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Formula for pre-calculating radiator vent size?

Motorapido Member Posts: 307
edited January 2018 in Strictly Steam
Is there a formula for pre-calculating the ideal size for a radiator vent, or is trial and error the only way? Specifically, I am curious if a formula might exist to indicate the maximum vent size usable on a radiator based on its volume to prevent the dreaded rush of steam across the top or bottom and the closing of the vent before all the air has been pushed out.

I am doing some re-balancing to warm up a first-floor bedroom addition from the 1940s that is over a crawlspace (rest of house is over basement). This room, despite the radiators heating, has always been five or more degrees colder than all the other rooms, since the crawlspace is colder than the basement and this bedroom addition is just one story (rest of house is two story plus attic) and there is more heat loss through the roof than the other first-floor rooms experience. I put Vent-Rite #1 vents on three radiators closest to the thermostat in the main part of the house, and I have them set between 0 and 1 to slow them WAAAAYYYY down. I did that to increase the time required to satisfy the thermostat so there is more time for the colder bedroom radiators to continue condensing steam to warm that room to a temperature equal to the other rooms in the house. I use Maid-O-Mist throughout the rest of the house, including that colder first floor bedroom. On all the MOM vents, I am using the smallest orifice, except for in the colder bedroom addition, where I am using the third-from-smallest orifice. This has helped quite a bit so far in warming the cold room to within a few degrees of the other rooms.

The Vent-rite vents at their minimum setting vent a good bit slower than the smaller orifice.

In the cold room, I can easily increase the Maid-o-mist orifice size up another size or two, but I wonder if that will cause steam to race to the vent and close it before all the air has been pushed out. I would be interested to know if a formula exists which would show a maximum venting rate for a radiator of a given volume at a given PSI of steam above which the steam would race too fast and close the vent prematurely.

I should mention that my mains are very well vented with Big Mouths, all are insulated, and I have my vaporstat set to cut out at 9 ounces. My boiler is 50% oversized, so I confess before you, my brothers and sisters, that I do cycle on pressure. I'm one-pipe steam. 10 year old Peerless. Despite the cycling, my gas bills look great compared to neighbors' bills. Water is clean. No surging. Needs makeup water pretty much only when I do a blowdown.


  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,521
  • BobC
    BobC Member Posts: 5,477
    How many outside walls does this room have?

    How well is it insulated and is the floor well insulated?

    Is the runout to that radiator insulated (if so what kind of insulation) and does that feeder have good slope back to the boiler? Also make sure there aren't any sags in that pipe.

    If the radiator heats up at about the same time as other radiators it could just be too small to do the job.

    You need to calculate the volume of air in each radiator and all the piping that connects it to the steam main. calculate the CFM you will need to vent so the air could be expelled in a given time - say 5 minutes. It will take longer than that time because the steam has to bring everything up to temperature as it works it's way down the radiators feed pipe.

    Each radiator has to be proportionately vented according to the amount of air to be vented. There are some variables that you may not be able to quantify like pipes going up outside walls so you will have to juggle things from there.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • FinishGuy
    FinishGuy Member Posts: 31
    @Dave0176 created a nice chart and has made it available for use. Thanks Dave!

    1916 two-family, now condo. Top floor. 970 sq. ft. of ‘well ventilated’ space. One-pipe, parallel flow, gas fired steam heat. 27’ of 2” main (un-insulated) vented via Gorton #2. 27’ 1 1/2” dry return (un-insulated) vented by Dole #5. 7 HB Smith Princess 2 col. radiators (38” tall) & 1 ARCo 30s era thin-tube 6 x 8 sec. (32” tall) = total radiator EDR 244. Using Maid-o-Mist radiator vents, sized by calc. & 14 winters tinkering. 1980 HB Smith G210-S-5 rated output 120,000 btu, poor near boiler piping.
  • Motorapido
    Motorapido Member Posts: 307
    Thanks for the comments, links and chart, everybody. I had already worked through Gerry's venting chart last season when initially replacing all my radiator vents. After using that chart last season, all the rooms except this colder first-floor, one story bedroom have heat that is well balanced throughout the house.

    What I am trying to zero in on right now is still not addressed by anything I've read in Dan's books, Gerry chart or posts on this forum, and that is the question: Is there a tipping point when you start to vent a radiator too fast and at which you are guaranteed to rush the steam too quickly across the radiator and prematurely close the vent before all the air is pushed out? Is there a way to calculate that? Or is the exercise purely trial and error by putting increasingly large orifices on my cold-room's Maid-O-Mist vents until I have crossed that threshold and the steam rushes too fast and prematurely closes the vent?

    To gain the incremental improvement that I achieved this week, I have slowed the venting on three radiators closest to the thermostat and increased the venting speed on the radiators in the cold bedroom, and I am seeing positive incremental results, but I'm not yet at the point where that bedroom is precisely as warm as the other rooms throughout the house.

    A prior owner remodel job in that big, colder bedroom resulted in the removal of one relatively small radiator, and the room configuration does not allow me to reinstall the removed radiator. It might turn out that no amount of vent slowing in other rooms and vent speeding in the colder room will be enough to overcome the temperature differential. I will find out. But I'm still curious to know if there is a way to calculate the tipping point at which steam will rush too fast and prematurely close the radiator vent.
  • Fred
    Fred Member Posts: 8,542
    edited January 2018
    I'm, not sure there is a formula to determine when a vent is so large that it will allow steam to race across a radiator and close the vent. You can calculate the amount of air in the radiator run-out plus the air in the radiator and vent to that size. After that, it is probably trial and error, stepping the vent up a size at a time, until you reach that tipping point. No one tries to intentionally vent a radiator that fast so I doubt that anyone has developed such a formula. I would have to say there are so many variables that each radiator probably responds a little differently, given size, type, thickness of the cast iron (each brand is probably a little different, maybe even within a brand) which will affect condensing rates, etc.
    I would suggest you do a heat loss calculation for that room and determine if that radiator is simply too small. Then you can determine if you want to make envelope improvements or change that radiator out for a larger one. You can calculate and determine needed BTU's to required radiator EDR.
    If you still believe that radiator isn't getting steam as fast as you want, you can add a vent at the radiator but on the supply pipe to vent the air out of the run-out very quickly and then let your radiator vent handle just the air in the radiator itself.
    May not be the answer you want but the best I can tell you. From what you have said, my guess is the radiator is just too small. You can't heat a room as an after thought and based on minimizing the size of radiator to keep the remaining floor space for other purposes. You can find other types of radiators/convectors/ baseboard styles that may give you the amount of radiation you need.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,521
    I've never seen a formula for that, and I think that's why the vent manufacturers came up with the adjustable vents.
    Retired and loving it.
  • Motorapido
    Motorapido Member Posts: 307
    I do have a little more tightening work to accomplish and that should help. Unfortunately, all my first floor radiators are recessed and I don't want to tear open a wall to add another recessed radiator, plus the accompanying work of adding new piping to feed it, so if a little more venting tinkering combined with some envelope tightening gets me closer to matching the temperature of the other rooms, I'll consider that the finish line.
  • Bio
    Bio Member Posts: 278
    An old poster jpf321, had an excel venting chart worksheet, find it by his signature on this link

    Erin Holohan Haskell
  • jpf321
    jpf321 Member Posts: 1,568
    Exactly what I was looking for .. Thanks!
    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
  • gfrbrookline
    gfrbrookline Member Posts: 753
    Have you looked into adding insulation to the cold room since it is an addition? Your rad might be to small but that might be able to be offset by adding insulation in the crawl space and above the room if possible.