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Vacuum or Not off cycle?

Southboy Member Posts: 2
edited November 21 in Strictly Steam
I have a 2 pipe steam system with a gas fired HB Smith Boiler. On the system is a WEBSTER VENT TRAP 0023T that blows off air when the system is heating up. On the top vent of the Vent Trap is a ball bearing, about the size of a large marble that blocks the vent hole when the system is off cycle and cooling down. This created a system vacuum which prevents air from being sucked back into the system. This Vent trap has a float in it which closes when water reaches a certain level to prevent water and stem from discharging through the vent hole. This vent trap at times had water remaining in it off cycle, closing the vent and preventing air from being blown off. As a result, some radiators did not get hot. To resolve this, I installed a supplemental air valve at the highest point in the system piping. This supplemental valve can be vacuum set with a steel ball or non vacuum with the steel ball bearing removed from the top of the vent (the very top screws off).

Question: Should this system be vacuum or not? I read somewhere that this was a remnant of the old coal burning system and that the system should NOT be a vacuum; that the steel ball should be removed. Any experience or thoughts on this ? Would the vacuum interfere with the condensate being returned to the boiler?

The Trap looks VERY similar to this picture.


  • Jamie Hall
    Jamie Hall Member Posts: 17,049
    One can get well off into the weeds on this one. There are two critical things to remember: steam systems work on pressure differences, and the boiling point of water varies with pressure.

    OK. Pressure differences. Depending on the steam system in question, the exact pressure difference between the steam mains and the dry returns may or may not be critical. In any system which depends in whole or in part of calibrated valves, orifices, or certain types of traps the pressure difference is critical, and further must be kept very small -- on the order of a few ounces per square inch -- for the system to function properly.

    In addition, many systems depend on loop seals (which may or may not connect to a wet return) to keep steam out of the dry return. These are also restricted in pressure difference, but the difference usually can be greater -- up to a couple of pounds if the seals are tall enough.

    It should be clear at this point that upsetting the pressure difference will be detrimental. But what about absolute pressure? So long as the pressure differences aren't altered, one can run the system at any absolute pressure one wants to, and this is where it can get confusing. It may be simplest to consider two different approaches which were used in early steam systems: having the dry returns always at atmospheric pressure (often with just an open pipe!) and systems which allowed the entire system to drop into a vacuum at times. They both worked well. The advantage of the vacuum is that it allowed steam to continue to be produced as the coal fire died down and the boiler cooled, but it only worked if the entire system was able to drop into a vacuum. Uneven vacuum -- allowing some part of the system to go down but not others -- can indeed trap water in the pipes, and is most unwanted.

    So there is no one answer to your question. You will have to look at the whole system and determine where vents are on it and traps, and decide whether it can drop into vacuum uniformly, or whether the pressure differences can build up. You should be aware that the probability is very high that modifications have been made -- vents added or removed, traps added or removed, etc. -- which may unbalance it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 10,268

    As a general rule anything with an automatic burner either oil or gas you can't run a vacuum system. Some exceptions.

    And yes, vacuum can cause condensate to hang up in the system
  • Jamie Hall
    Jamie Hall Member Posts: 17,049
    duplicate post
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 1,696
    @Southboy, I've merged your two posts into one here.
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 757
    I would think if water is backing up high enough to get into the vent, the pressure is too high. This type of system is designed to run at a maximum of 8 ounces of pressure, so I would first address the issue of the pressure in the system.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Southboy
    Southboy Member Posts: 2
    Thank you for all of your considerations. As to the Steam Whisperer: The pressure in the system is less than 1 lb. max.--it cuts off the burners at just below 1 lb. of pressure. It rarely reaches this level (1 lb.) before the house thermostat cuts the system off as temperature reached. It seems to me that water is supposed to enter the vent trap--to operate the float in the trap which closes to prevent water and stem from exiting the system. It the float valve did not close after blowing off air, the system would never pressurize, would it ?
  • PMJ
    PMJ Member Posts: 1,122
    @Southboy ,

    I have run my 2 pipe system in vacuum during the off cycle 20 years or so. If I told my wife I was going back to the way the system ran when we bought the place open vented I'd find my stuff on the front stoop.

    Natural vacuum is not a popular idea around here, and you won't find many with actual hands on experience with it. You will hear plenty of reasons why you should take the check ball out and open vent.

    Probably better to discuss PM if you want to know more about my experience with it so folks don't get upset. I wouldn't consider running any other way.
    1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
  • Jamie Hall
    Jamie Hall Member Posts: 17,049
    I won't comment on vacuum or not; PMJ has all the answers on that. What I will comment on is pressure: the system isn't supposed to pressurize. Reread my comment on pressure differential; in your system the maximum pressure differential you ever want is 8 ounces -- half a pound -- per square inch, measured as the difference between the steam mains and the dry returns. If part of the system goes into vacuum, the entire system must do so, evenly, to maintain that differential.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England