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Unusual behavior when using different sized vents - system goes into vacuum with #5 & #6 but not

1 pipe steam.  Radiator EDR of 28 + 3 feet of 1.25" runout pipe.  Everything is pitched perfectly.

This is a 5 story building with 4 floors of apartments containing radiators (12 apartments total).  Restaurant on ground floor.  I have no idea what type of venting is on the main or risers (multiple riser pipes in each apartment), but I'm guessing not much based on the amount of noise my vents make throughout the entire cycle (quite a bit).  I'm also sure that the other apartments have a variety of random vents, some of which may not even be working (when I first moved in, my vents were stuck open and spewed steam everywhere).

I've been experimenting with vent sizes using Maid-o-Mist vents and different size orifices.  Here's the details:

#5 orifice - takes about 20 minutes from the time I start to hear any venting at the beginning of the cycle until the radiator is fully hot.  By fully hot, I mean including the bottom of the sections in the middle.  However, almost as soon as it gets fully hot, the boiler starts pulling air back in through the vent and it makes a loud whistle for 5-10 minutes.  The vent itself cools down and the radiator seems like it might also be cooling down a little quicker than usual as a result of the air being pulled back in.

#6 orifice - same as #5 except it oddly seems to take a few MORE minutes for the radiator to get fully hot. Probably closer to 25 based on watching the clock for a couple of cycles.  Same loud whistling vacuum for several minutes once the radiator is hot.

#C orifice - takes about 20 minutes to fully heat (same as #5) but does NOT allow the vent to open back up once the radiator is fully hot.  I'm assuming this is because the vent itself is hotter and therefore isn't opening back up.

I'm tempted to try a #D just to see what happens, but that would require another trip to the store.

Why Is the system sucking air back in as soon as the radiator gets hot?  Why would some vent sizes allow this but not others?

Also note that no matter what size or brand vent that I use, the vent always closes and reopens several times throughout a 10-15 minute window while the radiator is getting hot.  It seems that steam is getting ahead of the air and then the vent cools down a little and allows more of the trapped air out.


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    short-cycling system

    i do not think that your venting efforts will make much difference. obviously, with a strong vacuum , the main vents are not up to the job of allowing air out, or in.

    who pays for the heat? do they mind paying extra to have the fuel company pump the air out through little radiator vents, instead of having large main vents, and low pressure.

    about the time your radiator is getting hot, the system is cycling on pressure, and continues on and off. find out from the management about their service procedures.--nbc 
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