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Radiator vent placement

jeff_50 Member Posts: 13
What if some of the largest radiators in this system
needed more venting; where would the second
vent be placed? Or simply should a larger/faster vent
be used? And, how difficult is it to drill and tap
where there is no boss?


  • jeff_50
    jeff_50 Member Posts: 13

    I've read in Dan's books that air vents on
    radiators should be 1/2 to 2/3 up on the height radiator.
    Are these too low?
  • michael_15
    michael_15 Member Posts: 231
    they're fine

    I believe those radiators probably even have a built-in tapping hole where they're placed; that is, they were built with the intention that air vents could be added in that spot.

  • Dave_12
    Dave_12 Member Posts: 77
    Air vent placement

    You do not want the air vents too high because you want steam to enter the radiator, force the air out through the vents, then stop venting when the air is out of the radiator.

    If you install the vents near the top of the radiator, the thermostatic vents will close when the steam gets down to the level of the vent. The bottom of the radiator will still have non-vented air, blocking more steam from coming into the radiator. The result will be a partially cold radiator.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Right on both counts, guys

    by the time these "large-tube" radiators came out, it was standard practice to cast a "boss" into the radiator so one could drill and tap an opening for a vent. This way all you had to do was drill where the manufacturer wanted you to, and the radiator would work as designed.

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  • David Efflandt_2
    David Efflandt_2 Member Posts: 24

    If you have Dan's book "The Lost Art of Steam Heating", read Chapter 8 about venting. Do you have enough main venting in the right place?

    Sizing radiator vents may take some experimentation to get everything balanced. If you cannot vent the slow ones fast enough, an alternative is smaller or adjustable vents on ones that get too hot. But if upstairs risers are too small, too large of a vent can result in gurgling, sloshing and spitting, because rapid condensation from a cold radiator may be fighting the steam.

    I had that problem with a north 2nd floor radiator (riser too small, most distant from boiler). It previously had a Dole C vent which had become inoperative. A Dole 1A set wide open plugged up in week and stopped venting at all (other Dole 1A's that came with the home still work fine). Now I have a Hoffman 1A throttled down, and it has worked fine for a couple of years. The radiator does not heat completely in mild weather (shorter run time), but fully heats without spitting in cold weather.

    I do not have main vents (yet), but have a large vent on the 1st floor under the problem radiator, to get at least get steam to the riser quickly. And that 1st floor rad. is near an open stairwell, so its surplus heat can rise upstairs. Even though the radiators and risers are not perfectly sized, balancing the venting keeps everything at a fairly even temperature.
  • Steamhead
    Steamhead Member Posts: 16,714
    Jeff, regarding your main vents

    measure the length and diameter of your vents and we can tell you what you need. Proper main vents will make balancing the system very easy.

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