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Confused about radiator vents

Clance Member Posts: 53
I'm sure that Im over-thinking this but -I see that radiator vent air elimination rates are listed based on 3 different OZ pressure scales.
For example- Maid O' Mist #4: .028 CFM@ 1oz, .045 CFM@ 2oz, .060 CFM@ 3oz.
Maid O' Mist #5: .10 CFM@ 1oz, .158 CFM @ 2oz, .20 CFM@ 3oz
I'm confused about what the pressure in Ounces is referring to and how to figure out what mine is so that I can do the math to choose the vents... if I am supposed to use 1, or 2, or 3 oz?
In my mind I would divide the air CFM to be eliminated by the the vent's CFM capacity at my particular oz to see the total time it would take to vent each my radiators with different size vents.
So if am trying to vent .90CF of air (3 column, 6 section rad so 36sqX.025=.90CU Ft)
with a Maid O'Mist #4 then
@ 1oz it would take 32 minutes (.90/.028),
@ 2oz it would take 20 minutes (.90/.045,
@3 oz it would take 15 min (.90/.06)
The same .90CFm of air with a Maid O'Mist # 5 then
@ 1oz it would take 9 minutes (.90/.10),
@ 2oz would take 5.6 minutes (.90/.158),
@ 3oz would take 4.5 minutes (.90/.20)

What I can't figure out is what OZ I should be using. I realize that balancing is trial and error but I figure if I calculate the minutes that different vents will eliminate the air from each radiator then I can find a common ground of minutes across all of them and that would give me a fairly close starting point in choosing different vents that in theory would allow all the rads to heat in a close timeline.

I have and read Lost Art and also Gerry and Steve's Balancing Steam Systems (which is where I pulled the vent #s) I realize my process differs from Gerry and Steve's in that I'm not choosing a set arbitrary minute divisor but rather more trying to discover a common minute with different vents at each radiator.
I'm also a bit confused that Gerry and Steve wrote on P.4 paragraph 1: "So you may feel more comfortable venting slower, and if that’s the case, use the 2-ounce or 3-ounce scales." It looks to me like using a 2 or 3 oz scale increases the venting speed (over 1 oz) contrary to Gerry and Steve's statement. So I am Missing something in my interpretation.

Is the OZ scale just an arbitrary decision and I just pick one? Or dos my systen have a specific ounce (1, 2 or 3oz)?

My Pressuretrol is set for 2PSI with a 1.5 diff and the Vaporstat is set at 2PSI with 6oz Diff. and the round gauge above the Vaporstat reads about 1.25 PSI


  • KC_Jones
    KC_Jones Member Posts: 5,736
    To get pressure in the steam system, the vents have to be inadequate, the boiler oversized, or a combination of both.

    Personally I use the venting guide as a reference for which vents are faster and which are slower, not for the actual rate. When the system starts producing steam the pressure is essentially zero. So the venting rate for a main vent will be lower than the 1 ounce rating. To get to the 1 ounce rating you would have to under vent, slowing the steam down, to build that pressure. This is why Dan says low pressure steam moves faster. This is why Gerry and Steve say slower for the higher pressure, because while the vent is capable of venting faster at that pressure, you have already slowed the steam down to get that pressure.

    For what you propose you could use the lowest range since you seem to be describing a comparison across your system. Where the calculations would stop is using them to predict actual time, like I said you will vent much slower at low or zero pressure.

    To add clarity, or maybe confuse further. The pressuretrol or vaporstat aren't controls they are safety shut downs. They don't force the boiler to have pressure, they react after the system creates pressure and shut down the burner. I think this concept is critical to understanding these systems. As I said above it's a function of venting and boiler size, the pressure safeties react to that, not control it.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    There are way too many variables -- most of which are either difficult or impractical to measure -- to be able to use the venting rates as anything but relative guides, and if you look at the rates measured at various differential pressures, you will find that -- relatively speaking -- they are similar; a vent which is faster at 1 ounce differential will be faster at 2 and 3 ounces.

    Do not make the mistake of assuming that all the radiators -- never mind mains -- will be at the same pressure at the same time, as long as the boiler is making steam. They won't. Use the relative rates as a starting point, and plan to adjust as needed by your specific system and building from there.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • The best method of determining vent size is by reading the pressure in ounces as the main vents are allowing the air to escape. Under 2 ounces is best, and you need an additional gauge graduated in ounces.
    Choose slower radiator vents, so that the main supply pipes fill first, and then the steam will start to rise to the rads all at the same time. The aim is to balance the backpressure of the main vents to the aggregate of the rad venting, and have greater resistance in the rads.
    If there are some remaining imbalances in steam arrival time, then you can change some of the vents to be faster, (maybe in top floor rooms).
    When the air has been removed, all vents closed, and the boiler continues to steam, the pressure may rise depending on whether the boiler is over sized for the system.--NBC
  • Clance
    Clance Member Posts: 53
    So I'm definitely over thinking it - what I'm hearing is the CFM rating on vents is best used as a guideline for comparison rather than a basis for accurate calculation. And sounds like I should go with 1oz when thinking about it.
    The boiler is a week old - new MegaSteam 288 put in by NESW. Its small- 2 parallel flow (1 is 12' and the other is 30') and 1 counter flow (16' 1.5" pipe)- total rads = 288 EDR. Gorton # 2 was added to the end of both dry returns, no vent on the counter flow.
    All rads have Varivents installed and its's my tenants system so I don't have open access- so going by what I feel on her pipes in the cellar. What I'm feeling is that steam is reaching the end of the longer parallel flow close to 4 minutes after it reaches the end of the shorter parallel flow and the rad at the end of 30' is on a riser to the 2nd fl and is getting 2 of 5 sections hot at best. To make matters worse the tenant leaves her stat low (maybe 64) I think the system gets satisfied too early to heat the one rad on the 2nd fl.
    So, per NESW suggestion my plan is to eliminate the adjustable Varivents as they are way too fast and replace with all fixed #4s on 1st floor and a #6 on the 2nd floor as a start and see if that helps to balance out - then adjust from there. That sounds like what @nicholas bonham-carter explained too...vent the rads slow so the mains fill.
    Since I don't really have a lot of access to the apartment to keep balancing I figured I try the math system to creat sort of a guide for the next adjustments... and managed to confuse my self in the process. Thanks for the helpful info guys!
  • ethicalpaul
    ethicalpaul Member Posts: 5,701
    What I'm feeling is that steam is reaching the end of the longer parallel flow close to 4 minutes after it reaches the end of the shorter parallel flow

    These both have gorton #2 the ends of them? How are you timing it? I recommend:

    - let the mains get steam to the main vents during a call for heat
    - switch off the emergency switch.
    - wait 5 minutes
    - flip the switch back on
    - start your timer
    - go to the closer main vent and feel it let the air out. When it closes note the time
    - go to the farther main and feel it let the air out. When it closes note the time

    If you are timing from a system that has been off for awhile, then the mains have to get re-heated and that makes the steam take longer.

    But to answer your real question, I think when you get #4 vents on the radiators that are closer to the thermostat, and #6 vents on the upstairs, the whole game will change and you should have much better luck balancing it.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Clance
    Clance Member Posts: 53
    Thanks - Ill try your way next time. The vents are close enough that I can stand between and touch each. Answer to your questions -
    yes, each one has a Gorton #2 toward the end of the Dry returns.
    I was timing it from a sitting idle start with cold pipes to see if the mains were getting full at the same time - by holding onto the very end of the shorter vented main, then when it got hot running to hold the end of the longer vented main (only about 12' away)- and counting Mississippi to see how much longer it took to get hot. - it took about 220 Misissippi longer for it to heat to the end. Your way sounds better :)

    I tried to feel the air coming out of the #2 vents but I couldn't really feel or hear anything - but the the dry return after the Groton 2 vent didn't get hot as fast as the section leading up to it- so figure the vent is working. If that makes sense. I wondered if a lot of the air was going out all the radiator Varivalves instead of directly out to the Gorton 2s. (Guessing tenant has all Varivalves wide open- she would prefer I not come in due to pandemic)
    I just ordered Maid O'Mist Adjustable valves which may be delivered Monday and will start with the #4 orifices on everything on the 1st fllor and a #6 on the one on Fl 2. thx. Again! Rick
  • ethicalpaul
    ethicalpaul Member Posts: 5,701
    OK very good. I have often used the "touch test" to see when my pipes were getting steam. It's very convenient that they are so close, congrats on that :smile:

    Yes you may be venting a LOT of air out of the radiators. Things will be a lot better after you get those 4s and 6s on there. Then I bet you'll be able to hear and feel the air exiting the main vents.

    And I'm glad to hear you were testing it on rather cold pipes...that much time between the two mains getting filled with steam seemed much too long otherwise. Running the test with hot pipes will also give you a better chance of hearing or feeling the air exit the main vents.

    The Maid O Mists are my favorite, but I don't call them adjustable, but rather "swappable" :) If you buy a few extra 4, 5 and 6, then you'll have extra orifices to swap around and some spares in case of eventual failure.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Clance
    Clance Member Posts: 53
    since it's probably nice to hear how things end ... As a follow up to my swapping out the Varivents to the Maid O'Mists #4s on all the radiators (to start) - It definitely made a HUGE difference on balancing the Mains - what was close to a 4 minute differential between the short and long main branch ends getting hot is now almost simultaneous - the long branch is hot before I can even make the 6 steps over from the shorter one. Psyched!

    So, for all those as clueless as me and in need of help, what I learned is that "Balancing" the mains is an important task before attempting to balance the radiators...and this was accomplished by limiting the venting at the radiators as much as possible so that the main vents can do their job of allowing the steam to fill the mains first. Now on to balance the radiators/room heat.

    As to my 2nd floor rad not heating - identified a 23" horizontal pipe leading to its riser that is pitched the wrong direction. (pitched about 3/4")..so that issue is on my (and hopefully NESW's) to do list. :)
    Thx. Again to all!
  • ethicalpaul
    ethicalpaul Member Posts: 5,701
    That sounds great! Thanks for following up! When you try your radiator vent changes for additional balancing, make small adjustments slowly (like a few days between changes) and you'll be able to see how one change can affect things (possibly only slightly) in other rooms.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el