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One-Pipe Steam....Maintaining Vacuum?

Had a weird thought today. When my system generates steam and shuts off, a vacuum is induced by the steam condensing. The vacuum holds for some period of time (usually less than a minute) then I hear a vent open up and air rush back in, I think its my Big Mouth main vent.

Anywho...water boils faster in a vacuum right? What if I put one way check valves on the radiators and main vents so the system will maintain its vacuum when not running? Would this benefit the system at all and help it boil water faster?

Just my crazy thought of the day.


  • Jamie Hall
    Jamie Hall Member Posts: 22,918
    Not a crazy thought, really. As you note, water under a vacuum boils at a lower temperature. It also condenses at a lower temperature, which will reduce the output of your radiators. A number of steam systems in the coal fired days took advantage of this to continue to supply heat from the coal fire, after it had been damped down.


    Yes you could install check valves on all your vents. You would need to find check valves, however, for which the cracking pressure -- the pressure at which they will open -- was very low (compatible with the pressure your system should be running at -- a few ounces per square inch until the vents close) and those are not cheap. Further, if the system were to hold more than a few inches water column of vacuum for more than perhaps half an hour, if that, you would have to find and seal all the other vacuum leaks in the system -- every valve stem, for instance, or threaded joint.

    Could you do it? Yes. Would it be beneficial to the system as a whole? Not, in my opinion, unless the effort was part of a fully coordinated upgrade of the system, which incorporated modulating burners for the boiler to match the load better and a way to actually control the depth of the vacuum.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • AdmiralYoda
    AdmiralYoda Member Posts: 618
    @Jamie Hall thank you, great explanation! I think you are right though, maintaining a vacuum until the boiler fires and builds pressure is probably not going to do me much.

    I took a quick peek at check valves and I'd have to spend at least $500 to give it a try....and the return on investment would probably take way longer than would be worth it.
  • ethicalpaul
    ethicalpaul Member Posts: 5,608
    This topic has come up several times even in the last 3 years. I can picture in my mind the check valve I would need but no one manufactures it. It would be gravity operated with a small cylinder sliding in a fitted tube all of which would have to interface with the vent connection.

    I think the better way is the system where a small "vent" tube leads back to a common check valve/vacuum pump in the basement. I know there was a commercial implementation of this once but I don't know enough about it.

    It's fun to think about and I even have purchased some allegedly low-cracking-pressure valves to mess with but none were nearly low enough.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Jamie Hall
    Jamie Hall Member Posts: 22,918
    Paul system, among others, @ethicalpaul . Some of them used steam ejectors to produce a vacuum. Others used a vacuum pump (Warren Webster, I think?). Some of them were one pipe (like the Paul) with small pipes leading to a single ejector; others were two pipe and placed the whole system into a vacuum

    Then also many systems in the coal fired days used vacuum vents -- the Hoffman 15 and 76, for instance -- to allow the whole system to drop into a natural vacuum as the fire died down or was damped. This was advantageous, as a coal fire -- unlike gas or oil -- doesn't go from full song to nothing all at once, but continues to produce heat as the embers continue to burn, and that heat could be captured and transmitted at the lower temperatures involved in a vacuum. The efficiency at low temperatures was horrendous, and the emissions worse, but that was long ago!

    They also had no need for copious main venting, as a coal fire doesn't go from nothing to full bore quickly -- again, unlike gas or oil, so often only a single main vent on the dry returns was used and sufficed very nicely (even now such systems can operate very well with far less main venting than is often suggested -- Cedric, for instance, runs at between 2 and 4 ounces differential until all the radiators are full and the traps close using only one Gorton #2 and Hoffman #75).

    I suspect that running a steam system -- either of the Paul type or a two pipe vapour system -- with the system under an induced vacuum could potentially yield some real gains in efficiency on days when less heat was needed and it could be run at a high enough vacuum to operate at flue gas condensing temperatures. This would require, of course, boiler designs to allow that -- and boiler burners which could modulate to match the load... and outdoor resets to control the vacuum... and vacuum pumps. I do think about it. Then reality sets in and I find myself unwilling to trade the simplicity and reliability of a conventional vapour system for the complexity!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England