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Divided Circuit Condensate Return Piping

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SgtMaj
SgtMaj Member Posts: 76
I have a divided circuit steam system, one circuit feeds the front of the house, other circuit feeds the back of the house.  One header with two takeoffs.  Should my returns be joined together below the waterline of the boiler?

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  • SgtMaj
    SgtMaj Member Posts: 76
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    Both returns.
  • Jamie Hall
    Jamie Hall Member Posts: 23,313
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    That depends. There is no one right answer.

    In some systems -- in many vapour steam systems, the dry returns (true dry returns) not only should be joined together above the water line, they must be for the system to work properly.

    In other two pipe steam systems, if they have true dry returns, they may or may not be joined above the water line; it doesn't really matter.

    In any case, in two pipe systems, all dry returns must be vented before they drop below the water line.

    Now... can of worms time. In many one pipe systems, mostly parallel flow designs, the steam mains are continued back to the boiler, then dropped below the water line. These are often called dry returns, but they are not true dry returns. There have been some lively debates on what to call these pipes... In any event, in such a system, these pipes -- whatever you want to call them -- must not be tied together above the water line. They must drop individually below the water line.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SgtMaj
    SgtMaj Member Posts: 76
    edited November 2020
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    This system is on its third boiler, the piping was changed on boiler # 2 and near boiler piping was all goofed up. I.e. the equalizer was piped before the header takeoffs which allowed wet steam continuously down one of the steam mains.  Additionally based on the mailable fittings the returns were repiped as well.  These are two inch returns that join together above the waterline and reduce to 1-1/2” into the wet return about 34” above the boiler waterline.  The returns have 3 #75 Hoffman vents.  When the newer boiler was installed about 5 years ago none of the near boiler piping was corrected and the more efficient boiler was pushing wet steam on the steam run on the back side of the house about 40’ or so, which resulted in rusting out the 2” main which has since been replaced, along with correct near boiler piping which now includes a 1-1/2” Hartford Loop. I’m still getting a little priming or water pushing through the return from the boiler.  Boiler pressure controlled with PA404A main set at .5 diff at 1 which is what the boiler Spec calls for. The issue I’m having and several have had before me is that there are 3 convectors with no heat except for the ones where some one has drilled tapped and installed vents.  I believe where the returns of these convectors with no heat tie into the main returns there is not enough pressure differential to get the air to move.

      My thought was that there wasn’t enough space in the return to handle the expanding water or not enough downward pressure from the wet return to keep the water in the boiler and that separating the returns and joining them below the waterline would give the return enough additional pressure to keep the water in the boiler thus greater differential in the dry return to allow the steam to push the air through the returns of the convectors with no heat.

    anyway just thinking through the process.
  • Jamie Hall
    Jamie Hall Member Posts: 23,313
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    It's not rocket science, so don't over think it. However, I still don't know whether this is a two pipe system with true dry returns (and, possibly, wet returns) or a one pipe system and we're looking at continuations of steam mains by whatever name.

    If it is a two pipe system with dry returns, the dry returns are -- or should be -- always at atmospheric pressure (well, there are vacuum systems, but that seems unlikely). Further, although they must be vented, the vents should never close.

    There are two ways the steam mains are vented: they may have crossover traps at the ends, which allow air -- but not steam -- to pass into the dry returns. If that is so, the steam mains do not need venting. The other approach is no crossover traps, but then the steam mains must be vented at or near the ends, to allow the air in them to escape. The air in the radiators escapes through the radiator traps into the dry returns.

    Condensate is handled in various ways, but one way or another it is taken down -- eventually -- to a wet return or returns and thence to the boiler. It is common for these systems to be parallel flow, and then tere will be drips at the ends of the steam mains to the corresponding wet returns. There will also be drips at the ends of the dry returns in these systems, also to the wet return.

    It is absolutely essential that the wet returns at the locations of any drips be below the water line of the boiler at all times.

    The purpose of the equalizer is -- as its name implies -- to ensure that the pressure in the wet returns, to which it is connected, is the same as the pressure in the boiler, thus ensuring that the boiler pressure cannot back water out of the boiler into the wet returns.

    Since the water pressure jn the wet returns at the boiler is the same as the boiler pressure, it will also be the same anywhere else in the system (since there is almost no flow). This means that the water will back up the drips towards the dry returns a distance to balance that pressure -- about 28 inches for every pound of pressure. This must not get into the dry returns; therefore the dry returns must be at least that distance above the boiler water line. The pressure in the steam mains will be less than that in the boiler, but not necessarily that much less -- so water will stand in the drips from the steam mains somewhat higher than the boiler water level, but not as high as the water in the drips from the wet returns.

    Repiping to join below or above will not change the pressures in any way.

    Assuming that the problem convectors are connected at one end to the steam main and at the other to a dry return (almost always through a trap), there will be enough pressure differential to get steam into them. Provided... that there are no sags in either the supply or return piping, that the valves are open, and that the traps are open. If they are only connected to one end -- making them one pipe units -- they must be vented. Further, if the return connection goes to a wet return, not a dry return, they must be vented -- as, perhaps, you imply someone may have done.

    The water in a steam system does not expand, by the way, or rather very little (it does, but the amount is trivial compared with the volume difference between water and steam). Further, you have to remember that the pressure in water is dependent only on the depth of the water, not on volume.

    Rather long winded, but I hope this helps some.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SgtMaj
    SgtMaj Member Posts: 76
    edited October 2020
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    It is a two pipe system, and I thought that the main returns are dry due to their height above the BWL until it drops to the Hartford Loop. 

    Following the main Steam Run to the back of the house, it loops back, ties into the Main return where there is a vent, Hoffman #57 which works. 

    Where the convectors’ returns tie into the main return there is a 2”x2”x1” Tee laid on its side, I could see that being an issue, additionally no vent within 10” of the end of the main return.
  • Jamie Hall
    Jamie Hall Member Posts: 23,313
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    But there is no trap between the "steam main" and the "dry return"?

    If so, it is NOT a true dry return, but a continuation of the steam main, and thus is under steam pressure, not atmospheric. Such steam main continuations must connect to each other below the water line, not above.

    Where do the returns from the convectors and other radiation go?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SgtMaj
    SgtMaj Member Posts: 76
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    I would need to check for a trap where the steam run Turns back to the boiler.  all fittings insulated with asbestos.  I suppose there could be a trap at that location covered up.

    All returns from all of the convectors tie into the same 2” main return.

    So if the main run has no trap when it comes back to the boiler, join returns under water line.  If there is a trap change out cage unit.  I’m thinking a trap maybe covered up with asbestos.  If that is the case, the trap hasn’t been serviced since installation in 1932.
  • SgtMaj
    SgtMaj Member Posts: 76
    edited October 2020
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    The steam run returns from the right side of the photo. Drops and runs back to the boiler as a return as shown.  The other pipes on the right side of photo are other returns that tie into the return at the lower left further down as depicted in the other photo.  Confusing I know.
  • dopey27177
    dopey27177 Member Posts: 887
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    In any steam systems that do not have steam traps or crossover traps to vent air where the returns are dry return should have the drop leg tie into the wet return. Each separate dry return must have a vent valve to vent the air from that segment of the return piping.

    The one exception is where the system is a vapor system operating at less 3/4 psig.

    The reason for the separate drop legs from each dry return is to prevent steam from one dry return from back flowing and charging the other dry return with steam.

    Jake
    SgtMaj
  • Jamie Hall
    Jamie Hall Member Posts: 23,313
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    There might well be a trap under there. In which case, being insulated, it is a very unhappy trap. Take a look see...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    SgtMaj
  • JUGHNE
    JUGHNE Member Posts: 11,062
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    I would bet against a trap under there.....just an over application of asbestos plaster. The air vent location looks original....and the return pipe would maybe be smaller than the main.

    You can probe that with a very sharp small diameter ice pick to feel for fittings.
  • SgtMaj
    SgtMaj Member Posts: 76
    edited November 2020
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    The horizontal run Which drops I believe is now a steam return is 1-1/2” pipe.  It drops into a two inch return at the vent pictured above and this two inch return ties into the other two inch return from the front of the house.  This system has been piped incorrectly for 50 years.
  • SgtMaj
    SgtMaj Member Posts: 76
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    This is the only photo I have before the re-pipe.  The equalizer was piped before the steam takeoffs and it had no Hartford loop.  It was pushing wet steam down the 2” steam main that feeds the back side of the house. 
  • SgtMaj
    SgtMaj Member Posts: 76
    edited November 2020
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