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Tilting Main Lines & Short Cycling

Another question for you guys.

1. I checked my 2 mains and 1 actually is not tilted - perhaps even tilted the wrong direction. This one causes water hammer constantly. How does one go about tilting it correctly?

1. How many minutes is considered short cycling? ie. How long does it take your boiler to cut out?


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,573
    to tilt or not that is the question

    some systems tilt the main away from the boiler, and are known as "counterflow", while the majority are "parallel-flow" with the main sloped away from the boiler.

    a better explanation would be in "the lost art of steam heating" available from the shop here.

    as far as short-cycling, the best way to analyse the situation is to find out when it happens--beginning of the cycle [during venting through inadequate main vents?], or during the main part of the cycle [over-sized boiler?]. knowing the exact pressure in ounces would help as well.--nbc
  • TeachMeSteam
    TeachMeSteam Member Posts: 128
    Amount of Venting Required

    Is there a table or chart to determine how much venting is required in the main lines?

    I have 2 main lines. One is at most 40 ft in length and the other is at most 55 ft in length. I'm giving conservative numbers - it's probably a little less.
  • Jamie Hall
    Jamie Hall Member Posts: 21,106
    edited March 2011

    Also known as pitch...  Your steam mains (and runouts!) have to have pitch, so that condensation which forms in them can get back to the boiler -- and so that it won't get pushed into an elbow or something like that by the steam, and create water hammer (as you have noticed!).

    Runouts should, in general, pitch back to the steam mains -- the correct amount of pitch is given by tables in The Lost Art (there are a few exceptions -- but only when the far end of the runout is dripped to a wet return).  Mains can, as nbc noted, pitch either way, although parallel flow is preferred.  The question is, where is there a chance for the condensate to get back to the boiler?  If they pitch away from the boiler, there must be a drip at the far end to a wet return, which then goes back to the boiler (usually at floor level).  If they pitch towards the boiler, then the drip is at the boiler.  Again, there are tables for the minimum amount of pitch required in The Lost Art.

    The fundamental rule, then, is that any low spot on a main or runout must have a drip to a wet return.  No exceptions.

    So... for your main which may not have adequate pitch, or may even go the wrong way, the only solution is to somehow adjust things so that it has enough pitch -- or to put in a drip.  Sometimes this is as simple as adjusting the hangers for the pipe; either they -- or the whole building! -- may have shifted.  This is particularly true if there is a sag (that is to say, if the pipe has a low point somewhere along the length, but is OK taken end to end).  Sometimes you can't do that, if things have shifted enough so that fixing the pitch of the main would require raising radiators, for instance.  Then things get a little more complicated -- in order to raise the main, for instance, you may have to make corresponding changes to the runouts to the radiators.  Each individual situation is different, though, so without actually seeing your setup I'd be hesitant to say what needs to be done.

    On short cycling... How short is short?  And when does the boiler start turning on and off at a relatively short interval?  This has been covered before, but a short synopsis...  if all is well and the boiler is reasonably close to the correct size, generally it should run until the thermostat is satisfied except when you are raising the temperature from a setback.  Then it may, towards the end of the overall cycle -- say anywhere from twenty minutes to 45 -- begin to cycle on pressure.  That's not a problem, and is a consequence of the boiler being just a bit bigger than what is needed in relation to the amount of radiation.  If the initial run is much shorter, or if it cycles when just maintaining temperature, there are several possibilities.  First, make sure that it is the boiler cycling on pressure, and not the thermostat commanding too short a run.  If it's the thermostat, check the cycles per hour setting or anticipator, depending on the thermostat.  If the boiler is cycling on pressure, starting fairly quickly after it first fires up, the chances are that your venting is really inadequate.  It does happen, although honestly it's not that common.  The most common other possibility is that the boiler is significantly oversized for the radiation.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Rod
    Rod Posts: 2,067

    Hi- Everybody is giving you good answers though I'm still not exactly sure just what type of mains your system has. First of all are we working here with a 1 pipe steam system? (I'm assuming that as tilting radiators was mentioned earlier.)  My second question are the mains parallel or counterflow?  A "Parallel Main" will have a (dry) return line fitted at the opposite end of the main away from the boiler which will than be attached to the base of the boiler or wet return.  A "Counterflow Main" won't have a return line at opposite end of the main away from the boiler but might have one on the boiler end of the steam main.

    It is possible to have a system that has both  parallel flow and counterflow mains on the same boiler. (As it is also possible to have 1 pipe and two pipe radiators on the same boiler.)

    When looking at an old system, especially one that has problems and may have been "Mickey Mouse" modified by Knuckleheads over the years, the first thing you need to do is try and figure out what the "Deadmen" originally intended before you decide to "modify" it yourself.

    As for short cycling etc. I wouldn't worry about that until you get your piping straightened out.

    - Rod
  • TeachMeSteam
    TeachMeSteam Member Posts: 128
    try my best

    Thanks for all the helpful info.

    You guys are correct. The system is a 1 pipe. The line that is banging is a parallel main but it looks like the house has settled and is slightly tilted in the wrong direction. In addition, a runout in the early portion of the main is counterflow because the previous owners added an addition. In addition, the mains are not insulated and they can't be because they finished the basement and they are behind drywall now. So, I am thinking that the wrong tilt plus this counterflow that brings water back to the main, and non-insulated piping is gotta be the reason for all the water hammer.

    I guess there is nothing that I can really do here but live with it. I'm going to try to insulate as much piping as possible - the parts that are exposed. nothing I can do with the counterflow or the wrong tilt. I guess i have to live with some noise. the only other solution would be to take down the walls and re-pipe the entire system which would be ridiculous? no?

    I'll give you guys an update on how things go after I add more main vents and do more insulating. also, i'll be adding a vaporstat since it looks like it's the way to go for a small home like mine.
  • Rod
    Rod Posts: 2,067

    Hi- Not being there I'm not sure quite what advice we can give you. My immediate thought is that dry wall is dirt cheap and insulation, especially with the direction fuel prices are going, gives you the biggest bang for the buck. With that in mind I'd be thinking in the near future to be cutting out a section of dry wall and straightening /insulating my mains.

    Here's a good link to an article of Dan's on the benefits of insulation


    My mains had insulation but my near boiler piping did not and when I finally did insulate the near boiler piping it did make a noticeable difference so now I'm a great believer in insulation.

    I've also attached crude drawing of a jig to measure pipe straightness. It was setup to measure through insulation. Bare pipe makes measuring  much easier.

    If you consider adjusting the piping perhaps you might post some pictures first so we can see more of the new pipe additions and how they might be affecting the system.

    - Rod
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