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Radiator Steam Vent Sizing Assistance Needed

Hey guys,

I replaced what appeared to be 2 faulty vents in the 2 small radiators in my living room of my apartment.  I live on the 3rd floor of a 5-story walk-up and the boiler is in the basement.  The radiators are single-pipe units and are both very small.  There is one located under each window, for a total of 2, and they are connected to the same pipe.  The radiators are probably only 24" x 18" each.

The old vents were whistling like CRAZY and releasing hot steam almost the entire time the boiler was running.  It was literally like a boiling tea kettle going off in my living room all the time and they weren't retaining heat like they should!  This is my first winter in this apartment so I did a lot of googling about radiators today to try and determine what the problem might be which led me to try a vent replacement (and how I found this forum).  This appears to have worked!  (read on)

My old vents were both the maid-o-mist/jacobus style and were size "C" (fairly large vent holes according to their website <a href="http://www.maid-o-mist.com/jacobus.html">http://www.maid-o-mist.com/jacobus.html</a>), so I replaced them with the same.

Now, things are SIGNIFICANTLY improved with the new vents, but I still get a little noise for a few minutes when the boiler first kicks on (at much less annoying volumes).  The 2nd radiator in the chain now whistles softly for several minutes longer than the first one however.  It also gets hot a slower pace since the steam reaches the other radiator first.

Which brings me to my question.  Should I be using different vent sizes than I'm currently using for:

A.) Both radiators

B.) Radiator #1 in the chain

C:) Radiator #2 in the chain?

I'm really not sure how to properly size these, and I'd ideally like radiator #1 and #2 to get warm and stop whistling at about the same time.  I don't mind spending another few bucks for a different size if it will get me better results.  Note that I rent this apartment and have no control/access to the main vents closer to the boiler.

I really appreciate any assistance anyone can provide.  Thanks in advance!!


  • Rod
    Rod Posts: 2,067
    edited October 2011
    Radiatior Vents

    Hi- If it was releasing hot steam the vent was shot. It is supposed to release air but then should close when steam reaches it. Radiator vents don't last for ever and should probably be changed every 3 years though some will last longer than that.

    Since you are already familiar with Maid -o- Mist  I think I would try a #5 or a #6.

    Getting the radiators to operate where you want them is a matter of trial and error.

    Here’s a comparison of venting capacity per Gerry Gill’s excellent venting chart.

                                             Cu.Ft per min.

                                             @ 1 oz. PSI

         Maid-O-Mist   C             0.283

         Maid-O-Mist  #6             0.150

         Maid-O-Mist  #5             0.100      

    As you can see the #6 is has about half the venting of the “C” and the #5 has about 1/3 the venting of “C”.   These are available from Pex Supply on the internet.   


    Another possibility would be a Hoffman 1A which is an adjustable vent. It’s range is between 0.020 to 0.145

     Be sure to change the vents when the boiler is not operating  and the radiator is cool.

    - Rod
  • jkozlow3
    jkozlow3 Member Posts: 18
    edited October 2011

    Thanks Rod.

    Just so I understand...

    So the smaller vents cause the hot steam to start filling the radiator a little later, right?  So if the boiler runs for 20 minutes and the steam gets to my apartment on the 3rd floor after 2 minutes, I can only theoretically get 18 minutes of heat with the largest vent, right?  If I have a smaller vent, I might only get 16-17 minutes of heat since it will take longer for the cooler air to leave the radiator and be replaced with warmer air, right?

    If I wanted the maximum possible amount of heat (assuming my landlord is cheap and sets the thermostat as low as possible), I want the largest sized vent, right?  If I find that the apartment is getting too warm, I put a smaller vent in to shorten the amount of time that hot air is inside the radiator? (maid-o-mist sells a model that comes with multiple sized vent caps so this would be easy)  Does the size of my radiator affect this theory at all?  Remember, each unit is quite small.  Smallest radiators I've ever seen actually.  They're about 24" x 18" x 2" little boxes - not the pipe-looking kind.

    Does this all sound right?  Now, what about my 2 radiators in the living room which are 5 feet apart on the same line, with a single supply valve on rad #1, connected with horizontal pipe from the outlet of radiator #1 to rad #2?  Rad # 2 hisses longer when the boiler kicks on and gets hot a little slower.  They have the same vent size at the moment.  Should I put a larger vent on #2 in the chain than #1 or will #2 always get warm after #1 does regardless since it's fed from the outlet of #1?

    Thanks again!
  • Rod
    Rod Posts: 2,067
    Radiator Vents

    It a bit more complicated than what you are proposing. This isn’t hot and cold air.  The basis of steam heating is latent heat.  Here’s a good description of the basics  of steam heating. Read just  the first part on how steam works in a heating system.   Farther on it gets into piping and boilers which is probably of more interest to the homeowner with a boiler in his basement.


    What happens in a one pipe steam radiator is that when the system comes on and is steam is available, the steam being under a slight pressure forces the air in the radiator to exit through the radiator vent.  The radiator vent allows the cool air to pass through but when steam reaches it, the vent closes.  The steam, when it touches the cool inner surfaces of the radiator walls, collapses (condenses) changing back to water and releasing the latent heat it is carrying to the cast iron radiator.  When steam collapses it instantly goes from a volume of 1 cubic foot of steam to 1 cubic inch of water. This creates a huge vacuum which then draws more steam into the radiator. The condensate (water) drops to the bottom of the radiator and flows back to the boiler through the steam pipe.

    As to what would happen with changing vents is hard to speculate even if one could see the system. At an initial look it would seem that using a bigger capacity vent would be a benefit.  However using too large a vent can cause problems like noise and water hammer and in some instances slow the system down.

    Just putting on a smaller vent of the faster heating radiator may allow the slower one to heat faster as it results in a more even distribution of the available steam.  As I mentioned it is more trial and error.

    Another thing I might mention is that on a one pipe steam system the valve on the steam pipe should always be either fully open or fully closed. Having it halfway causes a collision between the steam coming in and the water leaving the radiator and often results in water hammer.

    One item  you might be interested in is TRVs (thermostatic radiator valves) as these can be set so they can control overheating. There are separate models for two pipe and one pipe steam systems. Use the search function and you will find info on them on this website

    - Rod
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    All landlords cheap?

    All property owners are interested in reducing expenses of renting space-residential, or commercial. Many do not know how to reduce the cost of one of the major expenses of building ownership-heating, nor do they know how wasteful their systems are now through lack of maintenance.

    This situation of yours seems like such a case of the system pressure raised to compensate for some venting problem, which has caused your vents to fail. Can you shake the upper part of the tree? If the owner knew he could burn less fuel, and make you, and others more comfortable at the same time, would he listen?

    I suspect this a system-wide problem, and if you can get the management to take care of the system as a whole, only then will you be comfortable. Until then, the replacement of radiator vents can only be a stop-gap solution.--nbc
  • Alain
    Alain Member Posts: 1
    Heat steam not reaching our 2nd floor bedroom

    Out of the 3 bedrooms, 2 with the big radiators Single pipe steam system give off good head but the smaller one in our bedroom hisses without giving heat.

    Appreciate getting some experienced advice as to how to correct problem.
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