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Donnelly System? Air vents where?

Finally finding some time to try and learn more about my steam system in our house. After looking around this amazing forum and looking through The Lost Art of Steam Heating I believe we have a Donnelly System that has been monkeyed with over the years. We have a two pipe system. Supply is at the top and return down low on the radiators. Every radiator except one (it has a Hoffman 17C) has a bulbous 90 degree elbow on the return with a "C" stamped on it (see photo). Cracking one of these open I found a check valve (see photo) which makes me believe it is a Donnelly system and so the question is how to optimize it.

Most radiators have air vents and I assume I should remove all of those? What is the best way to plug the air vent hole? A short properly sized bolt?

Also attached is a rough schematic of the mains and returns with a call out of where air vents are currently located. Black lines are mains, gray lines are returns - I did not include any radiators but hopefully it is enough detail to get the point across. The system air vents are Hoffman 76 vacuum valves. The ones on the mains are broke (ones on returns still work). I suspect I should upgrade the air vents on the returns and want to confirm that I should replace the broken vents on the mains.

Also attached are a few photos of the near boiler piping with attempts at labels scribbled in red.

Any recommendations or information would be appreciated. It is fascinating learning about these old systems and I hope to dial ours in before the pending heating season.

Thanks!

Comments

  • gerry gillgerry gill Posts: 2,954Member
    air vent placement looks proper on schematic.
    It may take some experimentation with the removal of the radiator air vents to see how fast the radiators heat up. Here is what i'm seeing in my mind- the system is going to have to build up some back pressure, in order to push open the check valves and vent the radiators air. Sitting here at my desk, i couldn't tell you how long that will take. Use an 1/8th inch pipe plug to plug radiator vent holes to see.
    gwgillplumbingandheating.com

    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • Jamie HallJamie Hall Posts: 10,809Member
    Indeed the vents on the mains must be operating; nothing wrong with the Hoffman 76s, but you don't need the vacuum feature, so the 75s will do just as well and are a lot cheaper -- but it is even more important that the vents on the returns be functioning and functioning well, since without them the air can't get out of the radiators. If you need to replace them, Gorton #2s or Barnes ^&Jones BigMouth should do nicely.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • lunchboxlunchbox Posts: 3Member
    Thanks for the responses. It sounds like the main vents are the place I should start first. I am thinking a Barnes & Jones Big Mouth for each return and use my still functioning Hoffman 76's on the end of the mains. My thinking is that the faster venting on the returns may help pull steam through the radiators but I guess the check valves may mess up that idea.

    Interesting thought on the back pressure Gerry. If I am understanding correctly perhaps I will remove all of the radiator air vents and then if there are any slow to heat radiators, I could add back on the air vent to help speed up that radiator or would that defeat the purpose of the Donnelly System?

    Regardless, I will likely wait a few weeks to do extensive testing as we are still having temperatures in the 80's here in Missouri.
  • Jamie HallJamie Hall Posts: 10,809Member
    Faster venting on the returns won't pull air out -- but it will let air out. The pressure drop across any checks or traps will still be there, and will show up as a slightly increased pressure in the mains, but that's not a problem. If the air can't get out of the returns, the system will work poorly if at all, so you are on exactly the right track at this point.

    I wouldn't consider putting vents back on slow radiators. First place, you will probably find you won't need them if everything else is working as it should, and second you can always slow a radiator down on a two pipe system with the inlet valve or with an orifice.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • lunchboxlunchbox Posts: 3Member
    Poor wording on my part with the word 'pull'. My intent was that potentially the path through the radiators to the returns will be the path of least resistance with the larger Big Mouth vents on the returns versus the mains with the 76's.Time will tell.

    Good to know on closing the inlet valve. I will try that first if needed. I forgot that was an option on a two pipe system.

    Exciting for cold weather to come so I can see what these changes will do!
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