Sizing vents on dry returns

delcrossv

Am I correct that a vent on a dry return needs to account for:

1. The volume of the main + risers; and
2. The volume of the rads; and
3. The volume of the dry return ?

Initially it seems so, but I'm happy to be schooled in this.

Big Ed_4

Just the main ..

delcrossv

Ok, because everywhere else steam is condensing?

Danny Scully

Well…is it 1 pipe or 2 pipe? If it’s 1-pipe and the main vent is at the end of the dry return, I would include the dry return piping in your calculation.

Big Ed_4

Condensing should only take place in the radiator is the goal … insulation

The vents allow the air to escape allowing the steam to enter … Resistance is used to direct the steam . The mains should fill first, .Main vents just vent the main and are free opening , keeping resistance on the radiators is important … Once the main vents shut closed , then radiator vents take over … To balance size the radiator vent to the radiator ..

Jamie Hall

It is both simple — and not. First, let's be clear on one thing: are we looking at a dry return in a two pipe steam system, or are we looking at a steam main extension, often, but mistakenly, referred to as a dry return, in a one pipe system?

If it the latter, you size the main vent for the return exactly as you would any other main vent for a steam main.

Now for a true dry return, there are two possibilities: first, there are already main vents on the steam mains. In that case, you are venting only the radiators — but you are venting all of them at once. The other possibility is that there are crossover traps from the steam mains to the dry returns. This is very common on low pressure (vapour) systems and some others. In that case, you are venting all the radiators plus the mains.

But there is an important point to keep in mind: except for some limited situations in vapour systems, the dry returns should always be at or very close to atmospheric pressure. Indeed, in some older systems there was no vent device on the dry returns — just an open pipe, often to the chimney flue. The reasonable (and correct) conclusion to that is that it is simply impossible to overvent a two pipe system dry return. On the other hand, one really doesn't need more venting capacity than would be suggested by considering the mains only if there are crossover traps, and only minimal venting if there aren't, as once steam enters the radiators it will displace the air in them only slowly, since it is mostly condensing instead.

There is one other consideration. On a number of vapour type systems, there are devices which limit the differential pressure between the steam mains and the dry returns. These function by closing the vent on the dry returns and allowing enough steam in to the dry returns to limit the differential pressure. On these systems, there can be one, and only one, main vent location, and that will be at the control device — usually right near the boiler. Crossover traps are always used, and no other vents are permitted anywhere on the system.

delcrossv

@Jamie Hall and @Big Ed_4

To clarify, 2 pipe with crossover traps.

No fancy differential devices. Just gravity.

Imo, one pipe doesn't have returns, just mains. 😉

Sorry if there was confusion.

delcrossv

"The other possibility is that there are crossover traps from the steam mains to the dry returns. This is very common on low pressure (vapour) systems and some others. In that case, you are venting all the radiators plus the mains"

So, that being the case, more is better?

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1834616#Comment_1834616

Yes — up to a point. Vents aren't free, particularly large main vents, and beyond a certain point there's no point to adding more. Not that it will hurt the system or the system performance — it won't — but it isn't worth spending the money.

In my opinion — and limited experience — it would have to be a very large system indeed to warrant more than 2 Gorton #2 vents at the boiler.

ethicalpaul

Too much main venting will hurt performance. A Gorton #2 is physically massive (as in mass as a unit of measure). A Big Mouth is even more so I believe. That mass takes steam and time to heat up in order to close.

Therefore, if you put on too many main vents, that will increase the amount of time that it takes all the vents to close, delaying the time before steam starts getting pushed to the radiators.

There is such a thing as too much main venting.

Jamie Hall

Aye, I might agree — with one pipe steam or with two pipe setups with vents on the mains. But not for systems with crossover traps.

May I remind various people…

In two pipe steam with crossover traps, the vents — if all is working as it should be — will never, ever see any steam, and are never expected to close. There are certain two pipe low pressure (vapour) systems where the main vents are part of the system protecting against excessive pressure differential between the mains and the returns, such as the Hoffman Equipped system — but as a general thing, those main vents will never see steam and never close.

The distinction is not trivial.

ethicalpaul

I guess I don't understand why not just use an open pipe then?

delcrossv

https://forum.heatinghelp.com/discussion/comment/1834664#Comment_1834664

If I recall correctly, they often did.

ethicalpaul

There you go, where's your courage? 😅

delcrossv

https://forum.heatinghelp.com/discussion/comment/1834671#Comment_1834671

They were piped into the chimney. I think I'll go with a BM. I've had good luck with those.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1834668#Comment_1834668

They did…

There were — and are — two reasons why vents were added when reliable ones became available, and both are related to malfunctions.

First, if a trap fails in the system and allows steam into the returns, the vent will eventually close (if there's enough of it) and prevent steam from getting into the boiler room. Hopefully someone will notice that the system isn't working right and find and fix the problem (yeah, right…).

The other is that in many vapour systems there was a mechanism of some kind — and of varying complexity (Hoffman Equipped is about the simplest) to prevent an excess pressure differential between the steam mains and the dry returns, thus guaranteeing that condensate would return to the boiler and that traps wouldn't be damaged. Those mechanisms all depended on being able to pressurize the returns with steam in the event that the pressure control on the boiler failed, which meant that they needed to be able to get steam into the return in the first place — and that a vent would close when it got there.

Then things got fancier — the two Hoffman "vacuum" main vents (#76, still made, and #11 — good luck finding one of them!), for instance, are a little weird: they are normally open, but both of them close and hold a vacuum. The idea there was to make better use of the residual heat of a coal fired, large mass boiler The pressure differentials in a system with those closed on the dry returns is a little counterintuitive in that the pressure in the steam mains may actually be lower than that in the returns… but it works. Not worth the money with low mass on/off firing boilers. They open again as soon as the pressure in the system equals or exceeds atmospheric.

ethicalpaul

Why not now? Is it because when something fails, then steam can appear there?

Jamie Hall

Not sure what you're after there, @ethicalpaul — but the basic thing about two pipe systems (wherever the vents are) is that steam should never get into the dry return at all. Murphy, however, being alive and well, it sometimes does — and so that you don't have steam all over the place (and, incidentally, the related water loss/new water feed!) you can put a vent on it, rather than just an open pipe.

If you have crossovers, then you only need the one vent where all the dry returns come together before dropping to the boiler. If you don't, you still need one there — but you also need them on all the mains.

I might mention that the only time I've ever known Cedric's main vents to close (a Hoffman 75 and a Gorton #2 on a little antler) was the wonderful day when the run contacts in the boiler control fused closed and the boiler couldn't be shut off by any of the safeties… that one was fun.. as I say, Murphy is alive and well…

ethicalpaul

Thanks, I'm not after anything except to understand why there would be a vent on a pipe that is never supposed to see steam. I'm usually not trying to pick a fight, but people seem to think I am sometimes 😅

 Is it because when something fails, then steam can appear there?

sounds like the answer to my question above is "yes", thanks! 🙂