Main Vent for Wet Returns

bigtoddny · August 2022

Hi All,
I have a single pipe low pressure steam gas system. I need to have the boiler replaced due to a hole above the water line (the boiler EG-45 is about 35 years old). Before I get a someone in to replace the boiler I want to know about the piping in my house.

There are two main lines, one to the front of the house, and one to the back. Neither has a main vent at the end of the line. There is what looks like a wet return that runs from the end of the main lines back to the boiler. I've included pictures. I don't think there are valves at the transition from the main to the returns. My questions are 1) Is this piping ok? 2) Can I / should I put main vents on here?

Thanks for any help.

jhewings · August 2022

Wet returns are supposed to be connected below the water line. You may want to separate the returns and put main vents on each before dropping them down.

EBEBRATT-Ed · August 2022

Sometimes the vents are installed at the end of the steam mains before the mains drop to the wet returns (best location). If the returns come back to the boiler above the boiler water line the vents are sometimes installed there

Jamie Hall · August 2022

As @jhewings says, those aren't really wet returns at all -- in fact, to be somewhat pedantic, they aren't even dry returns (although they are often called that). Rather, they are continuations of the steam mains, and they are there mostly to carry the condensate back to the boiler.

And to cause several interesting varieties of mischief, if not handled properly!

As to main vents, you do need them, and you need one on each of those return lines. The good news is that you can put them pretty much anywhere convenient, provided they are after the last radiator takeoff.

The mischief comes from the fact that they need to be dropped below the water line, again as @jhewings says, before they join together. This won't be hard to do when you are doing the new near boiler piping for your new boiler, but it needs to be done.

BobC · August 2022

While checking that piping make sure it's all sloped so water can find it's way back to the boiler. Trapped water can lead to radiators that don't heat and water hammer as slugs of water get rammed down the pipes.

Bob

ChrisJ · August 2022

@Jamie Hall I may have asked years ago, please forgive me as my memory isn't great at times.
But what by definition is a dry return?

If a pipe is only big enough to carry condensate, and it doesn't feed any radiators, why would that be considered a continuation of a steam main rather than a return that is dry?

I do recall a dry return being something specific, but I cannot remember.

My single pipe system is setup similar to the OP's with just a 1" pipe coming back and then dropping down to a wet return. One of them is a good 15' long and I insulated both although neither are vented.

Jamie Hall · August 2022

I'm a bit pedantic... but to my way of thinking, a dry return is a pipe which is separated from any steam mains either by a steam trap (crossover trap) or a loop seal -- or both -- intended to carry condensate from the radiators and the air exhausted from them before their traps close, and which is well above the boiler water line. Many systems may have a pipe, also well above the water line, carrying condensate -- but since it is not separated from the steam main, it carries steam. I prefer to call those steam main extensions. The difference isn't purely academic, as if there are multiple steam main extensions they drop below the water line before they join, and must be separately vented, whereas dry returns can -- and often do (and in some systems must! --) join BEFORE they drop below the water line, and are usually master vented at the boiler. Additionally, steam main extensions operate at boiler pressure, while dry returns are always at atmospheric pressure (or, rarely, even a slight vacuum). They should never see steam -- in fact, on many older systems they simply vented through an open pipe; no vent (vapour systems almost always have a vent there, but its purpose has to do with boiler water level control and it should, normally, nver close).

There is a slight joker in the deck which I only recently became aware of: it was not uncommon with older one pipe systems with coal fired boilers for the steam main extensions to join at the boiler, effectively making one or more complete loops. We've always said that was a no-no, and that (as I said above) each extension must have its own vent). But -- in the days of coal firing, steam built slowly and the steam mains may not have been vented at all, with all the venting at the radiators -- and that does work for slow firing, but not for the fast on and off of oil or gas.

ChrisJ · September 2022

@Jamie Hall. But mine have main vents at the end of the mains so those 1" pipes don't carry steam, there's an air trap.

Main Vent for Wet Returns

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