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Steam radiator vent size question.

Nick11 Member Posts: 11
My current setup is a main vent in the basement,
maid-o-mist vent #4 on the radiator in the living room (where the thermostat is),
#5 on other radiators on the first floor,
#C on the second floor ones.

Reading up, seems like the vent is just kind of a timer function to heat rooms evenly.

So wondering what if I increase all the vents by 1 size. If all the vents go up by the same amount, they should still warm up in the same order right? Just the whole house will heat up faster.

Thoughts? Are there any problems with putting #D vent on a radiator?


  • ethicalpaul
    ethicalpaul Member Posts: 5,580
    That is not guaranteed. You really can't predict exactly how the steam will react to changes in your venting. That's why it's best to only change one thing at a time and leave several days in between to observe what goes on.

    In my opinion your current setup sounds pretty good. I would always recommend starting with a #4 on the radiator closest to the thermostat and going from there only where needed. Most of my radiators are #4. Upstairs ones often need less heat due to heat rising up there and/or them being in bedrooms where people like it cooler.

    I really wouldn't put anything larger than a #6 on any radiator. A #D is the same size as a Gorton #1 main vent, you don't want your upstairs radiator winning that fight :sweat_smile:

    If someone has the urge to put a C or D on a radiator, it's usually because they are trying to make a radiator win by going faster, when in almost all cases, the better way is to make the other radiators go slower.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Jamie Hall
    Jamie Hall Member Posts: 22,891
    How fast your house heats up is determined by the total radiator size (and sometimes by the boiler -- but rarely)-- not by the vents. If the radiators fill with steam evenly now, you will gain very little if anything.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Chris_L
    Chris_L Member Posts: 334
    I'd say go for it. Since these are Maid O'Mist vents I assume you are just changing the orifices. You can always switch them back if it doesn't suit you.

    I have six radiators, four of which have Maid O'Mists vents with C or D orifices. They work fine. In my system, the mains are short, the runouts are long so for some of the radiators, most of the air to be vented is in the 1" runout, rather than the 1.5" main. Using the larger vent does lead to faster heating.

    Where you might run into problems is if your boiler is undersized or properly sized. But if you are like most people, yours is ovesized and you more than likely have enough steam go go around.
  • bburd
    bburd Member Posts: 870
    edited December 2022
    In a one pipe system, the steam and condensate must flow in opposite directions through the shutoff valve and radiator piping. If the radiator is vented too quickly the steam and condensate will interfere each other, which can lead to problems like water spitting from air vents, gurgling in the pipes and inconsistent heating.

    If it works well now, I wouldn’t mess with it.

  • Nick11
    Nick11 Member Posts: 11
    Thanks for all the replies.

    Like Chris_L my main is a short one, most air probably gets vented via the radiator.

    Takes about 15 mins from a cold start to get all radiators warmed up. So thinking maybe bigger vent size will help.

    Guess will try replacing one orifices at a time to see how it works.

    Thanks all.
  • gfrbrookline
    gfrbrookline Member Posts: 753
    edited December 2022
    Vent your radiators based on the @gerry gill venting rate guide which can be found on this site. When doing your calculations include the size of the radiators, the diameter of the runouts feeding them and the pick up factor the way he describes it. This worked very well for me. Don't be alarmed if you wind up using very large Gortons. A large size Gorton may be required on the radiator below the top floor to vent the air out of the riser to allow the entire system to vent properly. The alternative would be to tap a Gorton D on the top of each riser.