Should I insulate return pipes on a two-pipe steam system?

SteamCT

I had asbestos removed from my heat supply pipes and are going to get them re-insulated with fiberglass jackets. Should I also get the return pipes insulated?

Jamie Hall

That depends. If the returns are actually a continuation of the steam main back to the boiler -- no traps or what have you at the ends, then yes. If, on the other hand, they are actually true dry returns -- separated from the main by a trap and a water seal, and taking only air and condensate returning from the radiators, no. Not needed. If these are wet returns, below the boiler water line, no.

SteamCT

I think they are are true dry returns. The return pipes are separated from the main by traps on the "return" side of the convectors.

Jamie Hall

Sounds like it. What's at the ends of the dry returns and mains?

If they are true, then they don't need to be insulated -- but they do need to be vented.

SteamCT

I know there's one main vent, though aside from that I don't see any other vents.

Jamie Hall

SteamCT said:

I know there's one main vent, though aside from that I don't see any other vents.

They may have been removed -- or never been there, if it was originally cold.

At the ends of the mains, if there is an adjacent --probably slightly lower -- take a look and see if there is a plug in a T on the main and one nearby on the dry return. That may have been a crossover trap location at one time. If the mains are parallel flow (away from the boiler) there may also be a drip to a wet return at that location -- and possibly one from the matching dry return.

SteamCT

SteamCT

Now I’m second guessing myself.  Here’s an “end” of a heat main, though it seems to connect right into the return pipe.  Should there be a trap or something between the main and the return?  An F&T trap maybe?  Sorry, I know this is getting off topic.

Jamie Hall

Somewhere along the line someone did you absolutely no favours at all -- and I'd bet it was whoever did that return in copper.

Bottom line -- yes, there should be a trap, and it should be a crossover trap.

They didn't understand what they had. Let's look at this. You have the steam main, higher on the right. It has a coming off to the left. That T should have been connected to the inlet of a perfectly ordinary radiator trap -- like a Hoffman 8C or something of the sort. The outlet at the bottom of the trap would have gone straight down into a T -- at least the copper guy put one there again, but didn't know why. That way air would get from the steam main into the return, but not steam. Continuing the "what have we here", at the end of both the steam main and the return there is a vertical pipe which goes down to a drip leg and then on down to the wet return. If that wet return is low enough, that forms a water seal which lets the condensate get back to the boiler, but again keeps the steam out of the dry return.

As it stands now, steam will pass over from the steam main and pressurize the dry return. If your system works, it's not working well.

What to do about it? First, check and make sure that that wet return is really and truly below the water level of the boiler. If it isn't, there are ways to fix that, but it's tricky.

Because I prefer crossover traps to steam main vents, the next thing to do is cut that little copper pipe which goes from the steam main to the return on both the vertical and horizontal, install a couple of copper to pipe thread adapters, at least one union, and a trap (the trap may have a union on it; most do). Pipe it so the inlet is connected to the steam main and the outlet to the return.

Do this to all the mains.

Then go back to the boiler, and where the dry returns come together make sure there is adequate venting -- probably two Gorton #2s will be ample.

Make sure all your radiator traps are operational. When the system is going full bore, and the radiators are hot, the outlet from the trap should be somewhere between warm and very warm -- but not quite as warm as the inlet.

Also make sure that the person who redid the returns didn't mess up the connections from the radiator returns to the return!

And once your done, go back and insulate the steam mains. You will find that, if done right, the dry returns will never be more than warm, and there is no point to insulating them.

SteamCT

Great information, thank you very much,  I have some thinking to do!

New England SteamWorks

And some work!

EricBaisch

Is that maybe the main on the left and the dry return on the right, as I think I can see a trap in the picture?  It’s a little hard to see as it’s hidden by some of the other piping.

Jamie Hall

EricBaisch said:

Is that maybe the main on the left and the dry return on the right, as I think I can see a trap in the picture?  It’s a little hard to see as it’s hidden by some of the other piping.

Hmm... you might be right, @EricBaisch . My eyes aren't what they used to be... @SteamCT , could we maybe have a real closeup of that arrangement? I was going partly by the generality that the dry return is usually a bit lower than the main... but looking closer, it seems that the copper is larger than the iron, and usually the main is bigger than the dry return...

dopey27177

Jamie Hall Got that right.

Based on what I see it looks like you have a vapor system or a two pipe steam system from the old days

Jake

SteamCT

Hi all, yes, I confirmed that the pope in the left is the main and on the right is a return.  Will send a closer picture.

SteamCT

SteamCT

JUGHNE

Assuming that those drop into a wet return, where are your air vents, how does the air get out of the system?

Jamie Hall

Much better picture. Thank you! And there is a trap just above the return there. Inlet is the copper pipe and outlet drops straight into the return.

Now -- is that trap working? It is critically important. I it is working, the air (this is partly for @JUGHNE 's question above) travels up the small copper pipe, over to the trap, through the trap and into the return. Which is what it is supposed to do. When steam gets there, that little copper pipe should get steam hot -- but the return just there should stay no more than warm. If the copper pipe doesn't get steam hot, the trap may be failed closed. If the return gets hot, the trap may be failed open.

Either is easy to fix -- find out what make and model the trap is, and then both Barnes & Jones and Tunstall have repair kits.

Now to further pursue @JUGHNE 's question -- the air goes along the dry return back to the boiler, where there must be main vents.

JUGHNE

OK, so the return should slope down towards the boiler, venting air from the main, but why would the dry return be tied into the wet return if all condensate is headed back to the boiler?
Other than the drip from the steam main which goes into the wet return.

Though I see no harm in that situation.

Jamie Hall

Very very often -- I won't say always! -- the steam main is parallel flow, and the dry return parallels it -- which means the dry return is counterflow and also slopes away from the boiler, and therefore needs that drip at the far end.

JUGHNE

I see, thanks Jamie.
Just a point of interest; not far from me is a 2 pipe counterflow system with both supply and return lines hanging side by side with the same slope back towards the boiler.
I looked and never saw that design in any book, but it has advantages of head room in the basement.