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Understanding working steam pressure

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So let me see if I've got this straight...

A one pipe steam system will humming along at <em>some</em> relatively low (under #1) pressure, but it is not possible or particularly advantageous to try to lower this working pressure (short of repiping). Pressure controls like vaporstats merely ensure that the system does not grossly exceed this ideal pressure- it does not "make it" work at lower temps.

Right? Or wrong?

Thanks,

Patrick
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Comments

  • Steam Pressure

    I'm not quite sure of what exactly you are asking. Ideally you want to run your steam system at as low a pressure as possible.This may have limiting factors due to design faults in the system. If they are major, you fix them, if mino,r it's usually not economically worth it to re configure the system. Building pressure wastes fuel as the boiler  is making more steam than can be used. However until they get a full modulating burner, we are stuck with the "on and off" system. The Pressuretrol and Vaporstat are limiting devices. They control the on and off points of the burner. A Pressuretrol's measurements are fairly course and they are considered unreliable at low pressure. The Vaporstat is designed to make finer measurements and therefore works better at low pressure (measured in ounces) If you look over at Gerry Gill's recent posts under Controls you will see that the recent quality control of these apparently have a lot to be desired.

    - Rod
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited December 2009
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    As I understand it . . .

    When a steam boiler operates at a lower pressure, it makes steam at a lower temperature.  This, in turn, extracts more heat from the combustion of the fuel, resulting in lower stack temperatures.  The net result is greater combustion efficiency.  The difference in temperature from 0 psi to say . . . 3 psi is only a few degrees.  Whether or not the decrease in combustion efficiency is really significant at the slightly higher pressure isn't all that clear to me.  The gage readings of 0 and 3 psi are also a little misleading.  At a gage pressure of 0, the steam is actually at atmospheric pressure . . . about 14 PSI.  Raising it to 3 psi gage pressure raises it to about 17 psi . . . only about a 20% difference in actual pressure.



    Additional advantages of operating at a lower pressure seems to be quieter vents that work better because pressure doesn't hold them shut, and the potential for less leakage in the system.



    Lower pressure steam also travels faster because it is less dense.  As a system fills with steam, the pressure limit switch probably hasn't come into play yet anyway, so it's setting irrelevant at that point in the cycle.  Once the system is full of steam, I'm not really sure it matters all that much how fast the steam velocity is, since the mass flow rate of the steam would remain relatively constant at any pressure.  Since the vents probably close the first time before all the air is completely out, they need to be able to open repeatedly.  Low pressure helps there . . . but only because the typical radiator vents aren't designed to open again under much pressure.



    Basically, the main advantage I see is that lower pressure equals lower steam temperature, resulting in less energy wasted up the stack.
  • David Nadle
    David Nadle Member Posts: 624
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    Advantages

    A vaporstat is more precise than a pressuretrol. If it takes less than 0.75 psi to fill all your radiators with steam--and it probably does--there is no reason to burn fuel until the pressure reaches 1.5 psi. That's where the advantage comes in.

    However... if your system is like mine it will almost never cut out on pressure. You might want to wait for a cold day and try to observe your boiler cycling on pressure before investing in a vapostat.
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited December 2009
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    Cycle delay comes into effect.

    If your burner is only on/off, and not modulating, then there will be a delay from the time that the controls call for burner ignition and the time the flame actually comes on.  On intermittent pilot systems, this delay can be almost a minute, due to the safety factors built into the ignition sequence.  You'll want to have the low pressure setting high enough to make sure that the system doesn't go into vacuum from the time the system hits the low pressure setting on the pressure control switch, and the time the boiler starts making steam again.  On my boiler, If I set the low pressure setting much below 1 psi, the system is sucking air before the boiler starts steaming again.  That can't be good for efficiency . . . and venting an already hot system happens a lot more rapidly than a cold one.  Not only are the vents opening more in that case, they are noisier.
  • Jamie Hall
    Jamie Hall Member Posts: 23,428
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    Flavours of steam system and pressure

    In residential heating -- and in most (but not all) large buildings, there were and are two basic flavours of steam heating systems: "conventional" systems, which might be one or two pipe, and "vapour" systems, which were always two pipe but had some important differences from conventional two pipe systems.



    Conventional systems were usually designed so that there was about a half psi loss in the steam mains and risers, and an allowance for a half psi loss in the rest of the system.  Thus they were intended to operate at 1 psi, more or less.  There is no advantage to going to a higher pressure.  It takes more fuel to heat the boiler to a higher pressure, and the small amount of extra heat from the radiators at the higher temperature doesn't compensate (almost all the heat in the radiator is from condensation, or latent heat).  Further, going to a higher pressure puts extra stress on the whole system -- and some components, particularly vents, don't work properly (if at all) at anything over their working pressure, which is usually 2 to 3 psi.  In fact, they can be destroyed by significantly higher pressure.  Thus you would run a conventional system at around 1 psi -- which is done nowadays by using a pressuretrol or vaporstat, set to cycle the pressure around that value.



    Vapour systems, on the other hand, were designed to operate on even lower pressures -- in fact, in the old days with coal fired boilers, they were designed to operate at a partial vacuum for part of the time.  Most vapour systems will not operate properly above a pressure of 8 or 9 ounces per square inch, and will run very nicely at 0 (there is a pressure difference still, as the condensation in the radiators continually creates a slight vacuum).  Unfortunately, for practical reasons, vaporstats have a lower limit of pressure for cutin around 2 ounces or so psi.  So if you have a vapour system, you use a vaporstat (a pressuretrol is useless at those pressures) set to cut in at 2 ounces and cut out at 8 or 9.



    You are partly correct in saying that the purpose of the pressuretrol or vaporstat is to control the system pressure, but it isn't so much to avoid grossly exceeding the maximum pressure as it is to limit the pressure both high and low to the optimum range for the system.  It does that by cycling the burner on and off (or, in some systems, between high and low fire).  In the old days, the same purpose was accomplished with pressure regulated draught dampers on the coal fire -- as the pressure rose, the damper closed, and vice versa.  In both cases, until the pickup load and initial load of the system is satisfied, the fire goes along at its rated value.  It's only when the initial loads are satisfied that it is necessary to limit the fire to the amount of heat which the radiation can condense -- and that's when the pressuretrol or vaporstat (or, in the old days, the damper) comes into play.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Big-Al_2
    Big-Al_2 Member Posts: 263
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    I Wish I Could

    operate my boiler from 0 to 1 PSI. (It was originally a Trane vapor system.)  My pressure limit switch set at to run between 1 and 2 psi results in an actual operating pressure from 2 psi to close to zero.  Given the lag times involved in the gas train on my Burnham IN-6 boiler, I can't figure out a practical control scheme to lower the pressure.  If the burner cut out at 1 PSI, the pilot ignition sequence would have to start again IMMEDIATLY to stop the system from sucking air.  If I could find vacuum system radiator vents, then maybe the system could exclude air long enough to get the burner back on . . . but as far as I can tell, they are not made anymore..



    Any practical suggestions on how to operate at a lower pressure would be appreciated.. 
  • Patrick_North
    Patrick_North Member Posts: 249
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    Should have been clearer...

    ... in my question, but I think I got the answer, anyway!

    If my 3# gauge is to be believed, my one pipe steam system seems to settle in at about 8oz of pressure. My original question should have been: "is this 8oz more or less fixed because of the way my system is laid out? By adding a skyhook vent or removing a board stretcher gasket could I lower this "happy" pressure to, say, 4oz? And if I could, would my boiler run better/more efficiently (assuming that my vaporstat can cut out at the optimal point in either case)?"

    From your responses, I gather:
    1. The 8oz (in my case) is "baked in" and there's nothing to add or remove to change it.
    2. Moreover, as long as my vaporstat prevents me from running up pressure much beyond my system "ideal," 4oz is no better than 8 oz. In other words, the only critical point is to keep your boiler in a pressure zone that surrounds yours system's design point, whatever value that is.
    3. I ought to set the vaporstat to cut out at not much higher than 8oz (this much I knew), but don't set the cut in much lower, either (I hadn't thought of this being as important, but I see the rationale now). 
    When I stop learning...

    Thanks, all,

    Patrick 
  • Big-Al_2
    Big-Al_2 Member Posts: 263
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    If your system stays at 8 oz . . .

    when the system runs continuously for a very long time, then that means that that's thepressure/temperature  at which the system is in thermal equilibrium . . . the amount of heat leaving through the radiators, etc. equals the amount of heat entering the water in the boiler.  If you shut off a radiator, the equilibrium pressure would be higher, and vice versa.



    At 4 oz of pressure, saturated steam is at 212.8 degrees F.

    At 8 oz of pressure, saturated steam is at 213.7 degrees F

    At 2 lb of pressure, saturated steam is at 218.5 degrees F



    Would you gain measurable efficiency by lowering your boiler temperature 9/10ths of a degree?  I doubt it.  Would I gain  measurable efficiency by lowering my boiler temperature 5.7 degrees?  Maybe.  However, in both cases I'm sure that the major portion of each call for heat has the boiler operating at just above zero pressure, and the higher temperature is only realized intermittently for a very short time near the end of the call. 



    My boiler is somewhat oversized.  That's why it can reach 2 PSI or more.  I could lower the equilibrium  pressure  by adding more radiation . . . but my boiler is off more than it's on already.  I could also try to down-fire it somewhat, but operating it too far outside of its design parameters might actually use more fuel.  I would benefit from a smaller boiler, but the break-even point might not occur in my lifetime.
  • jpf321
    jpf321 Member Posts: 1,568
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    how to determine optimum pressure?

    for 1-pipe steam ..

    how does one determine optimum system pressure?



    when Patrick says "settles in @ 8oz" what does that mean? does it mean that at a particular outside temperature, his boiler will run forever and never increase or decrease pressure? what happens @ 10oz? or 6oz? that is thermal equilibrium has been reached heat-loss =  heat-fired



    when folks here say, for example, adjust v-stat to 8oz/4oz as a starting point what does that imply .. how does one determine whether to raise or lower the cut-out or cut-in ..



    should pressure be goverened by something like an outdoor reset?



    it is my understanding, that with my oversized boiler, I should set it at a pressure which ensures all rads heat all across. then cut-out. then cut-in before too much outside air is sucked into the system. am I way off base?



    is there a tried and true methodology for determining optimum pressure? has it anything to do with EDR and main-length? I'm sure that pipe insulation comes into play.



    sorry for all the questions .. i guess i'm trying to get my head around this rather than "pick a number between 2 and 16oz for cut-out and a number 2oz less for cut-in".. and maybe there are indeed perfectly sized boilers that will get to 4oz and never stray from there for a given indoor/outdoor heat-loss condition and it's so hard for me to grasp b/c my boiler is so grossly oversized.
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • FJL
    FJL Member Posts: 354
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    Sucking Air

    I'd like to hear more about the relationship between the pressure setting and the system sucking in air when the boiler cycles off due to pressure, and whether this is detrimental or not.  My system sucks in air when the boiler cycles off due to pressure, and I read somewhere that this is a normal condition and nothing much to worry about.  If this has some engery efficiency consequence, I'd like to hear more about that. 
  • jpf321
    jpf321 Member Posts: 1,568
    edited December 2009
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    some more thoughts...

    so i went down and made some observations and played a bit with the v-stat .. i think the setback has much to with it as well. for a given setback and a given outside temp .. pressure comes into play.



    i think the main goal is this:

      -- keep the pressure high enough to heat all rads across but no higher (higher pressure adds only fractional heat and slows down steam and causes longer burn)

      -- keep your fire going as long as possible (for efficient burn and less wear and tear)

      -- limit the fire downtime as much as possible to keep cold air out (less sucking) since sucking leads to super fast condensation, vacuums and pressure loss

      -- pressure lost (thru sucking cold air or ignition delay) will require fire to burn longer to get back to pressure to keep rads heated all across.



    i appreciate any corrections or criticism.
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited December 2009
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    I would say that the absolute optimum . . .

    would be to operate a boiler at just enough pressure to overcome any

    pipe friction and keep all the radiators full.  It might vary from

    system to system, but would probably be no more than a few ounces.







    That said, the boiler needs to have enough excess steam capacity to overcome the "thermal inertia" of the system and fill it with steam fairly quickly.  That would necessitate it to be a little oversized.  If it doesn't switch to a lower firing rate after the system is full, then it's probably going to have to cycle off and on.



    As far as the system sucking in air, much like jpf321, I can think of four reasons why it's bad.



    1) Each time the system fills with air it brings in fresh oxygen.  More oxygen means more internal corrosion.



    2) Filling the system with cold air means that the boiler has to fire that much longer to get steam to the radiators again.  Longer firing means more fuel used.



    3) The sound of vents breathing or clicking open and shut can get annoying.  I'd rather minimize that whole procedure than have it start up over and over every time the burner fires up.



    4) Every time the system vents, a little water vapor gets out with the air.   Even a properly operating vent also lets out a wee bit of steam right before it closes.  If more steam gets out, more water needs to be added.  More feed water means more corrosion from more dissolved gasses, and more dissolved solids entering the boiler.



    So what is the optimum pressure in the real world?  I'd say as low as you can go and not suck air back into the system until the thermostat is satisfied.  If the burner cycles more on a lower pressure setting, but the boiler keeps making steam, it might wear things out a tiny bit quicker, but it should actually save energy by keeping the steam temperature lower  . . . but hey, I'm just a homeowner/engineer who is trying to figure this out too.
  • Jamie Hall
    Jamie Hall Member Posts: 23,428
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    Latent heat, gentlemen, latent heat

    In the various comments about heat output of a steam system vs. steam pressure and condensing temperature, there is a serious point being missed: in fact, the elephant in the room.



    Steam delivers heat to a radiator by condensation, which occurs at a constant temperature (the boiling point at that particular pressure).  All that is required is that the radiator radiate away the heat (or convect it away).  Now granted, the BTU output of a radiator does vary with temperature -- but rather weakly.  Therefore raising or lowering the temperature of the steam -- that is, raising or lowering the pressure, since we are talking saturated steam here -- will have at best a very small effect on the BTU output of the radiator, and therefore on the required BTU input of the boiler.



    Nonetheless, it is true that a steam system will come to an equilibrium condition, if you have a way to vary the firing rate.  If the design of the system were such that it was intended to operate at 15 psi, you would attempt to control it so that it ran at 15 psi.  If the design pressure was 1 psi, that's what you'd go for.  If it was 4 ounces, that would be the target.  The design operating pressure is built into the hardware of the system, and has absolutely nothing to do with the temperature of the steam.



    Although it sometimes seems as though the chant is "lower the pressure" without regard to the system, it really shouldn't be.  It really should be "adjust to the optimum pressure, taking into account the design of the system".  The reason that one hears "lower the pressure" so often, is that so often the pressure is too high -- over 2 psi! -- for any residential steam system to operate correctly, regardless of flavour.  It is possible to run too low a pressure on a conventional system as I defined the term in my previous post -- but not with a pressuretrol (they won't go that low). 



    In an ideal world, we would modulate our burners to maintain the design pressure, plus or minus whatever small variation was needed to make the servo system stable (the old time coal burning boilers could maintain pressure to less than an ounce variation -- all day, until the fire burned out -- although some of them, for vapour systems, were deliberately designed to allow the pressure to droop instead).  We don't have an ideal world, so we use on/off or sometimes 2 stage burners, and get the desired effect by allowing the pressure to vary over a small band (cut in to cut out, or stage to stage).  If you had the time and the money, you could probably set up a test rig to determine the optimum cut in and cut out pressures for best efficiency -- for a specific system.  We don't, so we make generalizations, and it will be a balance between wear and tear and slightly lower efficiency on starting the burner vs. heating a slightly cooler (been off longer) boiler.  Probably change day to day...



    But it has nothing to do with steam temperature -- only optimum operating pressure for our specific system.



    On sucking air -- a steam system will only bring in air when the pressure drops to zero (assuming we don't still have vacuum vents).  That's once per thermostat call, regardless of how many times the burner cycles on and off.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • FJL
    FJL Member Posts: 354
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    Sucking Sound

    I notice that my vents in my second floor apt suck in air even when the boiler cycles off due to pressure, and that will be more than once a t-stat call for heat.
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited December 2009
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    Jamie, latent heat does carry the energy from the boiler to the load,

    but first the latent heat needs to get into the steam.  If the boiler is operating at a higher pressure, therefore a higher saturated steam temperature/boiling point, then it does stand to reason that the stack temperature will ride up right along with the overall boiler temperature.  A higher stack temperature means more energy going up the chimney, less heat going into the steam, and less overall combustion efficiency.  Lower boiler pressure leads to higher combustion efficiency.  Am I missing something?



    As far as sucking air . . .   When my burner shuts off on pressure, the boiling rate starts dropping off, and the steam pressure starts dropping.  When it hits the cut-in pressure, the ignition sequence starts and the race is on.  The flue vent damper opens first.  Then there seems to be a fixed delay before the igniter is energized.  Then the pilot valve opens. Then the lit pilot heats up the flame sensor  . . . then the main gas valve opens a little . .  then full-bore, and the iron in the boiler finally starts conducting burner heat from the flame to the water.  All that time, the boiler pressure keeps falling off.  If it stops making enough steam before the burner starts adding heat again, then the system can drop below atmospheric pressure each time the burner cycles on pressure, and the vents suck in air every time . . . unless the cut-in pressure is set high enough for the burner to fire up in time.  Again, am I missing something?
  • Jamie Hall
    Jamie Hall Member Posts: 23,428
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    Don't think you're missing anything...

    You are quite right, of course, that there is a marginal gain in efficiency in lower stack gas temperature (up to a point).  Since there is a minimum stack gas temperature, though, for most installations (we'll leave condensing units out of this!) it may be marginal.  But it's there, quite true.



    I love your race to the finish on your burner!  I have oil -- and when the vaporstat says "now" the burner gives a 15 second purge and off she goes, full bore.  Also, I have a vapour system, and although the dry returns are at atmospheric because of the vents, the radiators can -- and do -- drop into a vacuum once the traps close, if there is steam coming in -- particularly if the room in question is cold.



    I must say that in your system I would be very inclined to set the cut in high enough (heresy!!!) so that by the time the burner woke up and got steam up, you still had an ounce or two at least of positive pressure.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • David Nadle
    David Nadle Member Posts: 624
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    Collapse

    My system fills the rads and heats well at around 6 oz, but it's a bouncy 6. It's more like 5 to 7 with occasional bursts to 8. It goes from 0 to ~6 oz in about a minute, and stays there for a long time. I set my cut-out to around 10 oz. I felt that any lower might cause a false trip. But the boiler takes more than 40 minutes to get up there, and by then some of the downstairs rads are hissing. The vents stay hot, closed, and quiet for a long time, but they eventually give in and I don't know why. 

    When the thermostat or pressuretrol does cut out the pressure collapses to maybe 2 oz vacuum and stabilizes at 0 within 1 minute. I think that my system cannot be controlled effectively with a vaporstat. This year I changed my thermostat to 2 cycles per hour and so far it's been both quiet and comfortable.
  • lutorm
    lutorm Member Posts: 78
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    My pressure question

    This thread is connected to something I've been thinking about, too:



    What is the normal cycle time when an optimal system cycles on pressure?



    In my small apartment system (7 rads) it takes about 1.5 min for the pressure to fall from cut-out at 1.6 to cut-in at 0.6 (and it's fallen to .4-.5 by the time the vent has opened and the burner kicks in.) Then it takes about 2min 20s for the burner to cut out again.



    From reading "the lost art", I got the impression that in a perfectly designed system with dry steam (and no leaks, obviously), the pressure should stay up for a while even after the boiler turns off. That just doesn't seem likely to me.



    Using the handy excel sheet that was posted here, I estimated that my system has a total volume of 6 cubic feet. If we fill that with steam at 1.5psi, it holds about 0.25 lbs of steam. If the pressure is 0.5 psi, it holds 94% of that (because it's the ratio of absolute pressures that matter), ie when we've condensed 6% of those 0.25lbs of steam, the pressure has dropped to 0.5psi. To condense that amount, the system has to lose 32BTU of heat, which is not much. If that happens in 90s, like in my system, that's an equivalent power of 1300BTU/h which doesn't seem much at all. There just doesn't seem to be enough mass of steam in the system to keep it overpressured against the condensation loss for very long at all. (Unless I screwed up the numbers, which is entirely possible.)
  • Mark N
    Mark N Member Posts: 1,115
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    LAOSH CH. 2

    Patrick if you have LAOSH reread chapter 2. One pipe steam systems were designed to run on pressure drops of 1oz per hundred feet. So if your furthest rad is 100ft from your boiler your system should be able to run at slightly more that 1oz at the boiler. I have a small system 252sqft EDR. I use Gorton's for all my vents. When my system is running my low pressure gauge reads .1psig. That equates to 1.6oz.. My last rad is about 50ft from the boiler. Dan suggests you double that to take into account the loss through fittings and valves. That equates to 100ft. The gauge reads 1.6oz. I'll drop 1oz. over the 100ft so I should have .6oz at the rad. Plenty of pressure to vent the rad.

    Pressure in the system will start to rise as the rad vents close under temperture. No reason to make steam if the vents are all closed. So once the pressure rises to the cut-out it cuts off the burner. Hopefully by then you satisfied the the tstat. If not when the pressure drops to cut-in the burner will start. The vents should have reopened and you'll be able to vent air again.



    Mark
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited December 2009
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    Retained Heat

    When the burner stops firing, the retained heat in the iron of the boiler keeps the water boiling for a little while.  As the steam in the radiators condenses, the pressure drops a little, and the boiling point of the water falls with it.  This allows the boiler to keep boiling a little longer yet, since the iron may still be slightly warmer than the new boiling point.  We're only talking a few degrees of swing here from 1.6 to .6 psi . . . but it might keep boiling for a minute or two . . . depending on the thermal mass of the boiler.  If you had a vacuum system, the water would continue to boil at lower and lower temperatures as the system went into vacuum . . . but in a normal system, the vents would open not too long after the boiler lost pressure. 



    Your system seems to be behaving much like mine.  The quick pressure recovery on burner restart might indicate a somewhat oversized boiler . . . like mine.  Not much we can do about that, unless you can downfire the burner.



    As far as operating at only a couple of ounces of pressure, I'm sure that the piping and radiators would be perfectly functional, perhaps optimally functional, at those pressures.  However, because of the reasons in my previous posts, I've come to the conclusion that the internal control scheme on some intermittent pilot gas-fired boilers just won't work well at those low pressures.   It just takes too long for the burner to re-start. I'm not about to start tampering with the ignition sequence.  That probably wouldn't be safe.   If the boiler itself was designed to operate with a cut-in pressure of .5 to 1 psi,  then that's where it needs to run.
  • lutorm
    lutorm Member Posts: 78
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    I guess I'm confused...

    ... about how the system works.



    There seems to be 2 regimes. One is "open-air", when it cycles on the thermostat which gets satisfied before all the radiator vents close. I guess this is most common unless it's really cold? In this case the pressure is set by the size of the boiler and the size of the vents and there is no pressure control at all. The burner just goes until the tstat is satisfied.



    But there's also the second regime, "steady-state" or "sealed", where all the radiators are filled, the vents are closed and steam is circulating purely because it condenses in the radiators and makes room for new steam. In this regime, the system is sealed from the atmosphere and pressure is controlled by the pressuretrol/vaporstat, which cycles the burner on and off to maintain the pressure limits. It's this case I'm talking about.



    You said "No reason to make steam if the vents are all closed.", but that can't be true. In the old days of steady boiler fire, the vents must have been closed most of the time. Or even these days if it's really, really cold.



    Now in this case, if you set the vaporstat to cycle between 3 oz and 1 oz of pressure, it seems it will have to cycle the burner on and off every 10 seconds or something to maintain the pressure within that narrow interval.



    My system takes 90s to drop 16oz (from 1.6 to 0.6 psi), if I had a vaporstat and set it to 3-1 oz, it would turn off the burner for only 10s before turning it off. That kind of cycling must be really hard on the hardware, it's not even time for the vent damper to close before it has to start opening again.



    So, we're back to my original question: how long are the burner cycles supposed to be in a well-tuned system that is cycling on pressure control? I just can't see how they can be anything longer than 2-3 minutes.
  • lutorm
    lutorm Member Posts: 78
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    Good point

    Ah, I forgot about the "induced boil" from the lowered pressure. That's a good point. It shouldn't be hard to estimate how much that will add.



    I figured the boiler metal is thin by design so it wouldn't have much heat capacity compared to the water.



    The vacuum idea is interesting. I heard about these systems, but I thought they were only 2-pipe systems with a bunch of hardware necessary. Is there any reason you can't operate a 1-pipe system under vacuum? You need to run at positive pressure to get the air out, of course, but once the air is out, couldn't you in principle just have the vents seal shut and run the system under negative pressure?



    Fun stuff.
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited December 2009
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    Have you ever tried to pick up a boiler?

    Those guys are heavy!  Lots of thermal mass in there.
  • jpf321
    jpf321 Member Posts: 1,568
    edited December 2009
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    i think I need...

    a 0-3psi gauge on a rad or two to better understand the cycle of my system. it may be a while before i have a spare one in hand. maybe even a -1 - 3 to understand sucking forces at work.



    BTW .. I think The Wall should start recommending the following low pressure gauges:

    for folks with vaporstats - 0-20oz [url=http://www.gaugestore.com/prodinfo.asp?number=33015]http://www.gaugestore.com/prodinfo.asp?number=33015

    for folks with p-trols - 0-32oz [url=http://www.gaugestore.com/prodinfo.asp?number=33016]http://www.gaugestore.com/prodinfo.asp?number=33016

    not only b/c the gauges are scaled in OZ rather than PSI, but also because the secondary scale is INCHES OF WATER .. which will give people a good undrestanding of how high up their "A" dimension the water is. I'm ordering a 0-20oz now.

    They are the same price as the normally noted 0-3psi gauge. (#33020)



    Please note again that gaugestore.com DOES NOT WORK with Firefox web browser. Your order will never go through (but you will think it did). You nee to use Internet Explorer.
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • FJL
    FJL Member Posts: 354
    edited December 2009
    Options
    Low Pressure Gauge On Radiator

    I have a 0-20oz gauge hooked up to a radiator in my apt.  You can view it in this link:  http://www.heatinghelp.com/images/posts/1113/resize_DSCN0697.JPG.  Recently, I saw the pressure in that gauge get up to 14 ounces during a long run in which the boiler was cycling off due to pressure. 

    FYI:  I have since put teflon tape on all of the connections. 
  • Radiator Gauge Setup

    Attached is a picture that might be of help to you.  Depending on which direction the ball valve is placed, make sure the nipple under the handle (when the valve is open) is long enough to allow the full swing of the handle. I managed to get a ball valve with a short handle normally the handles are longer. I just had an extra 0-5 PSI gauge as you mentioned,  I would now go with a lower range gauge.

    - Rod
  • jpf321
    jpf321 Member Posts: 1,568
    Options
    great pic rod .. however ..

    should there be a pigtail prior to the gauge to protect the gauge from over 140degF temps?
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • FJL
    FJL Member Posts: 354
    Options
    Pigtail or Not

    Can you please explain when a pigtail is needed and when a brass tee is needed?
  • Mark N
    Mark N Member Posts: 1,115
    Options
    Chapter 8 LAOSH

    Lutorm



    If you have LAOSH reread chapter 8 especially pg 108. When the vent closes and the steam condenses the remaining air in the rad expands to fill the vacuum. Steam can't move into that rad until the vent opens and allows the air to be vented. In the old days they used coal that allowed them to use a vacuum vent on the rads. Doesn't work today with oil or gas.



    Mark
  • lutorm
    lutorm Member Posts: 78
    Options
    Yes, but...

    I did read that part. However, it doesn't quite make sense to me. Yes, when the pressure drops the air will expand. It does that precisely because the pressure drops. As long as the pressure in the radiator is lower than in the boiler, steam will move in that direction.



    Though you are right that if you were to drop the system into a vacuum, then that air could concievably expand enough to fill the entire radiator, at which point it won't work anymore. I guess that's why you need a 2-pipe system, not to evacuate it but to "draw" the initial air out and fill the radiators with steam?
  • jpf321
    jpf321 Member Posts: 1,568
    Options
    pigtail & brass

    this is what I believe to be true:

    pigtail are used so that water condenses in the pigtail and there is then a section of the pigtail with water in it. the water then protects downstream equipment from direct steam. the pressure pushes against the water "bubble" and then the water bubble pushes against the gauge so pressure is still accurately indicated. the water bubble will never grow too large since when it gets too big, it drips down rather than raises up you want to be careful that you don't get a bubble of "rust" or particles in the pigtail rather than simply water .. this would cause a "plug" or "clog" and needs to be cleaned out .. compressed air can work for that perhaps....at least this is how i understand things to be.



    regarding brass .. i think this is used in order to prevent corrosion .. brass doesn't rust like iron. the problem being that brass is more expensive than iron so brass parts are used here and there where corrosion prevention is desired. I believe that in this instance the reason you want to use a brass tee is b/c you don't want to rust the threads going to the multiple testing valves.



    as always, I welcome corrections and criticism
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • jpf321
    jpf321 Member Posts: 1,568
    Options
    i was going to ....

    comment regarding a possible air-lock condition earlier .. but then it did occur to me that the air HAS been removed .. the condensed steam thereby DOES create a mini vacuum into which new steam can move. if the condensing steam caused a strong enough vacuum that it pulled in air outside the system rather than more steam internal to the system, i could see an air lock problem .. but in my mind .. the condensing steam does not directly cause outside air to be drawn in. 
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • Mark N
    Mark N Member Posts: 1,115
    Options
    2 pipe

    You don't need 2 pipe. 1 pipe works fine. If your boiler is properly sized for your system, the tstat should shut off your boiler long before all the vents are closed and the pressure builds.



    Mark
  • jpf321
    jpf321 Member Posts: 1,568
    Options
    depends on ..

    this is true .. but you still have to include setback amount and active heat-loss (outdoor temp, wind, are the windows open?) .. so in cases where it's as cold or colder outside than system design day, or if the setback is deep (perhaps greater than 2degF) the t-stat may not be satisfied before pressure controls kick in.



    deep setbacks seem to be discouraged for steam. I have seen posts that mention 5degF maybe too much and 3degF isn't too bad.



    don't forget that in the dead men days, setbacks weren't as easy as setting your 21st Century digital t-stat. the coal fire burned long and hot and it didn't "turn off" and wait for the temp to drop to setback value before refiring. certainly they had damper control of the fire, but i don't think they had much option for t-stat setbacks. present times usually include a simple on/off fire so we can alter the t-stat settings much more liberally, but this may be contrary to system design and intention.
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • Jamie Hall
    Jamie Hall Member Posts: 23,428
    Options
    Believe it or not...

    first building I was responsible for was a vapour steam system.  Coal fired boiler (by hand, friends).  The dampers on the boiler were controlled by a combination of steam pressure (plus or minus 12" water gauge) and a thermostat contraption with a big bimetal in it and a cable...



    Setback?  ho ho ho.  The only time we had a setback was when I or my helper forgot to stoke the boiler...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Unknown
    edited December 2009
    Options
    Pigtail

    Let me first state you DO need a pigtail to protect a gauge,  It's just in the case of the radiator vent test gauge setup I don't think a pigtail is necessary as most of the contact is with vented air and also being in place for a very short time the exposure to the steam is very temporary. If a pigtail was used, the only benefit would be the length of the pigtail cooling the steam as you'd drain any water in the pigtail's loop screwing the setup on and off the radiator vent hole.



    This thread has been an interesting discussion. If any of you haven't read Boiler Pro's article, it may be of interest to you.

    [url=http://www.heatinghelp.com/article/11/Hot-Tech-Tips/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell]http://www.heatinghelp.com/article/11/Hot-Tech-Tips/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell



    I'm also uploading a very interesting chart I saved from a post  on the "Old Wall". Unfortunately don't remember the name of the person who created it so I can credit them for it.  - Rod
  • nicholas bonham-carter
    Options
    short-cycling on pressure

    this how i see it:

    let's imagine a perfectly sized boiler for the radiation load, with completely inadequate main venting. as the boiler fires, it builds up pressure faster than the air can get out, and so it will short-cycle, until all the air is out. when the radiation surface is thus completely available to the steam, condensing will take place at the same rate as the production of steam, and voila! no short-cycling. when the burner stops firing, the steam is no longer "fed", and will fairly quickly collapse into a vacuum-first in the pipes, then in the boiler. if the vents are not able to let the air back in, then you could have some water sucked up into the wet returns higher than when it is simply pushed up by boiler pressure, because of the larger pressure difference.

    that's why i say, "money spent on one more main vent is never wasted"!--nbc
  • David Nadle
    David Nadle Member Posts: 624
    Options
    don't think so

    Short cycling has nothing to do with backpressure from inadequate venting on my system. The vacuum develops almost instantly after cut-out. That is just a characteristic of the load I imagine. Something breaks the vacuum quickly, but the mains and rads are all hot. When the boiler fires again there is little energy expended on reheating iron, so cut-out pressure is reached again quickly.
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    Options
    Not Mine.

    The 0-30 psi gage on my boiler is threaded directly into a fitting on the boiler . . . no pigtail.  The installer did it that way eight years ago, and it was that way when I bought the house.  The gage still seems to work fine, and the readings correspond well with the settings on the Pressuretrol. (The Pressuretrol is on a pigtail.)  One of these days I'll get around to buying a better gage and installing it on a pigtail.  Somehow, I think the absolute need for a gage pigtail might be a bit exaggerated . . . but obviously it wouldn't hurt anything..
  • nicholas bonham-carter
    Options
    pigtail or no?

    your gauge may be labeled "internal syphon", and so therefore has a sort of built in pigtail. the only problem is that, being smaller,  "the internal syphon" can get plugged up, more easily without an external pigtail, and would be harder to flush out

    and i still think the pressure-cycling [depending on vaporstat settings] can start before the pipes are filled with steam, when poor/no vents are in place.--nbc
This discussion has been closed.