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Does Lowering the Steam Pressure . . .

Kool Rod
Kool Rod Member Posts: 175
I've got a cooperative board meeting coming up and I want to have some basic principles understood.

If all else is constant in my one-pipe steam system, does lowering the cut out pressure for my gas boiler (from 16 oz to 12 oz for example) effect how fast it will take to recover from a nighttime setback or otherwise influence how fast radiators and apts will get hot when the boiler cycles on?



  • Supply House Rick
    Supply House Rick Member Posts: 1,404
    to set back or not to setback

    that is the question! do a search here for "setback" and similar terms for discussions on fuel savings using setbacks. many times the fuel used to recover temperature will equal, or exceed the fuel saved by running at a lower temperature. certainly, steam at lower pressures moves through the system faster than overpressured steam. if your main line venting is increased to the point of steam "arriving more quickly", then that would mean less run-time for the burner trying to squeeze air out through the constipated little openings of inadequate vents!

    reduced pressure, even down to 2 oz. in some cases, results in greater comfort/economy as the radiators are heated to a slightly lower temperature, [using less fuel] and yet still warm the room. as you solve your venting bottlenecks, why not try getting your pressure down to 2/5 oz. as an experiment?--nbc
  • Brad White_191
    Brad White_191 Member Posts: 252

    Lowering the steam pressure does primarily two things.

    Firstly, it speeds up the delivery of steam. Because the volume is larger (and the pipe sizes are fixed), the steam simply has to move faster.

    Secondly and a fairly minor point, lower pressure steam has a higher latent heat content (the useful heat given up by condensing, the whole objective). This is also known as the heat of evaporation.

    Steam at sea level atmospheric pressure and 212 degrees has 970.3 BTUs per pound.

    At two pounds pressure, roughly 215 degrees, this drops to... (hold your disappointment now...) 968.4 BTUs per pound.

    By way of amplification, steam at 100 PSIG (do NOT do this at home!), rougly 338 degrees, has a latent heat value of about 880 BTUs per pound.

    Not a huge difference in the scheme of things at the low pressure ranges you are using, just a fact of physics.

    Mind you, all this reduced BTU per pound data means is that the steam will condense at a higher temperature before giving up the proverbial ghost. The remaining condensate is still much hotter and if the pressure would suddenly drop, it would "flash" back into steam at a lower pressure...

    You can tell I have very few hobbies :)
  • Jackchips_4
    Jackchips_4 Member Posts: 4

    "You can tell I have very few hobbies :)"

    We like the one you have here.

  • Jamie Hall
    Jamie Hall Member Posts: 20,467
    and the bottom line is...

    not much, going from 16 oz to 12 oz. If there is adequate venting, things will heat -- initially -- a little faster, but if the venting isn't adequate... on a one pipe steam system, the radiator vents are the number one determiner, although main vents can and sometimes do help a lot (at the ends of the mains -- one pipe steam doesn't have the luxury of cross-over traps!)

    Now had you started off at 2 pounds or more -- not an uncommon problem -- then yes, it would make a significant difference.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • FJL
    FJL Member Posts: 354
    This means . . .

    OK. I want to be sure I understand the information you've provided.

    The general answer seems to be that lowering steam pressure should increase the delivery of the steam to the radiators and increases the amount of latent heat. This means, I gather, that the building should heat quicker at lower, correct?

    But I read nbc to say something contradictory to that, which is that radiators are heated at a slightly lower temp yet still warming the room. This suggests that lowering the pressure will get the steam to the rads quicker, but it might take the rad longer to get as hot if the steam was more pressurized.

    I am trying to clarify because one of the complaints of a neighbor is that the building is taking longer to get warm after a setback under the "new boiler." Now I understand that this complaint could be caused by a host of other things unrelated to the pressure of the steam (and there might be another building condition responsible for this). But I just want to be sure whether lowering the pressure could be contributing, in any conceivable way, to how quickly or slowing the building heats.

  • Supply House Rick
    Supply House Rick Member Posts: 1,404
    lowered pressure

    i think that i am trying to say that the system, if at proper low pressure, and if properly vented, would be comfortable, and economic enough to keep set at a constant temperature [say 68 deg]. maybe the temp could be a degree lower than the present high setting, if constant, and that would save money. also the constant temperature setting will reveal any other "envelope" problems, which should be addressed, in conjunction with the search for heating economy.--nbc
  • ttekushan_3
    ttekushan_3 Member Posts: 944
    if I might add...

    Yes you can get more heat with increased pressure but its a bad idea for a low pressure system. As pointed out, the steam won't get distributed any faster.

    Yes, the latent heat is lower with increased pressure, but the radiators internal volume is now holding more steam due to the compressability of the steam vapor. High steam pressures can deliver more heat into a space. This may be a distinction without much difference in this case, because the pressures that take a steam radiator from "warm" to "bake" (or even "cinder") usually create the situation where we run into those nasty overshoot problems, like coming out of setback to a new setpoint of 70 degrees and finding the space 75 degrees and rising with occupants opening windows!

    The proof that pressure directly affects a radiator's heat output, is a subatmospheric system which operates under vacuum delivers steam with a high latent heat content and allows the radiator to run cooler and deliver less heat. There are fewer pounds of steam in the radiator because the vapor expands under lower pressure.

    I've run into systems in industrial settings that were designed for higher pressures to deliver the required amount of heat, but thats the key: "designed for higher pressures." Its almost frightening how much heat something like a little unit heater can deliver using 10 # pressure! The steam is 240 degrees at that point!

    At the end of the day, (to overuse a term) we agree that the system in question was undoubtedly designed for no more that 1 lb pressure and nothing more should be used. For all the reasons innumerated here, pressures beyond the system's design requirement are wasteful and put undue stress on vents, valve packings, traps, etc. and sometimes introducing unpredictable knocks, spits, bangs and clunks.

  • FJL
    FJL Member Posts: 354
    I See What You're Saying

    You're saying that we'd use less fuel if the temp was set at 68 degrees for 24 hours instead of at 70 with a night set back. That might be true, but last year we set the t-stat at 68 and people complained about a lack of heat when the boiler was cycling, and we decided to set it to 72 (w/a nighttime setback of 6 degrees). This year, we set the t-stat at 70, but had a daytime setback (9am-3:15pm) and a nighttime setback (11pm-5:30am) during the week. We had a change of residents that allowed for a daytime set back.

    But "to setback or not" was not the focus of my query. My neighbors are ok with a setback. My only question is should a lowering of the pressure reduce or extend the run time after recovery from a setback (all else being equal) as that was the complaint from a neighbor. And the answer to that, I gather, is that lower pressure should = shorter run time, with the caveat that the difference might be negligible when reducing from 16 oz to 12 oz.
  • FJL
    FJL Member Posts: 354

    I'm a bit confused. Maybe this question clarifies what I want to know.

    The boiler turns on a 6:00 a.m. and runs for 60 minutes to satisfy the t-stat. All else being equal, if the pressure had been set lower, would it take more or less time to satisfy the t-stat?
  • Bill_110
    Bill_110 Member Posts: 52
    Night setback

    I use a big night setback myself, mainly cause everyone in my household sleeps much better under cold conditions, so it's efficiency isn't the prime consideration. I just got Dan's set and finished the We Got Steam, so I'm eager to put some of the info to work in analysis. I can't address the pressure question with the expertise you've been given the benefit of, but I noticed you mention the tenants complain of a longer warming period with the "new boiler".

    I would expect the new boiler is quite different in BTU output than the old one, with a different volume of water to heat to steam. If it is a considerably smaller boiler than it's going to take that much longer to heat all the cold piping leading up to those radiators to steam heat temps. And the cold pipes are going to turn some of that more limited steam volume to condensate. So it just may not be possible to get things heating as quickly after the setback as they did with the old boiler, even with lower pressures. It's interesting though that the old boiler I had was so grossly oversized that it took a long time to heat up it's huge volume of water to steam, so much so that the boiler I have now with half the BTU output (and water) actually heats up as fast if not faster than the old one did after a night setback. I might just be a knucklehead thinking he's learning about steam though.

  • Kool Rod
    Kool Rod Member Posts: 175
    Good Point . . .

    but I also lowered the pressure during the middle of this month, so I want to know whether that could be the cause of her complaint, and she is just attributing it to the "new" boiler. Plus, as I recall, the BTU output of the new boiler is not much different than the old one, although I'm sure it is smaller.
  • Jamie Hall
    Jamie Hall Member Posts: 20,467

    that your tenant found out about the new boiler. I have found that keeping tenants in the dark about the mechanical side of things often works in my favour (I'm a building superintendent).

    I really don't think that in the pressure range you're dealing with it is making much difference -- although you don't say where you started, or was that the 16 oz.? In any event, any difference it made should have been for faster, not slower, heat.

    And I doubt that the boiler change, if it really is about the same BTU, made that much difference either (again, if anything it should be faster, as newer boilers hold less water to get up to speed).

    The setback now, though -- that could really make a difference. Deeper setbacks take much longer to come up to temperature (the difference in time required is almost linear: if it takes 30 minutes to come up 2 degrees, it will take an hour and a half to come up 6); as someone noted above a really deep setback (more than say 5 degrees) with steam also can kill your efficiency.

    I might add that tenants are weird...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Kool Rod
    Kool Rod Member Posts: 175
    Thanks . . .

    for the tips. They're not tenants, but resident-owners. She paid about 25% of the cost for the new boiler, so she is well aware of if, LOL.

    Anyway, I think the set back is causing the issue. I think maybe it is too deep. I set it back to 62, but the temp almost never gets that low. I've seen it go to 64 or 65 on some cold nights. Maybe a setback from 70 to 66 would be best for us.
  • Kool Rod
    Kool Rod Member Posts: 175

    I loved the story about the tenant that complained so much that the steam pro finally gave in and installed a thermostat on the wall in the tenant's apartment so he would have his own heat control. The tenant was very happy. The thermostat wasn't connected to anything.
  • Brad White_191
    Brad White_191 Member Posts: 252
    Less Time

    because the steam will move faster. Not by much, but faster.

    The other "things being equal" is that the setback temperatures would be the same for your hypothetical comparison and that your building mass would be the same.

    If you are finding that some areas are falling short in the heating department with setbacks, I would compare the radiator sizes to those spaces heat losses. If those rooms have comparable heat losses and radiator sizes to other spaces which work well, I would then go time the radiators warm-up cycle compared to others and increase venting.

    A "late arrival" warm radiator has less time to do the work it needs to do, heating the space.

    Deep setbacks are over-rated. Especially high mass buildings with little insulation.
  • Supply House Rick
    Supply House Rick Member Posts: 1,404
    another suggestion

    i would move the visionpro you now have, and replace it with the indoor sensor, made for that model, and then remount the central visionpro in some area, which only you can access. when the v-p has the sensor connected, its internal thermometer is disabled and it runs off the sensor, but with the controls [and temp readouts] at your fingertips. a group of 4 sensors [maybe on each side of the top floor?] can be wired in to it each up to 100 ft. away and their temps are averaged by the v-p itself. the advantage would be in not having someone else changing the settings, while you are trying to tune up the system. in addition, i would give the apt. owners good digital thermometers. put them all together, and compare the temps, before you give them out, as you want them all reporting the same temp from one spot. some of them can remember the highs and lows which can be useful. when the owners call to complain, ask them what the thermometer says. feelings of being cold or too hot are subjective, [chances are when they see the actual temp, they will not call! this would also give you the opportunity to disable the night setback until you are sure you have even heat [steam arriving to each of the rads on the top at the same time] --nbc

  • Brad White_191
    Brad White_191 Member Posts: 252
    Dummy Thermostats...

    The secret is out now, Rod...

    We will have to find something else now :)
  • tim smith
    tim smith Member Posts: 2,498

  • Kool Rod
    Kool Rod Member Posts: 175
    I had a similar idea

    Right now, the VisionPro is connected to a sensor, which is in the top apt. The control itself is in the hallway, where all residents can get access to it.

    Just today I was toying with the idea of maybe putting a wireless indoor thermometer. I could put the sensor in the 3rd floor apt, and be able to monitor the temp from my apt. That way, I could be able to see what the temp spread is in each apt.

    During the recent really cold spell, my apt actually was colder than the top floor apt for the early part of the day. For some reason, since Wed, the spread has increased and my apt had been overheating. My bedroom hit 78 degrees by 11:00 a.m. We turned off the heat altogether for several hours after that to let the building cool off. We're having a coop meeting on Thursday. I hope to have a better idea of what people have been experiencing with the heat, and what they'd like to do for the remainder of the heating season.
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