Return Lines under Main Vents get Hot before the Vents

Ps44

Have a one pipe system and notice that the return lines under the vents get hot before the main vents do. That doesn't make much sense to me since the steam is coming from the steam pipes above so just curious if that is normal or not. Thanks for indulging my curiosity.

Jamie Hall

Return lines on a one pipe system? And which vents? Mains? Radiators? Can you clarify for me?

gfrbrookline

The steam has to travel trough the return pipe before it can reach the vent which should be on a nipple about 12" above the return pipe. It take a little time for the steam the go up the nipple.

Ps44

Return lines on a one pipe system. I am talking about the main vents which are on a nipple about 12" above the return pipe. I didn't realize that the steam would travel through the return pipe before it goes to the vent since it seems that the return pipe is hot well before the steam reaches pipes leading to the vent. Thanks.

Gordo

Is that pipe getting hot because of the hot condensate reaching it before the steam gets to the vent? I'm I understanding your question?

motoguy128

In a parallel flow one pipe system the condensate travels in the main in the same direction. If the piping is not hot, on startup, the steam from the boiler is fully consumed heating up the metal before it travels further down the main. As it heat the metal pipe it condenses. That condensate travels down the pipe and starts to heat up the main a little well before steam gets there.

Steel and iron consume about 1 to raise 1 lb by 10f. So 4000 of piping will take 56,000 btu. So assuming the main was perfectly insulated, it would take a boiler with an output of 140000btu, 24 minutes to heat that pipe from 70 to 210f, once it starts making steam.

But it will have made about 14lbs of condensate to heat that pipe, or just a little less than 2 gallons.

Fred

You have a one pipe system. Steam has to travel to down the pipe until it reaches the vent. It is that steam temp that closes the vent.

JStar

Is the return getting hot in the opposite direction that the steam is moving?

Jamie Hall

I still want to know... what is steam doing in a return? Or is this one of those cases where folks are referring to the extension of the steam main beyond the last take off?

JUGHNE

Jamie, would we consider the dry return actually a non heat delivery steam pipe until there is an air vent at which point the steam should stop moving?

EBEBRATT-Ed

I have seen this before where the steam main was double fed.....steam coming from both ends with the vent in the middle. Didn't work very well as soon as steam gets to the vent it shuts and then you have trapped air in the other pipe.

@Ps44 How about some pictures of the system?? That would help

Jamie Hall

JUGHNE said:

Jamie, would we consider the dry return actually a non heat delivery steam pipe until there is an air vent at which point the steam should stop moving?

This is an area of fuzzy terminology which often leads to problems -- so I am a bit of a fanatic on my interpretation of the words! Which makes me a nuisance, I suppose...

As I use the terms:
A steam main (perhaps better "line") is any pipe directly connected to the boiler header. Clearly it must be vented either with main vents or with a crossover trap to a dry return. It may, or may not, continue past the last runout.
A dry return is any pipe used to conduct air and condensate, but not steam. In a properly operating system it should never have steam in it, but it must be vented. It has no direct connection to a steam main.
A wet return is any pipe located below the system (usually boiler) water line which is used to conduct condensate from various drips from steam mains or dry returns (on some occasions individual radiator returns or risers) back to the boiler.

The problem is that many times the section of the steam main between the last radiator runout and the boiler is referred to as a dry return in parallel flow systems. While it does fit part of my definition of a dry return -- it conducts condensate -- but it's confusing. For instance, two or more of this type of "dry return" must not be joined together, while two or more true dry returns not only can be joined together, but sometimes must be and quite commonly are.

But I have been accused of being pedantic and fussy and admit that that is sometimes true...

JUGHNE

To me, the term "dry" or "wet" return pipes refers to them when the boiler is off and all water is done flowing/moving.

But, to me, while in operation the dry return moves air, condensate and steam up to the point where the air vent is.
Then after that point the air is compressed by the steam, but not pushed thru the wet return at the boiler or any loop water trap. (hopefully).

Again to me, the wet return is always full of water.....add a pint at one end and a pint comes out of the other end. Just like the water tube water level.
(hopefully the piping does not raise above the water level of the boiler).

Hap_Hazzard

To me, a dry return is a pipe that runs horizontally from the end of a parallel flow main, remaining above the "A" dimension, until it drops somewhere near the boiler and connects either to the wet return or the Hartford loop. If there's more than one main, there has to be at least some amount of wet return so the dry returns can be connected below the level of the Hartford.

Jamie Hall

One pipe vs. two pipe, I suppose. In a one pipe system, yes, the continuation of the steam main does move air, condensate if parallel flow, and steam to the vent. In two pipe, air and condensate, but never steam unless there's a bad trap.

ethicalpaul

Now I see why I was confused by this term for so long. It’s because no one knows what it means 😂

I come in on @Jamie Hall’s definition. But I wish there was an easier name for “that part of the main after the last radiator and before the wet return”

Hap_Hazzard

How about "wasted pipe"?

To my way of thinking, the main vent should be immediately after the last radiator, and the dry return or drop to the wet return should come immediately after the main vent.

ethicalpaul

I agree! See https://forum.heatinghelp.com/discussion/178547/winter-return-repipe-project

But some dead man gave me that extra pipe and I assume I'm not the only one

Fred

In the case of this poster, my assumption is (was) that he/she has a pipe at the end of the main that turns and finds its way back to some vertical drop to a wet return. That may be all the way back to the boiler, before the drop. In any case, on a one pipe system, steam is going to travel to the point where it can push air out, until the vent closes. If that happens to be back at the boiler, that's where the pipe will be hot. Of course more info from the Poster would clarify his/her situation.

Hap_Hazzard

That was pretty much my assumption too. Of course, if there are multiple dry returns, and they connect above the water line, they'll fill with steam too.

Ps44

I did not think this so confusing but here are the pictures. One shows the top where the main vent is. On the right of that picture is the steam main line coming from the boiler. The other shows the bottom where the return line leads back to the boiler. Hope this helps.

Hap_Hazzard

Of course you didn't think it was confusing. You've seen it!

Yes, the pictures make it much clearer, thank you.

So, how much of that pipe is getting hot before the vent closes, and how hot is it getting? It might be hard to answer that question without a non-contact thermometer. Do you have one?

I suspect what you're feeling is just hot water running down the drip line, not steam. There'd be no reason for steam to enter a vertical pipe filled with air and sealed by standing water at the bottom, but when the condensate starts to return from your radiators, it's almost as hot as steam. The effect of condensation is to turn 212° steam into 212° water, so it's still pretty hot for a while, but I think you'll find that, if you measure the temperature, that pipe isn't quite as hot as the main gets when it's full of steam.

Ps44

Yes, yes and yes. What you say "Hap_Hazzard" does make sense. Not much of the pipe is getting hot, just the top portion. It is definitely not as hot as the main gets because I can hold it while with the main I have to let go right away. Interesting conclusion and I must say I learn something new every day. Thanks so much to you and to everyone :-).

Hap_Hazzard

No problem. And, by the way, that water is actually hot enough to provide a modest amount of heat in a basement or garage via a small radiator or baseboard if the logistics work, and since the water is already condensed, it doesn't add to the EDR load, but it does add to the time it takes the boiler to turn the water back into steam (called the pickup factor).

motoguy128

It will reduce steam output. But it’s not a lot of flow. .33gpm for a 200k inlet boiler. At a 50f delta T you get about 8000btu.

I thought pickup was just heat lost heating the piping and radiator metal itself. Not necessarily the heat needed to reheat feed water. That would add to pickup.

Hap_Hazzard

motoguy128 said:

I thought pickup was just heat lost heating the piping and radiator metal itself. Not necessarily the heat needed to reheat feed water. That would add to pickup.

You're right. I've been misusing the term. I wonder what the correct term is for the energy it takes to start steaming?

acwagner

I always thought the term was "piping and pickup factor" which refers to piping losses and extra output for initial system start. 15% for each for a total of 30%.

Hap_Hazzard

Yeah, I think that's how it's used. Of course a condensate radiator doesn't just add to the system start cost, because it keeps extracting heat from the condensate while the boiler is running too. It definitely contributes to the energy consumption, but it doesn't affect EDR.

Dave in QCA

@Jamie Hall Hall, I feel your pain and I agree with your comments. Adding my 2 cents...
Regarding terminology of steam system piping such as dry returns, wet returns, etc. I find it easiest to use Dunham's terms. They refer to the end of a steam main in a parallel flow system or a 2 pipe system as the "End of Steam Main". At that point there is a drip leg that goes down to the wet return, which is below the water line of the boiler. Depending on the location of the Main Vent, there may not always be steam in the end of main, but it is subject to the pressures of the steam main and if a cap or plug is removed, it will certainly deliver steam. "Dry Returns" on the other hand, are return mains on a 2-pipe system which are located above the water line. They will never be full of water and will never have steam in them, unless something is malfunctioning.