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Non-Thermostatic Vacuum Vents?

HarryHarry Posts: 60Member
Gentlemen (and Ladies): I'd like to put this topic out for discussion - especially among you old farts experienced in the art.

In a vacuum-vapor system there is always some kind of a vent at the end of the dry return. I'm thinking, for example, of a Trane, which has separate vents at the ends of the steam and return mains, or a brand such as Dunham, which has a crossover trap and a single vent at the end of the dry return. In the case of Trane (and others) they used a normal float and thermostatic vacuum vent at the end of the steam main and a thermostatic only vacuum "quick vent" at the end of the dry return. My question is this: Why does it need to be thermostatic at all - or does it? No steam should ever get to the end of a dry return (I'm not including trapless systems (like, say, Broomel) where vapor was expected to occasionally make it past the little water seal or tiny check valve. My recollection is that WarrenWebster, for example, and I think even Dunham, had only the vacuum check atop the float assembly that dealt with abnormal conditions, and no thermostatic element at all. This leads me to wonder why we don't just use a (suitably light) check valve at the end of a dry (vacuum) return, and if it's not vacuum, just an open pipe. Your thoughts?
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Comments

  • SteamheadSteamhead Posts: 12,302Member
    Because steam traps leak sometimes, and we don't want steam leaving the system.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
    https://heatinghelp.com/find-a-contractor/detail/all-steamed-up-inc
  • PMJPMJ Posts: 656Member
    @Harry ,

    Your question is exactly the one I asked myself many years ago and concluded that in two pipe no vent was required at all on the main and then that no thermostatic vent was required on the dry return. I have been trying to make that point here for years now. If I weren't pulling natural vacuum with the burner off I would go with just one open pipe on the dry return for sure. Then, standing next to that pipe anyone listening when the burner goes off would be asking themselves why in the hell they were letting all that air back in .... again and again and again!

    These systems were designed to heat on design day with the radiation partially full. No steam need be anywhere near a trap or therefore the dry return. Very simple timer setups can guarantee this. I have Mouat traps on most rads which would allow some passing of steam if the rads ever filled which I don't allow them to. Other rads where thermostatic traps had been installed by others I removed the seats so there is no trap at all now. I think I have 4 actual functioning thermostatic traps on 4 small bathroom rads that mission control here wants to fill totally. I'm not sure even those are needed really.

    Less fill over much longer periods - that is the ticket to much nicer heat. Once you start down that road steam will never get close to the dry return at all, let alone the end of it.

    Now, from a contractor's point of view running a standard control to a pressure stop (which means many full radiators) and steam ready to enter the dry return with one failed trap on most burn cycles. Well, I suppose they need to worry about callbacks for steam in the basement. I suppose they better put on the thermo vent. Too bad but I get it. It's just that the heat overall is so much nicer when the boiler never runs long enough in one stretch to put steam anywhere near the dry return.
  • Jamie HallJamie Hall Posts: 8,535Member
    Several types of vapour steam systems (vacuum was an added bonus), notably Hoffman equipped, had various ingenious schemes to limit the pressure difference between the returns and the mains to some low value -- for a Hoffman equipped system that difference is approximately 8 ounces per square inch. This made sure that condensate could reliably return, and also made sure that the various schemes such as orifices or calibrated valves -- or nifty little outlet gadgets -- would function and function reliably. These schemes all functioned without moving parts, for the most part -- but did depend on allowing steam at boiler pressure into the mains if the pressure difference got to high. This required quick acting, thermostatically actuated vents to be located very close to the boiler (the Hoffman 76 is a fine example) and that there be no other vents anywhere else on the system.

    There has always been a requirement to modulate the heat input to the boiler. Various schemes have their ardent admirers, and all of them have advantages -- and disadvantages. The advantage of the schemes noted above -- with their complete lack of moving parts for pressure differential control except for the vent itself -- is that they promote very long life and very reliable running for the system as a whole; they don't supplant or compete with the heat input control schemes at all.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • PMJPMJ Posts: 656Member

    8 ounces is such an incredibly high number to me now - like 8 times the highest header/ dry return differential I would ever expect to see in my system. Partially full radiators are condensing steam and creating vacuum so fast that almost no header pressure is required to deliver enough steam to them even on design day. For want of a simple limiting timer we instead deal with the expense of many pressure devices. I admit, I am surprised at this given our ability to simply turn the fire off when it obviously isn't needed.


  • Jamie HallJamie Hall Posts: 8,535Member
    Remember that that 8 ounces is the maximum limit set by the gadgetry. Not the operating pressure -- which in the case of the principle system I care for is around 1 to 2 ounces.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • PMJPMJ Posts: 656Member
    > @Jamie Hall said:
    > Remember that that 8 ounces is the maximum limit set by the gadgetry. Not the operating pressure -- which in the case of the principle system I care for is around 1 to 2 ounces.

    That is an average? If that is a max range then all you need apparently is a boiler and a tstat. You list a vaporstat as part of your control but if you are never over 2 OZ it wouldn't ever operate.

    I admit I couldn't run at max 2 OZ with nothing but a tstat. Burns would go on too long, rads would fill too much, and pressure would build past 2 OZ and I would overshoot some amount - even with vacuum.
  • ChrisJChrisJ Posts: 9,060Member
    1 - 2 ounces.
    And here my cheesy single pipe system operates between 1/8oz - 1/4oz.

    During long recovery I'll see upwards of 1/2oz.

    But don't mind me. :)
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • PMJPMJ Posts: 656Member
    ChrisJ said:

    1 - 2 ounces.

    And here my cheesy single pipe system operates between 1/8oz - 1/4oz.



    During long recovery I'll see upwards of 1/2oz.



    But don't mind me. :)

    I believe it. Mostly due to your Ecosteam which stops the boiler from making more steam than is needed - which is never enough to build pressure. My point exactly.
  • Jamie HallJamie Hall Posts: 8,535Member
    I simple can't run at much less than 2 ounces -- there is just that much head loss in the steam mains... it's kind of fun, though -- in a slightly twisted way -- to watch the pressure rise to that and then just simply sit there for the rest of a run!
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • PMJPMJ Posts: 656Member
    One ounce or two, to me that's down in the range where it's not important which it is.

    So Jamie what is the max pressure you ever see and under what conditions? From what you are saying you simply don't need any controls beyond just a tstat.
  • Jamie HallJamie Hall Posts: 8,535Member
    On a very long run -- say if the power has been off for a while or some other idiot event -- all the radiation will fill. At which point the pressure will begin to ramp up. The v'stat will shut it off before the Differential Loop trips... then everything sits for a while and condenses happily. Usually -- end of story. If it's right at design, Cedric may fire up again. But that's rare - @Charlie from wmass and I have it pretty well dialed in as to heat input vs. load matching...
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ChrisJChrisJ Posts: 9,060Member
    > @PMJ said:
    > 1 - 2 ounces.
    >
    > And here my cheesy single pipe system operates between 1/8oz - 1/4oz.
    >
    >
    >
    > During long recovery I'll see upwards of 1/2oz.
    >
    >
    >
    > But don't mind me. :)
    >
    > I believe it. Mostly due to your Ecosteam which stops the boiler from making more steam than is needed - which is never enough to build pressure. My point exactly.

    You're 100% wrong.
    I can run for an hour straight and not exceed 1/2 an ounce. My boiler is sized correctly and my system is balanced very well.

    I'd run the same pressures without the PLC.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • ChrisJChrisJ Posts: 9,060Member
    > @Jamie Hall said:
    > On a very long run -- say if the power has been off for a while or some other idiot event -- all the radiation will fill. At which point the pressure will begin to ramp up. The v'stat will shut it off before the Differential Loop trips... then everything sits for a while and condenses happily. Usually -- end of story. If it's right at design, Cedric may fire up again. But that's rare - @Charlie from wmass and I have it pretty well dialed in as to heat input vs. load matching...

    Cedric?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • PMJPMJ Posts: 656Member
    ChrisJ said:


    I'd run the same pressures without the PLC.

    I see. Pressure only comes from mostly filled radiation. So you are telling me that running a full hour your boiler can't fill your radiation no matter what the temperature is outside and no matter how long since the boiler last ran correct?
  • ChrisJChrisJ Posts: 9,060Member
    > @PMJ said:
    > I'd run the same pressures without the PLC.
    >
    > I see. Pressure only comes from mostly filled radiation. So you are telling me that running a full hour your boiler can't fill your radiation no matter what the temperature is outside and no matter how long since the boiler last ran correct?

    It'll shut most if not all of the vents but it'll take more than an hour to go above 1/2 an ounce. The amount the radiation condenses is very close to what the boiler produces.

    I should try to do a 5 degree recovery when it's below zero out just to see what happens.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • PMJPMJ Posts: 656Member
    edited June 10
    @ChrisJ ,

    I do get your approach of installing a boiler that just isn't big enough to fill the radiation to the point of building pressure. Given my argument that most of us have more radiation installed than needs to be full to heat on design day your approach will in fact work ok.

    I on the other hand argue for extra boiler to be installed. Then I have more options. I tell those who already have oversize boilers installed not to worry. The solution is not a new(smaller) boiler! I argue this because managing an oversize boiler is so easy to do and in fact is what your Ecosteam would do too. It would shut off your boiler before any pressure was reached no matter how large the boiler was. It would control the cycles and run time such that radiators would never be more than partially full and never build pressure. What size the boiler is would make no difference to it.

    By reducing boiler size you lengthen cold start times and you do raise the effective design day temperature some amount (yes, a totally full radiator is in fact condensing more steam looking at four walls that are -20F on their outside surface than 45F). Are either of these things really significant? Maybe not, but I don't like giving anything up - I like extra. I want to minimize cold start times and recoveries and I want extra when the 100 year storm arrives. I want these things because I see no down side at all to the extra boiler.

    Beyond this, all my advantages in using vacuum rely on significant boiler off times for it to do its thing. The more "closely matched" the boiler is, there is less and less boiler off time the colder it gets for vacuum to work it's magic. It is during those times where we actually need a lot of heat that a smaller boiler would have to run more and deny me all the very significant benefits of natural vacuum.


  • ChrisJChrisJ Posts: 9,060Member
    > @PMJ said:
    > @ChrisJ ,
    >
    > I do get your approach of installing a boiler that just isn't big enough to fill the radiation to the point of building pressure. Given my argument that most of us have more radiation installed than needs to be full to heat on design day your approach will in fact work ok.
    >
    > I on the other hand argue for extra boiler to be installed. Then I have more options. I tell those who already have oversize boilers installed not to worry. The solution is not a new(smaller) boiler! I argue this because managing an oversize boiler is so easy to do and in fact is what your Ecosteam would do too. It would shut off your boiler before any pressure was reached no matter how large the boiler was. It would control the cycles and run time such that radiators would never be more than partially full and never build pressure. What size the boiler is would make no difference to it.
    >
    > By reducing boiler size you lengthen cold start times and you do raise the effective design day temperature some amount (yes, a totally full radiator is in fact condensing more steam looking at four walls that are -20F on their outside surface than 45F). Are either of these things really significant? Maybe not, but I don't like giving anything up - I like extra. I want to minimize cold start times and recoveries and I want extra when the 100 year storm arrives. I want these things because I see no down side at all to the extra boiler.
    >
    > Beyond this, all my advantages in using vacuum rely on significant boiler off times for it to do its thing. The more "closely matched" the boiler is, there is less and less boiler off time the colder it gets for vacuum to work it's magic. It is during those times where we actually need a lot of heat that a smaller boiler would have to run more and deny me all the very significant benefits of natural vacuum.

    Yes, I lengthened start up time some, but it's moot as its burning equally less fuel.

    Your approach causes cycling we all know each time the burner lights there is fuel wasted.

    Overall, I feel my way is more efficient.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • PMJPMJ Posts: 656Member
    ChrisJ said:


    Yes, I lengthened start up time some, but it's moot as its burning equally less fuel.



    Your approach causes cycling we all know each time the burner lights there is fuel wasted.



    Overall, I feel my way is more efficient.

    I am quite confident that the savings in having each and every start in 60+ inches water vacuum outweigh the costs of pushing air out every time - even at more than 3cph. I am quite sure if you had a two pipe you would be here already because it is so easy. There is simply no way if my system were vented it could compete with what I have now no matter what size the boiler.

  • SteamheadSteamhead Posts: 12,302Member
    ChrisJ said:

    > @Jamie Hall said:

    > On a very long run -- say if the power has been off for a while or some other idiot event -- all the radiation will fill. At which point the pressure will begin to ramp up. The v'stat will shut it off before the Differential Loop trips... then everything sits for a while and condenses happily. Usually -- end of story. If it's right at design, Cedric may fire up again. But that's rare - @Charlie from wmass and I have it pretty well dialed in as to heat input vs. load matching...



    Cedric?

    Cedric is the name of the boiler Jamie cares for.

    Naming boilers is more common than most of us think, especially when there is more than one boiler on a job. For example:

    Ward and June (for Cleaver-Brooks boilers)

    Kirk and Spock

    Kirk and Picard

    Huff and Puff

    Pete and Re-Pete

    Use your imagination- "Boiler 1" and "Boiler 2" are soooooo boring............
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
    https://heatinghelp.com/find-a-contractor/detail/all-steamed-up-inc
  • HarryHarry Posts: 60Member
    I knew this would start fascinating discussion! As to the initial comment about failed traps causing steam to exit through an open vent . . I like to think of that as an indicator that a trap has failed and needs to be tended to, rather than concealing the trouble.
  • SteamheadSteamhead Posts: 12,302Member
    It would still "indicate" because the vent would close and the system wouldn't heat well.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
    https://heatinghelp.com/find-a-contractor/detail/all-steamed-up-inc
  • HarryHarry Posts: 60Member
    True, but I was thinking more of the open pipe version. I think live steam coming out of a pipe in the basement is a pretty good indication that a pro should be called. Or. . . it will just result in damage before anyone notices! It's not a perfect approach.
  • ShalomShalom Posts: 107Member
    Steamhead said:


    Naming boilers is more common than most of us think, especially when there is more than one boiler on a job.
    .

    How about Boiler-Bulge and Clang-Dickens?

    (note that the latter was rather close to swearing by 1902 standards.) Then there were their three friends: Pounder-Gratings, Nick-Nickel and I can never remember the fifth one. Oh, it's Bang-Rattler. https://archive.org/details/larkyfurnaceothe00broo
  • PMJPMJ Posts: 656Member
    Harry said:

    True, but I was thinking more of the open pipe version. I think live steam coming out of a pipe in the basement is a pretty good indication that a pro should be called. Or. . . it will just result in damage before anyone notices! It's not a perfect approach.

    In your own place? - my vote goes for just the open pipe. For sure you learn the most that way about what is going on. If rads are full enough to leak enough steam past a bad trap AND heat and fill the dry return all the way to the end things are way off track.
  • HarryHarry Posts: 60Member
    PMJ said:

    Harry said:

    True, but I was thinking more of the open pipe version. I think live steam coming out of a pipe in the basement is a pretty good indication that a pro should be called. Or. . . it will just result in damage before anyone notices! It's not a perfect approach.

    In your own place? - my vote goes for just the open pipe. For sure you learn the most that way about what is going on. If rads are full enough to leak enough steam past a bad trap AND heat and fill the dry return all the way to the end things are way off track.
    My thinking precisely.

    On the matter of naming boilers: At a previous employer's we had two 300 ton Lithium-Bromide absorption chillers we named "Miss Jessie" and "Miss Bessie," after two elderly cleaning ladies who used to work in the building. It's not just boilers!
  • the_donutthe_donut Posts: 369Member
    Downstream of the hoffman steam trap is an open pipe with a check valve that acts as the vent in a 1920’s Webster I get to spend time with. Some pretty wicked vacuum builds after burner shuts off ~1mbtu of radiators condense steam like crazy.
  • HarryHarry Posts: 60Member
    Yes, I've seen Webster systems with just a garden-variety check valve connected to a float separator (which of course never operates in these days of pressure controls), and they produce a very nice vacuum, thank you very much. I'm thinking of doing this on a system I'm rehabbing - just using a check valve vent at the end of the dry return. If a blown trap allows steam to get all the way to the end of the line without condensing, something needs serious attention!
  • PMJPMJ Posts: 656Member
    .
    Harry said:

    Yes, I've seen Webster systems with just a garden-variety check valve connected to a float separator (which of course never operates in these days of pressure controls), and they produce a very nice vacuum, thank you very much. I'm thinking of doing this on a system I'm rehabbing - just using a check valve vent at the end of the dry return. If a blown trap allows steam to get all the way to the end of the line without condensing, something needs serious attention!


    Bravo @Harry.

    I've had a plastic drinking water backflow check valve on my dry return for many years now. It cost $14. In two pipe this whole thing is really simple and inexpensive. One hole in the whole system with this check on it is all that is needed. What is not to like?
  • Jamie HallJamie Hall Posts: 8,535Member
    I'm tempted... when I win the lottery... to replace the "hole" in my system (which is a Gorton #2 and a Hoffman #75) with maybe three or four Hoffman #76s -- I'd keep the functioning of the Differential Loop, which I want to do if only for historic correctness (which counts in this place!) but I'd get the vacuum... when I win the lottery. Or someone comes up with a nice grant (right...).
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • HarryHarry Posts: 60Member
    It's kind of amazing that some manufacturer doesn't make a larger capacity vacuum vent. The #76 is only about a third the capacity of an old Trane cast-iron "bullet" vent. I guess we should be happy that anyone makes a vacuum vent at all, despite the fact that there are millions of two-pipe vapor/vacuum systems out there. Just imagine if someone like, oh, I don't know, Barnes and Jones, made a check valve version of their "Big Mouth." Oh, wait! One could put a check valve on one of those and, assuming you located it so that it didn't need the float function, it would be the same thing! A little less elegant looking, perhaps . .


    I did just use a half-inch check valve on the dry return of a system yesterday (without a thermostatic element), but at about 3" of vacuum I could hear air leaking in. So I'm still searching for a decent, inexpensive, check that is air tight. I might have to go looking for that plastic back flow preventer!
  • PMJPMJ Posts: 656Member
    Harry said:

    It's kind of amazing that some manufacturer doesn't make a larger capacity vacuum vent. The #76 is only about a third the capacity of an old Trane cast-iron "bullet" vent. I guess we should be happy that anyone makes a vacuum vent at all, despite the fact that there are millions of two-pipe vapor/vacuum systems out there. Just imagine if someone like, oh, I don't know, Barnes and Jones, made a check valve version of their "Big Mouth." Oh, wait! One could put a check valve on one of those and, assuming you located it so that it didn't need the float function, it would be the same thing! A little less elegant looking, perhaps . .


    I did just use a half-inch check valve on the dry return of a system yesterday (without a thermostatic element), but at about 3" of vacuum I could hear air leaking in. So I'm still searching for a decent, inexpensive, check that is air tight. I might have to go looking for that plastic back flow preventer!

    @Harry,

    Confession - I did get two of the plastic preventers from McMaster-Carr many moons ago and put one on top of the other in series. Can't remember if just one actually leaked or not. Anyway, that assembly has been in place for 15 years now and holds my 60-80 inches of water vacuum just fine. Don't spend a lot of money on this.

    Also, one point I think gets lost here is that once you run in vacuum venting capacity is no longer a consideration because there is so little to vent. On first warm up a very small hole gets it done because it takes longer to heat up pipes than to vent air. Once warm since you don't let air back in there is very little that needs to go back out. There is no similarity at all to the venting that is required in an open system.
  • HarryHarry Posts: 60Member
    True.

    We should also take a moment to recognize McMaster-Carr as the greatest supply house on the planet. Make a call, get a human, place an order, the item is delivered the next day. There is nowhere better.
  • PMJPMJ Posts: 656Member
    > @Harry said:
    > True.
    >
    > We should also take a moment to recognize McMaster-Carr as the greatest supply house on the planet. Make a call, get a human, place an order, the item is delivered the next day. There is nowhere better.

    I'll second that motion. Where I am it is order online by noon and it will be deivered by 4. Never the slightest hassle about anything. They have dragged Grainger screaming and kicking over the last 10 years into something more reasonable - but still nobody is close in my view. I will gladly pay a slight premium for this level of service.
  • FizzFizz Posts: 391Member
    Does Mepco still make their vacuum quick vent(can be either/or)? I have them on the end of dry returns in my Richardson System, no problems in 3 yrs.
  • SailahSailah Posts: 677Member
    edited June 18
    Yes the Quick Vent is still available.

    We sent that to Gerry Gill for testing along with a bunch of other vents I had devised. The issue is that the Delrin ball that acts as vacuum check against the o ring severely restricts flow and many of you run very low pressure. That pressure needs to lift the ball.

    I could easily create a better solution, but it would not be a “Vent” as you’d typically think. Would also be considerably more expensive as there are some components involved that cost more than a vent.

    Price, reliability & function. Pick 2.
    Peter Owens
    SteamIQ
  • PMJPMJ Posts: 656Member
    For whatever it is worth, the Mepco literature claims that the ball "provides no resistance to the discharging air".

    But beyond that I point out again that operating in the natural vacuum state nothing like the "typical" venting people are used to with open systems is ever required - not in volume or rate. Those with two pipe systems really need to experience this once. It will cause you to wonder why so much continuous processing of large volumes of air has gone on in all this time since coal went away.
  • HarryHarry Posts: 60Member
    What I have seen be an issue, is the same reason the original manufacturers started using orifices after thermostats came into use. In moderate weather, with short boiler run times, if you don't have a large vent, there is sometimes not time for the more distant radiators to start heating, as all of the air hasn't always had time to escape. The system I was just working on had a short run on one side of the house and a 50% longer run on the other side. Distant radiators at the end of the long run were thought by the owner to not be working, since the venting was so slow (original Trane vents were pretty plugged with paint) that the boiler shut down before the steam got there. Thermostat was, of course, close to the radiators nearest the boiler. In the cold of the winter things always operate more the way they should.
  • PMJPMJ Posts: 656Member
    @Harry,

    In the case such as you describe, assuming both dry returns are connected somewhere above the water line(mine is one big loop), you put the single vent/check hole in the most remote place on the longer run. This will favor the longer run on first heat up significantly by lengthening the escape route for the air on the shorter run from what it was. I had a "maids quarters" loop off the main dry return loop that was very slow. I put the one vent hole for the whole system at the remotest place on that.

    Again, this is significant only on a real cold start which is limited by time warming pipes and not air removal rate. What is important is only that some partial fill be achieved everywhere on the first run from cold. After that, the vacuum takes over and will do wonders evening out the fill on all following cycles. It isn't quickly obvious but with natural vacuum all the (re)filling on subsequent cycles after the first cold one takes place entirely below atmospheric with the system completely closed. Vent location has nothing to do with it after the first true cold cycle. In my experience vent size never has anything to do with it.
  • Jamie HallJamie Hall Posts: 8,535Member
    There are many ways to go after getting even heating with a steam system. There are also many ways of improving the efficiency of badly operating or designed steam (or knuckleheaded!) systems.

    There are several problems with any of the discussions (not the least of which is that everyone is, within limits, correct!). One is that the physics of a steam heating system are not obvious: one might think, for example, that if a boiler comes up to temperature and starts to generate steam at some rate -- say for example 20 cubic feet of steam per minute -- that for the pressure in the system to stay very low one would have to release 20 cfm of air through the vents. Not so. As soon as that steam hits metal -- a pipe or a radiator -- which is below the condensation point, it promptly condenses. The rate at which air would have to be released is less - in fact much less -- than the rate at which steam is being created. It can be computed, but there are a lot of rather poorly known variables involved, and at the moment I'm too lazy to do it (although I might point out that pipe insulation has a lot to do with it!). This is also why, at least in part, it is usually better to undervent a main or radiator which is heating too fast rather than trying to increase the venting on a slow main beyond a certain point; once one has enough venting (or, for the vacuum folks, vacuum) on a main, one is dealing with the rate at which one can heat the main -- not the rate at which one can push air out of it (or suck steam in, as you please).

    Another less than obvious point has to do with the thermodynamics of saturated steam. Understanding one of the quirks, though, helps to explain why orifices or calibrated valves work so well. If one expands it, such as by going through an orifice, it's not like air, which just cools off. No, it condenses. Which is just fine if it is going into a radiator at that point -- but not so good if it is going to a further steam main (case in point: reduced port valves on saturated steam lines are a complete no-no, as are most globe valves). The pressure difference across the orifice or calibrated valve will govern the amount of steam entering, and hence the heat output. Obviously, for balance, one wants to maintain that pressure difference within rather narrow limits.

    The dead men -- at least the good ones -- had most of this figured out pretty well, based on experience... it is worth studying older systems which are working well and figuring out the physics and thermodynamics of them!
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • HarryHarry Posts: 60Member
    Bravo! Couldn't have said it better, myself.

    For years I was the chief engineer at the largest of the old 1920's movie palaces in the country, and for all of those years I studied the old text books, and even articles in the trade magazines from the period, in order to understand what the original builders were doing. Our building was at the cutting edge of HVAC technology in 1928 and took advantage of all the recent research in comfort and technology (v-belts on the fans, not flat belts!), and on those occasions when I saw something that I thought must be a mistake, it was always that I just didn't understand their thinking. We bought district steam from Edison, used PRVs to regulate the pressure, and had vacuum pumps to maintain vacuum. We always ran at around 5" or so, except in the very coldest weather when we would run it up as high as 2#.
    Even our air conditioning was steam driven, since we used Lithium-Bromide absorption chillers to cool the water that ran to our air washers. No coils for the a/c, all water sprays with dew point humidity control. They cleaned the air, cooled it, dehumidified, and in the winter, humidified. Except for the chillers (originals were CO2 compressors) we kept everything as original as possible. 1928 brass Johnson Controls thermostats, Babbitt bearings on the fans, Nash vacuum pumps. . . Save what is best from the past, even technology. Those machines will still be doing their jobs long after yours-truly has gone to his reward.
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