Thank you for your response and suggestions. We are using one boiler and alternating between the two. We have been switching boilers each month. When we switch boilers we flood the dormant boiler to prevent excessive rusting(anything else?). Do you think that switching on a monthly basis is the right time frame? We repaired all the traps on the 2-pipe system and are working our way through the F&T traps (thanks for the gasket sealing tip) Since we turned down the pressure the building is heating really evenly should I turn it down more yet? When we redo the header do you think that we can get rid of the zone valves on the 2-pipe system (serevices 1st floor)and leave the one on the 1-pipe system(services 2nd and 3rd floors). When the 1 pipe system valve is open we typically do not build up pressure because the zone is satisified too soon. When just the 2-pipe valves are open we we build pressure fairly fast. Thank you for your help.
Thanks for reaching out. I just want to point out my experience is purely as a user and somewhat more knowledgeable amateur steam system aficionado. That being said:
As to flooding and storing the boiler that way - I would not flood it completely. Raise the water line by 1-2-3 inches (and alternate with each down time), and make sure you boil the water before shutting the boiler off for that month. Boiling water is to get the O2 out. Just bring it to boils for a few minutes and turn it off. Raising/changing this water level will move the regular water line and prevent the corrosion at the regular op water line. This is the most stressed spot when the boiler operates (although, arguably, water bounces when boiling, so it's more about moving the waterline away from regular between-cycles and set-back idle water line).
Or, do nothing until summer when both are shut off for the summer. Doing it monthly may increase the longevity of the section at the water line, but then, it also adds more make-up water each time, thus depending on water quality, may be more harmful than letting the boiler just sit as is. Testing water will make this known. Make-up water adds TDS (Mg and Ca) and chlorides, which reduces water quality and can decrease overall boiler life. More make-up water also reduces pH of the boiler water, and this needs to be in excess of 9. I strongly suggest getting SteaMaster tablets or some other boiler water conditioner to get pH up to 9+ range (but not over 10.5-11, when water will start to foam - bad for steam). Google Steamaster tablets. I have 2 tablets of SteaMaster in the boiler, and I watch water use like crazy. Our water is horrendous - TDS is at 220 mg/l, and chlorides from the waterworks reading are at 67.6 mg/l, which is considered ultra HIGH! Boiler water should have less than 100 - 150 mg/l TDS ideally, and no more than 20 mg/l chlorides for optimum performance and boiler longevity. I am now considering water treatment options - this too is deceivingly hard to pin down. Noone wants to be blamed for a boiler failing.
As to pressure, think of it this way. Per Dan Holahan's LAOSH - 'Lost Art of Steam Heating', regular pressure drop across the system should be in the neighborhood of 1/4 psi (4 oz) from the boiler to the end of the main. In theory, this means you should be able to operate your boiler (with FT traps and condensate receiver tank with pump) at, give or take, 1/4 psi (4 oz) and get the best heat. My system fires at 4 oz or below when all zones are open and calling, from overnight set-back or during the day. I've set low burn cut-in at 8 oz to give some room for colder/warmer days and for when one zone is closed. When all zones are open, op pressure NEVER gets to 8 oz (1/2psi), regardless of outdoor temps and stays at that 4 oz level.
Keep one thing in mind: steam compacts and slows down the higher the pressure gets, so in some systems where piping is wrong it helps to dry out the steam by reducing its velocity through higher op pressure. However, this increases fuel costs and does not address the fact that in order to get to building pressure, the system at first doesn't operate well until it reaches higher pressure. Also, when you have a zone closed and system becomes oversized, you have no options but to keep higher pressure, other than installing 2-stage gas valve and firing down the burners. I have this done to our boiler.
In your case, try to get the pressure down as much as is practical for your particular system when all zones are open. This will give you benchmark for how low you can go (but have to decide what to do when closing a zone - more on that below). All mains need to have adequate venting, so just make sure you have as good of venting at the end of mains as you can: Big Mouth, Gorton #2, or Hoffman 75. There is a chart Gerry Gill did (search on the wall for Gerry Gill Venting Chart). This will give you air capacities of pipes and vents and you can do calculation as to how quickly main of your diameter and length should take to vent through (usually) 3/4 vent opening. Then, match it with number of vents to exceed the open pipe volume. Big Mouth vent has 2.3 times capacity of Hoffman 75 (or close to 2.3 times), and unless you are dealing with water pooling and pushing up the vent (thus needing a float vent which Big Mouth is not but Hoffman 75 and Gorton 2 are), Big Mouth is the best choice at 2.3x venting capacity at the same cost as one Hoffman 75 or Gorton 2.
As to zoning: when the zone closes, it essentially makes the boiler oversized by reducing the connected EDR (closes off all the rads on that main). When this happens, your low pressure setting discussed above will essentially shut off the boiler before the steam reaches all the rads because in this situation, the steam output at the boiler exceeds the capacity of the pipes left attached to the boiler to distribute that (now larger for the load) amount of steam, and the system will build pressure at the boiler sooner than with all the zones open. Your only solution to this is to have and keep higher op pressure, or to install a 2-stage gas valve and use it to downfire the boiler once the pressure builds to whatever the regular op pressure is (pressure when all zones are open). This downfire is usually factory-preset at the about 60% of a full burn, and my understanding is this too can be dialed in. I am unfamiliar with your boiler and if this is allowed on it, but technically, with a hood vented boiler you should be able to do it. My thinking is that it should be doable as our Weil McLain LGB 7 (1975 EDR rated), has in its gas train a 2 stage gas valve which allows our boiler to down-fire when one of our zones is closed.
I had a really hard time finding good info on 2-staging of our boiler, although the manufacturer has it as an option listed. I had to research it and then convince our mechanical company that this can be done, and that it should be done as we have a zone which I often close off which oversizes the boiler. (Our particular company is really good for good installs to spec, but are not versed in operations of systems like ours, and I find this is very common out there in the real world with heating experts lacking in steam knowledge. They just don't do enough of it to care how it operates and 'good enough' has become a norm.)
So, consult your boiler's manual, or better yet, call the manufacturer and ask if it can be 2-staged and how. If you have a hood vs. barometric vent on it, which you do, and it is a gas fired boiler, which yours is, I think you should be able to do it regardless of what the manufacturer says. Swap your gas valve for a 2 stage gas valve of the same capacity of your current one and add an additional vaporstat to control low-fire range. With low fire, you are in essence simmering water to lower boil from full, and reducing the steam volume. 60% downfiring with a hood should also not mess with combustion mix nor have issues with stack temps being low for condensing and dripping back and rusting out the boiler fire box. This will allow you to reduce BTU input when one of the zones is closed, which will in turn, make the boiler 'smaller' and more closely match EDR left on the system after one or more zones close off, and save fuel. There are a couple of threads I was engaging with when doing this where a few of the pros on there did mention doing this on some of their jobs. So, it's a matter of knowing what is done and how it's done.
NOTE: after this video was taken, I was able to further dial down high/low/high to 8 oz (from 10 oz on this video) with 0 oz cut-in to high fire. Once one of my zones is closed (the only one I close ~500EDR connected to it), boiler fires on high as it normally does, takes some time to build pressure to 8 oz, at which point it goes to low fire mode (low stage) and for the most part stays in 2-4 oz mode until t-stat is satisfied. It may undulate between low and high fire a few times after the condensate feeder kicks in, but this too is fine. This mimics the old coal fired damper action, and actually helps with steam distribution. In theory, this should also be saving some fuel. I unfortunately can't tell how this affected our use as I have another furnace on the same meter and it was sometimes on and sometimes off and has messes with my degree days calculation readings comps to this year's readings...
I hope this is helpful. I invite you to post this question to your original post on the wall and see what other people have to say. I have discovered that many people may agree on some things and then also disagree on others, but you'll get other feedback you can then pick through. I was told vehemently we can't 2-stage our boiler by many on the board, and it turned out - you could. One of the guys on the board (Abracadabra, I think) pointed me in the right direction. Just keep at it!