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Two pipe air vent system – banging in just one radiator

Paul3Paul3 Member Posts: 8
Hi – I love this site and have found it tons of help.

One radiator, 56” long, second floor, very slight pitch to return, with two-pipes with air vent. When the supply is CLOSED – no banging and some heat at the return side. With supply OPEN, lots of banging and heats like the dickens (207 degrees across the full radiator).

The supply and return pipes run about 5 feet horizontally between the first-floor ceiling and the second-floor floor. The return valve is open. The air vent is set at medium.

I’ve had this problem for a while, and cannot think of something that I changed to create it. Its only become an issue recently because the room is being used and needs some heat


  • Jamie HallJamie Hall Member Posts: 13,118
    Double check the pitch on the supply pipe. It may be just enough off to trap some water. Is the banging particularly at the beginning of a cycle, or all through?
    Br. Jamie, osb

    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
  • BobCBobC Member Posts: 5,118
    You could try lifting the whole radiator up by 1/2" or so. Use a lever and be gentle.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • JUGHNEJUGHNE Member Posts: 7,060
    Its a 2 pipe rad with an air vent on the rad?
    Is there a trap on the outlet pipe or another valve?
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,198
    @JUGHNE two pipe air vent has no trap. They are usually troublesome.

    check the pitch on all piping and radiator as others have said and adjust if possible.

    Maybe slowing the venting down will help
  • IronmanIronman Member Posts: 5,737
    Agree, slope and slow down the vent.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • JUGHNEJUGHNE Member Posts: 7,060
    Does the rad have a sag or a belly in it that would trap water?
    Or if a valve on the outlet, are you sure it is open completely?
  • Paul3Paul3 Member Posts: 8
    Thanks all for the quick feedback. Some replies to your questions.
    • The supply pipe is in a ceiling (runs for about 4 feet, with one 90 degree turn), so hard to check it for sagging, but will try.
    • Banging is mostly at the beginning of the cycle. And now that I think about it, not so bad early in the season (water has evaporated over the summer?)
    • There is very little room to lift the radiator, as the supply and return pipes are as high as they will go. I might need to remove a bit of the old (1/4" oak) floor under the return-side radiator feet to lower a slope it towards the return.
    • No trap the in return on this radiator (or any others in the house). There is a return valve.
    • I checked the return valve, and it had about 1/8 turn left to full open. Lets see.
    • I'll try slowing down the air vent in a separate test.
    • Its a long radiator - but I cannot see a sag in the middle. I'll check with a level.
    Other than this one radiator, the system works well.

    Thanks again for your suggestions. I'll comment back what I find.

  • FredFred Member Posts: 8,423
    Does the plumbing to that radiator look original (all the way down and into the basement)? Is that return tied into a dry return or wet return in the basement or is it tied back into the main? If it has been modified, It is possible that steam may be getting into that return causing the hammer, especially with the air vent on that side of the radiator.
  • Paul3Paul3 Member Posts: 8
    Yes, the radiator and piping looks original. Only this one radiator is on the pipes up from the cellar. There is a drip return off the supply main in the basement, down into a wet return (enters below the false water line). The supply shoots straight up to the second floor and over to the radiator. The return from the radiator itself goes under the floor and comes straight down into the cellar return in the floor then over to the boiler.

    I cannot see what is in the floor below the radiator (which is also above the first floor ceiling). If the suggestions above do not address this, then I might open up a hole via a closet, to see the piping between the ceiling and floor.

  • MilanDMilanD Member Posts: 1,114
    edited April 2017
    EDIT: No, you don't have to open the ceiling. What was suggested earlier is to get a short 4x4 or couple of 2x4s as a fulcrum, then a good sturdy piece of iron (breaker bar is good, or a pipe) and sloooowly and gingerly without causing anything to break, try to lift the whole radiator, one side at a time, by 1/8 of an inch or so, and until getting the suply side up 1/2 inch and drain up 1/4 inch. Use washers or quarters or checkers to keep the radiator feet elevated. If things were piped correctly, there should be space between floor and ceiling in which the supply pipe is centered. Rad being connected on both sides may make it super sturdy, but any swing joint in the floor should allow it to lift. If there are no swing joints and the supply riser is indeed dipped into a wet return, you may indeed not be able to raise the rad without repiping. Pitch of 1 inch per 10 ft is not much pitch on a short pipe so a slight settling of the floor can cause issues and hold the condensate.

    If I read it correctly, you also said you had a valve on the drain side and that's where the banging happens? Make sure it is operating properly and not broken off inside.

    A picture of the rad would be helpful.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,198
    @Fred two pipe air vent usually always has steam in the return
  • Paul3Paul3 Member Posts: 8
    Thanks MilanD for the suggestions. I will try (gently) again to raise the rad a bit.

    Correct the banging seems to be on the return side. I will also check the return valve - perhaps just replace it.

    Does the fact that the banging only happens when the supply valve is open give any clues? Does that mean the issue can only be on the return side?

    I added three photos.
  • MilanDMilanD Member Posts: 1,114
    edited April 2017

    To your question: supply valve keeps the steam out of that whole rad. When it's off, it's off - no steam and no condensate.

    And just to make sure - you said the drain pipe goes straight down and into the wet return?

    If it's your drain side that's hammering and pipe goes straight down into the wet return, your floor has sagged and return pipe is holding condensate. Also, in addition to rad venting in excess, you potentially also operate a high op pressure which is pushing steam into the return, where the water that sits in it from last heating cycle flashes to steam when hit with steam being pushed from the rad. High op pressure will do this.

    Unless you can raise BOTH sides of your rad, which I don't think is really your issue, your only recourse will be to slow down the venting and make sure your op pressure is low. You said the rad heats across all the way like a dickens and has its own dedicated riser and return to wet return. Remember that, in theory, all those rads were not supposed to be hot all the way across unless it's very very cold outside - design day - usually 0*F. So you need to slow it down.

    Try 2 things - slow down that Hoffman 1A vent itslef to 1. Take the cap off and make sure '1' lines up with the vent hole low setting properly. A few I have don't. Then see what happens.

    If vent alone doesn't fix it and the rad still heats all the way across (here I'd also check the op pressure and turn it down!), you may try turning down a bit the supply valve to slow down the steam entering the radiator even more. Leave drain one on all the way. Then play with how much supply valve is closed... Air vent will still let the air out, the supply valve will slow steam down (and the force/speed with which it will fill the rad). If you keep your supply on such level so that the last 2-3 sections of the rad heat but only on the coldest of days, you may solve your issue.

    If you return is truly going straight down to wet return, banging in the return line comes from the steam in your return line, and the only way it's in there is by steam going past the rad and into the return via too much pressure and/or volume of steam in the rad. Get it to not heat the last section or two, and you will solve your issue.

    What is your operating pressure? I have a feeling that despite everything else working fine, your op pressure can be turned down into ounce territory.
  • JUGHNEJUGHNE Member Posts: 7,060
    Usually the first question here is what pressure does this run at?
    What can gradually change is for the pigtail to plug up and not allow the pressure control to see the pressure rise.
    Simple fix and should be done as regular maintenance.
  • Paul3Paul3 Member Posts: 8
    MilanD and Jughne... thank you.

    I found that I could lift the rad up about 3/8" at the supply side and 1/8" at the return side. I will test over the coming days (altho my wife is after me about the heat being on as the weather warms!) :smile:

    And you are correct about settling, the early 1900 1/4" oak strip flooring has a small dimple in it at the rad legs.

    I will check to ensure the air-vents are really at slow/1.

    I'll try turning down the supply side valve over a few days. The room, even in cold weather, gets quite warm. So, lowering the steam coming in might be good regardless.

    The return does not go straight down. It runs about 6 feet in the floor, then down to the cellar into a wet return.

    Funny enough, the pressuretrol pigtail did get clogged last year - fixed that.

    The operating pressure is about 1.1 lbs (cut out) with cut-in about 0.5 lbs. After I check out the various suggestions above, and do not have success, I'll try lowering the op pressure a bit.

  • MilanDMilanD Member Posts: 1,114
    By pipe going "straight down" I meant as you described, horizontal 5 ft then into wet return. Should have said straight into wet return, by 'straight' meaning 'steaight away' or 'directly'. My point is there is no way for steam to reach the rad through the return.

    From what you said, rad heating across even in mild weather means it is overheating. Lowering the op pressure and slowing down venting via vent and/or valve will solve your issue. Steam is getting into the horizontal return in the floor and hammering. Prevent steam from going there, and there will be no more hammering.

    Keep in mind that once the op pressure is lowered, you may need to rebalance the whole system. Some rads may not heat as well as now and may need increasing venting rate, while some other ones may need slowing down.

    You are on the right track.
  • Paul3Paul3 Member Posts: 8
    Thanks again everyone. After raising the rad a bit (3/8" on supply and 1/8" on return giving me a pitch of about 1/4" over 4 feet), the banging is lessened. There is probably still some water in the pipes. I have also closed the supply valve about halfway and will test this.

    As the weather is warming, it might a while before I really know how things are going. Unless there are new ideas, I think I am set for now. I'll report back progress when we get some cooler weather (hopefully November :smiley: )

    I really appreciate all the suggestions and will check back in to see if I can offer advice to others (noting that am just a homeowner and not a pro).
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