Fuzzy language

Jamie Hall

Dry returns vs. steam mains vs. whatever.

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 -- 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.

Or look at it another way. If I drill a hole in the side of steam main with the boiler running I expect to get steam. If I drill a hole in the side of a dry return, I'd better not get steam. Or this way. If I have a one pipe steam system, I don't have dry returns -- although I may have a nice length of steam main after the last radiator runout back to the boiler. If I have a two pipe system, I'd better have a dry return or my radiators won't be vented.

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

ethicalpaul

I have unconsciously adopted your terminology, mostly by refusing to use the term "dry return" at all (which works for me because I'm pretty ignorant of two pipe systems).

Hap_Hazzard

You can call it whatever you want, @Jamie Hall, but if it's dry and it carries condensate back to the boiler, I'm callin' it a dry return. If it doesn't carry steam from a boiler to radiation, it's not a steam main. The number of pipes any given radiator might have is totally irrelevant to me.

Jamie Hall

The problem, @Hap_Hazzard , is that what you call a dry return must NOT be hooked to any other similar dry return above the water line -- as you have noted (quite correctly) a number of times. But... what other folks (the two pipe crowd) call dry returns can and often MUST be hooked together above the water line.

Many of us know what we mean. But... not everyone does, and I'm trying avoid confusion.

Hap_Hazzard

There are a lot of things you do differently in one-pipe and two-pipe systems. You don't put radiator vents on two-pipe radiators, but we still call them radiator vents, don't we? Sure, that might confuse some two-pipers into putting vents where they don't belong, as we've seen on more than one occasion, but that's too bad. We're not going to make things less confusing by refusing to call things what they are.

dopey27177

Slight correction.

The dry return does carry steam and condensate in it.

If it did not carry steam in it the vent valve on the end of the dry return would never close,

One thing to remember is never install an additional radiator connected to any part of a dry return.

The term dry return basically means under normal operating conditions the dry return unlike a wet return is not full of condensate, Steam and water in this case flow together in same direction until it drops down into the wet return. Hopefully the dimension a is correct.

See correct dimension A in supplied drawing

Jake

Hap_Hazzard

My vents are at the ends of my mains, so no steam enters the returns.

Jamie Hall

There is, or should be, no steam in a two pipe dry return. Ever. If there is, there is a bad trap or, in an orifice system, the boiler pressure is too high.

And, indeed, when a two pipe system is operating properly at normal pressures, the vent -- if any -- never closes. In fact, many vapour systems back in the day had an open pipe (often into the chimney) as the "vent".

Now there are exceptions. For instance, in a Hoffman Equipped System if the pressure differential between the steam header and the dry return(s) exceeds the trip pressure of the Differential Loop, the loop will trip and allow steam under header pressure into the dry return(s) which will close the vent and keep the pressure differential to the loop trip pressure or less -- this keeps water from backing out of the boiler into the dry returns (hence the slogan "Watchman of the Waterline"). Other systems had similar widgets. Also, again in a Hoffman Equipped system, if the original main vent (#15 or #76) is still in use, when the dry return pressure drops below local atmospheric the vent will close, allowing a vacuum to develop in the system. This has been found to be beneficial in some cases.

Comparing this with Jake's comments nicely illustrates why I feel that more precision in this nomenclature is needed. His comment is entirely correct -- for the section of steam main, which he refers to as a dry return, which continues past the last radiator runout in a one pipe steam system. It is simply wrong for a two pipe system dry return.

Chris_L

Sorry @Jamie Hall but I think you are fighting a losing battle calling a dry return (on a one-pipe system) a main. Just Google "steam dry return" and you'll see how common it is to call the return on a one pipe system a "dry return" as long as it is above the boiler water line.

It doesn't make sense to me to consider the dry return a "main." Here is how Webster's defines a main: "a pipe, duct, or circuit which carries the combined flow of tributary branches of a utility system".

That is just what the steam main does. It feeds the radiator run-outs (the tributary branches.) That is not want the dry return does. It does the same thing as a wet return--returns condensate to the boiler.

Heck, on my one-pipe systems, the dry returns are all of 3/4". They couldn't feed steam to much of anything

Hap_Hazzard

Jamie Hall said:

There is, or should be, no steam in a two pipe dry return. Ever.

There shouldn't be steam in a one-pipe dry return either. If there is, the main vent is in the wrong place. There's no reason to fill a dry return with steam. It's just a waste of EDR, especially if the return isn't insulated.

Jamie Hall

I know it's a losing battle, @Chris_L . I've been in them before...

Perhaps it will help remind all of us, though, to be a little careful when we make suggestions to people, since the rules for correct piping for "dry returns" on one pipe system are almost exactly opposite to those on two pipe systems.

JUGHNE

Going back to when the boiler is off and all water has gone back to from whence it came.
A return pipe in either a 1 or 2 pipe system would be dry.
A wet return is always wet and full.
Actually the steam main should be dry also, but would not be considered a return pipe even though it is returning condensate in one direction or the other when steamed up. I guess we call it only a steam main because it is providing steam to the radiation when pressurized.

I am thinking along the lines of electric relay switching nomenclature, NO or NC refers to the switching connections when it is still in the box or on the shelf, no power (steam) applied. Just like the pressure in an expansion tank...the precharge with it still in the box or not installed/energized.

Hap_Hazzard

Like I said, there are lots of differences between one-pipe and two-pipe. The radiators are piped differently for starters, but I'm not going to stop calling them radiators because some people don't know the difference.

I don't think it's that hard to remember that with one-pipe you have to run all returns drips separately until they drop below the water line, whether it's a dry return from a main or a drip line from one of those cast-iron baseboards with the ¾" fittings. You also need to remember that any horizontal return piping above the water line has to be above the "A" dimension. It's not rocket science, but there are certain things you have to remember.

EBEBRATT-Ed

@Hap_Hazzard
as far as air vents on a two pipe steam system the "two pipe air vent system" has to have air vents or it will not work.

On a 1 pipe system is is common to have the steam main reduce in size after the last run out, then it continues back to the boiler the air vent is placed near the boiler and the pipe drops down wet below the boiler water line.

I understand what @Jamie Hall means but most call this a dry return (from the last run out to the air vent). So it's either a dry return or a steam main extension.

The air vent should not be located immediately after the last run out....It needs to be located where the Dry return/main extension drops into a wet return or boiler

Reason being you have to get the air out, your not pushing it through the boiler.

If the vent is after the last runout the condensate and steam will force the air back upstream to the vent and slow the condensate return

Hap_Hazzard

EBEBRATT-Ed said:

@Hap_Hazzard
as far as air vents on a two pipe steam system the "two pipe air vent system" has to have air vents or it will not work.

Interesting. I've never heard of that, but my two-pipe knowledge is next to nothing.

EBEBRATT-Ed said:

The air vent should not be located immediately after the last run out....It needs to be located where the Dry return/main extension drops into a wet return or boiler

Reason being you have to get the air out, your not pushing it through the boiler.

No, you don't. The air can stay right where it is. The condensate just runs down the return and drops down to the water line. There is no reason to fill the dry return with steam.

https://forum.heatinghelp.com/discussion/comment/1235420/#Comment_1235420

EBEBRATT-Ed

@Hap_Hazzard

Not a big deal but once steam hits the air vent and the vent closes and condensate starts forming air will be trapped between the vent and the boiler. When condensate heads toward the boiler the air will be displaced (it has no ware to go) and will have to go back towards the vent. Just because the vent is closed steam will still work its way back toward the vent making it sputter. and steam will still get into the dry return

Hap_Hazzard

How does steam get into the dry return if the air is trapped between the vent and the boiler? You're trying to invent a problem that doesn't exist. I don't have any problem with my dry returns, and they're piped exactly as I've described.

Precaud

I agree, @Jamie Hall . The terms are often inconsistently used and confusing to me. I thought it was just because I'm a newb to this stuff.

Hap_Hazzard

Speaking of fuzzy language, I've been trying to avoid calling the outlet holes in radiator vents "orifices," because that term has a very specific meaning in vapor systems, and a lot of the documentation from vent manufacturers refers to these openings as "vent ports." Who's with me?

DanHolohan

As I understand it, there should be no steam in a dry return. The main vents vent and remain open all the time. Air travels backwards through them when the boiler shuts down. They close only if steam should get into the dry return (failed traps?). They're there as a safety device to keep the steam from getting into the space where the vent is.

Hap_Hazzard

@DanHolohan, I think we're all in agreement that there should be no steam in the dry returns of two-pipe systems. We're debating whether there is such a thing as a dry return in a one-pipe system and, if so, whether or not they should be vented to allow them to fill with steam.

DanHolohan

There is no dry return in a one-pipe steam system. It's steam main all the way to the end, even if no radiators attach to the final part.

Hap_Hazzard

Even if it

• carries no steam
  
• supplies no radiators
  
• only exists to return condensate to the boiler?
  

Also, TLAOSH has a table on page 91 titled "Use This Chart To Check The Size Of One-Pipe Dry Returns."

Fred

@Hap_Hazzard , I agree with you but I also see the confusion. In my mind, Main vents should be somewhere near the last radiator Run-Out and any piping beyond that I consider a dry return (one Pipe System). The remainder of that piping only serves to return condensate to the boiler.
Having said that, I know many systems have main vents at the very end of what we are calling a "Dry Return" only because that is the most convenient place to install vents. In that case, steam will fill that pipe and from my perspective, while the pipe still only serves to return condensate, I consider it a part of the steam main. It carries steam and condensate, just like the other parts of the main. There just isn't any run-outs on that section of pipe (but there likely could be a run-out added if desired).

Jamie Hall

DanHolohan said:

There is no dry return in a one-pipe steam system. It's steam main all the way to the end, even if no radiators attach to the final part.

Thanks, @DanHolohan !

Hap_Hazzard

I think people need to think carefully about where they put their main vents, and I think calling a dry return a dry return enforces a useful distinction. If you say, "put the vent at the end of the main," and your concept of "main" includes the return, you're going to waste a lot of energy filling that pipe with steam—especially since most of those pipes are not insulated. By using a term that describes what it actually does, it makes it more clear that you shouldn't be filling it with steam.

So I'm sticking with the definition Dan gave in TLAOSH.

Precaud

Hap_Hazzard said:

I think people need to think carefully about where they put their main vents

You're inferring they should be at the last run-out, yes?

Hap_Hazzard

Precaud said:

You're inferring they should be at the last run-out, yes?

Yes.

Or is it a take-off. Now there's a term that needs clarification.

Fred

Precaud said:

Hap_Hazzard said:

I think people need to think carefully about where they put their main vents

You're inferring they should be at the last run-out, yes?

They should be just past the last Run-Out, a foot or two is fine.

Jamie Hall

I wonder how much energy you really are losing if you put the vent in a convenient location at the end of the steam main at the boiler instead of just past the last runout. Let's see here...

Suppose we have 10 feet of 2 inch steam main, insulated. That's (very roughly) 400 BTUh. Is that significant? I suppose that depends on how fanatical one is about saving energy. it's somewhere around a 100 watt incandescent light bulb...

Hap_Hazzard

For me it would be 45 feet of 1" pipe. That's 15.45 ft². It's like adding a small radiator.

DanHolohan

Either way is fine. As Gil Carlson (way more than once) said, "A difference, to be a difference, has to make a difference."

JUGHNE

In most cases that "waste" heat is within the conditioned space of the building and perhaps the only heat in the basement.
If too hot down there then insulate the "unnamed" pipe.

Hap_Hazzard

JUGHNE said:

In most cases that "waste" heat is within the conditioned space of the building and perhaps the only heat in the basement.
If too hot down there then insulate the "unnamed" pipe.

It's a lot easier and cheaper to just put the vent in the right place: at the end of the main.

Jamie Hall

We all know the objective of the exercise... whatever works.

JUGHNE

Hap; Easier and cheaper if you are building the system from scratch.
I have a 100+ year old church that has 3" steam mains a 100'+ down each side. No main air vent in sight. Returns went from under floor wet to over head dry with boiler change out (thankfully). Air vent connection was probably eliminated with return repiping.

Then after the last run out/take off there is a 100'+ of 2" going back to the boiler. There is not a union within 100' of the EOM 3 X 2 eccentric coupling.
So cutting into 2", 10' off the floor, to add a tee was not in my idea of easy.
The two returns were bullheaded at the end with one drop to the wet return/HL at the boiler. That had to change. So I added 5 G2 vents on each side, split the returns to drop separately.

To me I would rather have the 10 air vents in the boiler room in sight than above a ceiling in a store room where one might never look at them.
All the pipes were never insulated. The 3" steam main was bare from day one. Only insulated until it got out of the boiler room area. This was the heat for the school rooms in the basement.
And no the insulation was not removed and pipes left bare because I recall sitting in the room as kids tried to throw junk to sit on top of the bare pipe.....60 years ago.
Now all pipes are insulated and basement is cool and needs small electric heaters for the occasional use to take the chill off.

But the extended steam main of 200+ feet of 2" return heats the area enough until some monthly event takes place down there. FWIW.

EBEBRATT-Ed

No mater the vent location, the end of the dry return/main is going to have steam in it weather it's vented or not.

I suggest holding your hand on the unvented pipe and see what happens when the boiler runs

gfrbrookline

If we are talking about one pipe systems parallel flow systems that don't have steam traps, I get the sense most don't, it seems to be common to refer to the pipe that returns to the boiler after the end of the main or a drip as a dry return until it drop to the water line and fills with water to become wet. Most of these systems seem to have the vents near the boiler on the "dry" returns because the end of the main is located in a enclosed ceiling and it is not a serviceable placement.

One pipe and two pipe steam are almost as different as steam and hot water, the terminology needs to be isolated to the system we are talking about and we should not try to blend it together as we are not comparing the same thing.

ChrisJ

DanHolohan said:

There is no dry return in a one-pipe steam system. It's steam main all the way to the end, even if no radiators attach to the final part.

My main vents are on the ends of my 2" mains right after the last radiator connections.

It then drops down to a 1" line that runs back 20 feet or so, and then drops down to a wet return.

You're saying those 1" lines are steam mains even though they do not have steam in them?

DanHolohan

Dry returns exist only in two-pipe systems. They are on the outlet side of steam traps. No steam should ever be in them if the system is maintained, only air and hot water.

ChrisJ

DanHolohan said:

Dry returns exist only in two-pipe systems. They are on the outlet side of steam traps. No steam should ever be in them if the system is maintained, only air and hot water.

Interesting.

I mean, in the end it doesn't matter to me. You can call them Fred for all I care.

I'm just surprised.

All this time I thought I had dry returns when in reality I've got really poorly vented unutilized steam mains.