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air collecting in one baseboard only

Our hydronic heating system is a tangle of copper and galvanized pipes running here and there. There are radiators on three floors. On the 2nd floor there was a Burnham Baseray that was recently replaced during bathroom renovation with a Runtal baseboard. The supply and return lines remain the same.

The Runtal emits low but distinct water flow noises. Every time it does that, I bleed it, and a little air comes out. The noise stops. Two days later it starts again, and I bleed it, and sure enough there is some air....

I don't understand how air can get into a system under pressure, and only in one radiator. I realize that the Runtal is more likely to be noisy, compared to the heavier Burnham Baseray and also the traditional radiators elsewhere in the house. But I have bled the other baseboards and radiators and there is no new air collecting there. This is the only one that keeps getting new air.

All suggestions appreciated.


  • your boiler

    Your replacment boiler was installed improperly
  • Nom_Deplume
    Nom_Deplume Member Posts: 91
    edited November 2013
    pumping away

    Additional information: the pumps are "pumping away" exactly as they should.

    Note also that the problem baseboard is on the 2nd floor; the 3rd floor radiator is not collecting air.
  • i've seen

    I've seen many boilers are pumped away still piped wrong... send pixs of ur boiler and pipings and we'll see...
  • Gordy
    Gordy Member Posts: 9,546
    Air coming out of solution

    In case you are wondering how air gets in the system. It's already there. If your system was completely drained, and fresh water added after new base board was installed, and properly purged on initial filll it's air coming out of solution as the new water is heated.

    Do you have an air removal device in the system at the boiler such as a spiro vent?

    Is the air removal device properly located, and piped?

    Dish soap added in the system will help facilitate removal due to the surfactants breaking surface tension of trapped air. A little dab will do ya teaspoon or so.
  • kcopp
    kcopp Member Posts: 4,395
    add the dish...

    detergent... dawn, Ajax , etc. I do 1/2 a cup.
  • Gordy
    Gordy Member Posts: 9,546

    Pipes sudsy.
  • Nom_Deplume
    Nom_Deplume Member Posts: 91
    yes, air scoop

    Yes, we have an air scoop.

    Regarding dissolved air in fresh water, I do understand that as the weather gets cold and the water gets warmer (we have outdoor reset), more air will come out of the water since the solubility of gases decreases with increasing temperature (think soda water).

    What I don't get is why all of it is collecting at one radiator. I have been going around trying to bleed the other radiators and there is nothing to bleed there.

    I also don't get why lowering the surface tension (with surfactants, i.e. dish soap) will affect the solubility of gases in the bulk water. This is something I know a little bit about; really, the two have nothing to do with each other.
  • Paul S_3
    Paul S_3 Member Posts: 1,257
    edited November 2013
    air scoop

    I believe if there isnt at least 18" of straight pipe before the air scoop its basically pointless .....check that out.....post pics of boiler piping will help alot...and are you sure your purging air??...guys, what about a undersized pipe?wouldnt that give a waterfall type sound similar to air? i could be wrong.....thanks Paul S
    ASM Mechanical Company
    Located in Staten Island NY
    Servicing all 5 boroughs of NYC.
    [email protected]
  • bill_105
    bill_105 Member Posts: 429

    Is this thing Monoflow?
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Yes, we have an air scoop.

    The plumbing on my system (a mod-con with outdoor reset) is just like one of the diagrams supplied by the boiler manufacturer. My installing contractor put a Taco microbubble resorber in the system where the boiler manufacture said. The boiler is piped primary-secondary. Let me call one loop the boiler loop, and the other, the sysem loop. There is also an indirect water heater connected across the boiler loop. I would have put that microbubble resorber in the boiler loop, since the temperature in that loop sometimes goes to 175F, where the system loop, where the manufacturer said to put it, never goes over 135F. It seems to me that it is less effective there. The Taco unit works much like the Spirovent.

    See pages 2 and 4 of this document to see what I am talking about:


    To complicate matters, the main load in the system loop is the radiant slab that is lower than the boiler. The other load is some baseboard that is above the boiler, and has no vents at all. The installing contractor said they were not required, and that has been proved to be true. There is a purge valve for each system loop, however, to get the big bubbles out.

    I do not know the flow rate through either system loop, although I have guessed it. The results depend on how good my assumptions are, and my guess is that they are not very good.

    So any air in the baseboard will have to be forced down by the high water flow in that zone, or it will have to be dissolved in the water, heated in the boiler, and removed by the resorber. I think it is the latter because once I had to have the circulator replaced, and that got some air in the system. The technician purged the zone, so heating it does work, but the noise of the air is annoying until it comes out, and that takes several months because the temperature rise in the system loop is fairly small. When it is warm outside (outdoor reset), the supply temperature in the system loop is about 110F and the return temperature may be 109F or 110F. So, since it is not heated all that much, not much comes out in the resorber on each pass, and it takes a long time. After the air is out, it stays out, however.
  • Gordy
    Gordy Member Posts: 9,546
    edited November 2013
    Not affecting solubility, It just may be

    That there is an air bubble upstream of that radiator that is hung up in the piping, or micro bubbles all over in the piping up stream of the problematic rad like those very bubbles in a soda bottle that cling to the sides of the bottle. The dish soap will set them free to a return trajectory to the spiro vent ....... Trust me it works. Think like air, and water , and become one.
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