that connect above the boiler's water line.

If steam is short circuiting on drips, look for leaking wet return.

Jamie Hall

the_donut said:

Steamhead said:

Bad traps, probably. Or maybe someone removed the guts from one or more traps. Look for steam in the dry (overhead) returns, trace it back and you'll find the problem.

Another possibility is steam short-circuiting through drip lines in the basement that connect above the boiler's water line.

If steam is short circuiting on drips, look for leaking wet return.

Or a new boiler which was installed with it's water line set lower than the original. When replacing a boiler, it's water line must match the original boiler's water line, even if you have to put it on a pedestal.

EBEBRATT-Ed

traps are the most likely cause

@the_donut , why do you think leaking wet returns?

Jamie Hall

EBEBRATT-Ed said:

traps are the most likely cause

@the_donut , why do you think leaking wet returns?

Good question. And I don't see how a leaking wet return -- unless it were leaking fast enough to really lower the water level -- could do it. And that would be quite the leak in most wet returns...

jumper

>>Or a new boiler which was installed with it's water line set lower than the original. When replacing a boiler, it's water line must match the original boiler's water line, even if you have to put it on a pedestal.<<

Your attitude is interesting,Jamie. Doesn't a lower water line improve condensate return? Creative piping can resolve other issues. But too high water line requires smart pumps.

Pumpguy

Can you tell us more about your 2 pipe system?

Specifically how and where it's vented and if there are any condensate return pumps?

Jamie Hall

jumper said:

>>Or a new boiler which was installed with it's water line set lower than the original. When replacing a boiler, it's water line must match the original boiler's water line, even if you have to put it on a pedestal.

The problem is drips from steam lines and dry returns into the wet return. It is not at all uncommon to have the end of a steam main drip into a wet return -- and right next to it a dry return drip into the same wet return. This is fine, provided the wet return is wet. You have a nice water seal. But if the wet return got dried out because the new boiler's water line is lower than the old one, you no longer have a water seal and steam blows right on through into the dry return. Presto, problems.

And, since this happens in the far dark corners of the basement, it's easy to overlook.

Raising the water line in the boiler will raise the water line in all the wet returns connected to the boiler -- assuming there aren't some odd valves, pumps, or other whiz-bangs in the way. It won't affect the way condensate returns. The pressure and head dynamics in a steam system are very interesting -- and not that obvious. Suppose we have an ultra simple system -- two pipe, main vent at the end of the steam main, drips from both the steam main and the dry return into a wet return. Nothing else. At the start, before the burner fires, the water in the two drips will stand at exactly the same level as the water in the boiler. When the boiler starts firing and pressure starts to rise slightly in the boiler (let's not get into how much!) the water level in the boiler will drop slightly. The water level in the drips will rise somewhat -- in fact, just enough to balance the pressure in the boiler (28 inches per psi). Eventually steam will reach the main vent, which will close. Now the water level in the drip from the steam main will drop almost to the boiler level, the difference being the friction loss to steam in the main and the trivial friction loss to the condensate now flowing in the wet return. The water level in the dry return, though, provided its properly vented, will stay higher since the dry return is at atmospheric.

But all the time the flow in the wet return is determined by the driving head -- which is the difference between the water level at one end (plus any pressure head) and the water level at the other, plus any pressure head. All that raising or lowering the boiler might do is change the absolute elevations, not the difference.

So... actually, one doesn't absolutely have to match the new water line to the old -- provided one has evaluated the entire system, even in that dark corner behind the old coal bin, to make sure that what is supposed to be wet is still wet, and what is supposed to be dry stays that way.

the_donut

Jamie Hall said:

EBEBRATT-Ed said:

traps are the most likely cause

@the_donut , why do you think leaking wet returns?

Good question. And I don't see how a leaking wet return -- unless it were leakitng fast enough to really lower the water level -- could do it. And that would be quite the leak in most wet returns...

I had one that was leaking a gallon a minute on a buried return. Couldn’t figure it out until the 100 year old mortar gave out after 2000 gallons had been passed beside it.

The symptom was steam in the wet return and hammering. Combination of condensate flashing to steam from pressure loss and eventually water line in return dropped below water seal on return side, so steam short found the leak to atmosphere before pushing air out of upper levels.

lostinheating

Ok little more about system and problem. It's existing boiler. 2 pipe system 21 floors, one line (kitchen c line) had a radiator with air vent w trap on it. Removed air vent everyone below lost heat. Tried looping it and still no heat. Reinstalled air vent but this time w valve on supply. Gave heat shut supply and was getting air blowing out vent. Everyone was getting heat again w vent reinstalled. All traps accessible have been changed. Lot of them have been capped on supply and return.

the_donut

Are returns vented? Gravity return or pumped? Pictures of boiler, returns, traps and piping would be helpful.

lostinheating

Hard to trace pipes all In chase behind walls and drop ceilings. It's a brand new vacuum pump system as well. Problem existed before as well.
Spoke to engineer and he suggested system is double trapped some where.

nicholas bonham-carter

They call the double trap a crossover trap!—NBC

the_donut

A sketch of layout might help, but pictures of piping in boiler room will be helpful, around vacuum pump, boiler, and another vents/traps in room.

EBEBRATT-Ed

Sounds like a mess. Sounds like the Engineer should figure it out. he designed it........from his desk

lostinheating

Building manager doesn't want to spend the money on anything. So I'm just going to put a trap and put it outside. Shareholder happy boss happy and I'll ha e to live with the system not being correct.

the_donut

Double trapping doesn’t let condensate drain. Might have other issues.

lostinheating

At this point it maybe tripple trapping but I won't know until I get ok to make probes in basement apt ceiling