Vacuum breaker for steam system

Onepipe

I am just starting to use vapor stats and I have a system that I installed a 0-30oz gauge to set the v-stat with. I am noticing that on a long run when the system cycles on pressure it is pulling a vacuum. I’m worried about damaging my gauge. I was thinking about putting a vacuum breaker on the main to help with that issue. Is there a recommended vacuum breaker or am I going to cause other issues? I was going to install it tee’d off from my main vent riser. 

Thanks

Jamie Hall

What kind of main vent do you have/ It should open on a vacuum, unless you have a Hoffman #76.

Onepipe

I have a combination gorton #2 and MoM #1’s. I had the system running for 45min before any pressure started to build. It’s a new install so I have been skimming, runny for a wile, skimming exc. so it’s running much longer than normal. I noticed it at the end of the long run when the v-stat shut the unit down it dropped to 2oz and there was a call for heat again BUT in the time it took the start up sequence to fire up it pulled below zero on my gauge. Eventually when I shut it down it took a good 5-10 min for the mains to open up. I’m wondering is some of this issue is because I am running it a lot longer than normal. All the vents have been replaced, system balanced and vents added to mains. During normal run it’s hovering at 1oz and every radiator is starting to get hot equally. So the system is doing great, just noticed the vacuum thing.

ethicalpaul

What gauge did you install?

EBEBRATT-Ed

No need to put a vacuum breaker on as long as the vacuum does not hold up the condensate return draining back to the boiler but the vacuum will do a low pressure gauge no good.

Maybe you showing a vacuum at the boiler but not out in the system where the vents are but that makes no sense.

Mad Dog_2

New install?  We wanna see the pictures!  Thanks Mad Dog 🐕 

dabrakeman

I understand the issue you are having because I experience the same issue. I had to reset the zero point on my gage a few times because if I did have a particularly long cycle that started to build more than 1 psi upon shutoff the system would create a vacuum and pull my gage well below zero, then it would never return to zero. Did not even have to cycle on pressure... Since then I installed a shutoff valve to isolate the gage. I leave it closed unless I want to for some reason monitor the pressure live time.

AdmiralYoda

My Single pipe counterflow system is about 50% oversized and will cycle on pressure when it is in the 20's or colder. When the call for heat is over I experience the same thing.

In my experience it is the weakest vent that will break the vacuum. In my case it is one of my Maid-O' Mist vents on the radiator closest to the boiler. I've run through some bad MoM's and wouldn't be surprised if it wasn't sealing well enough.

Ideally with radiator vents in good condition the main will vent first and break the vacuum. I also leave my shutoff valve to my low pressure gauge (0-20ounces) open all the time and it doesn't seem to care. My Vaporstat kicks in at about 12oz.

ethicalpaul

I'm not sure why the main would break first...in many calls for heat the radiator vents don't even close because steam hasn't reached them.

And in the case of the Gorton and Maid O Mist #1 vents, the interior is exactly the same as their radiator vents with just a larger port.

I wouldn't say the weakest vent breaks first, I'd say the one that gets cool first breaks first, regardless of "vent strength" (if that can be determined at all)

+1 to adding a valve to protect the gauge if you want, or select a gauge that is OK under vacuum.

It's funny that at any given time some of us are trying to break vacuum and some of us are trying to preserve it

dabrakeman

I agree that normally the main vents will always be hot at end of cycle whereas the radiator vents may or may not be. So unless you are filling every radiator and then likely cycling on pressure vacuum build should not be an issue. There will be some vacuum as the steam begins to condense but with all the open radiator vents the air is just sucked back into the system without the vacuum building to create any issues with ones gage (if not vacuum compatible). Sure would be nice if the main would open in this situation to avoid some inrush noises that can occur at the radiator vents but that is just not going to happen. The vacuum issue only occurs if all or most radiator vents are closed by end of cycle. The vacuum gets retained until a radiator vent cools enough to open. This seems to be always faster than the main vents at least in my case. I basically avoid the whole issue by controlling cycles so as not to be long enough to fill every radiator (and I do recoveries too).

Jamie Hall

Or if you are talking about a two pipe system, properly done with crossover traps, the main vents, which are at the boiler and never see steam never close in the first place. Problem solved...

ethicalpaul

If they never see steam, couldn't they just be open pipes?

Onepipe

I’ll post pictures tomorrow when I’m back over there. It’s a one pipe parallel flow system with one counter flow branch. All rad vents are new and system is balanced, the wet returns have been flushed out, and I added a lot of vent to the mains and risers. I have steam to all radiator valves in the house in 7 min. The boiler is sized correctly off of EDR and I plan on running it at 6oz cut out and 2oz cut in. That said right now I have it set at 14oz cut out until I have all the oil skimmed out. I had to so a lot of wet return repair and main piping so there is a lot of oil in the system. I think I have most of it out but in between skims I was running the boiler today for almost an hour straight (had the house to 72, it’s 30 outside) to just get the steam cleaning the system and bringing any oil back. The whole time is was at 1oz or so until the last 5 min when it climbed to cut out pressure. Thats when I saw the vacuum. I know every vent in the house was closed. When I run it for a normal cycle time15-20 min the vacuum issue does not seem to show up. I think that’s all the info asked but let me know if I missed anything. 

ethicalpaul

Sounds like you have a good handle on your system!

Jamie Hall

ethicalpaul said:

If they never see steam, couldn't they just be open pipes?

Yes indeed they could -- and in a number of older systems they were (some were ven piped into the chimney, to allow the chimney to pull a slight vacuum!). It is better in modern systems to have actual vents on them (and in one or two old systems they are also needed for the proper functioning of the overpressure protection gadgets -- Hoffman Equipped for one) because many of those systems were designed to run on very low pressure, and people will crank up the pressure... which may, on some systems, cause steam to get into the dry return...

Onepipe

Ya system seems to be in great order, sounds like no vacuum breaker is needed. You all have been a huge help! I have three systems that I’m working on at the moment all 100+ years old that the customers have truly embraced removing the last butcher work and getting the units back to working order as they should be. By the time you get all these old systems up to where they should be the I joke with the customer that they should give me a cot in the basement.HA! There is a lot of time in these restoration jobs. But it sure is nice when they fire up and are as quiet as a church mouse. The owners wife of this new install was telling me today how she has never had soft butter in winter on the counter for the past 30 years until I showed up. I can’t thank you all enough for helping me keep steam alive. 

Onepipe

Here is the pics of the boiler piping. You will see that there is a sight glass in the riser that I was given the specs from ethicalpaul. This is only temporary as I wanted to see if I would get dry steam using a 2” header pipe like the u.s boiler manual said. Sure enough it’s dry! I would have normally upsized to 2.5” but the steam chamber is huge on the steam max. I’ll be pulling the glass out tomorrow. Also you may notice on the hartford side of the boiler there is a vertical drop of 1-1/2” pipe that connects to wet return that is not hooked up to anything. I have a steam coil that I am putting in tomorrow that will tie into that. Sorry I forgot to take a pic of the controls side. Let me know what you think

mattmia2

You could get a compound gauge that can read vacuum. Probably have to order it from someone like Dwyer. I suspect that it is more that the last of the steam is collapsing faster than the air can get in than air isn't getting in through one or more vents somewhere.