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Water hammer worse after adding main vents

System facts: residential one-pipe parallel flow steam. A 2-inch branch main that goes into a crawl space to supply four radiators in a first-floor bedroom was not sufficiently heating the radiators. That 2 inch branch main had been the only uninsulated main in my house when I moved in last year. I have insulated it with about 1.5 inches fiberglass. I replaced the ancient, insufficient main vents, with three Big Mouths on the longer section (which includes this crawl space branch). My calculations show that I now have more venting with those three Big Mouths than the total capacity of the 3/4 antler, so venting there is excellent. On the shorter section I used a Big Mouth and a Gorton #1 which should also be venting about as much as I can get through the 3/4 inch antler pipe. I have pitched the radiators above the crawl space for proper return of condensate. I have replacd those radiator vents with Maid-o-mist 5l, with mid-size orifices as a guess at what would be a good starting point for those mid-size radiators. Those radiators now heat pretty well, but not as completely across all tubes and not as hot as the other radiators in the house. I also encapsulated the entire crawl space and sealed the foundation air vents and sealed the tops of the block walls and the sill band with closed cell foam panels, with spray foam to seal the joints of the closed cell foam. So I now have a well-insulated main in a warmer, drier crawl space than before, which should all be beneficial.

I installed a 0 to 3 psi gauge on the boiler, on a clean pigtail, a pressuretrol set at 0.5 and 1, and operating pressure of about 3 ounces when steam reaches the vents on the two mains, and about 32 ounces pressure when furnace shuts down upon satisfying the thermostat. The pressuretrol setting should shut off the gas at 1.5 PSI, but due to innacuracy of pressuretrols, it allows pressure to rise to 2 psi by the time the thermostat is satisfied. Eventually, when I get more of the main/return systems optimized, I will install a vaporstat in series and hope to run at well under 2 psi. But for now, it is what it is.

Oddly, after doing the work described above, the water hammer in the branch main in the crawl space became worse as I completed each component on the above-described work. Worst of all, this morning I saw that the three Big Mouth vents on the end of that branch main had dripped out about 1/2 ounce or maybe less of water, which dripped onto the basement floor directly beneath the Big Mouths. I do not know if the Big Mouths were hit by condensate that flew up the antler and out the vents, or if those gigantic Big Mouths simply condensed some water from the basement air when they heated up, and that condensed atmospheric water just dripped onto the floor. Two of the three Big Mouths are oriented so that their discharge openings face each other. Perhaps the warm air getting forced out of the the two openings that face each other was enough to cause the basement atmosphere to drip some moisture. Basement is about 65 degrees and relative humidity is about 35% in the basement, so at that low relative humidity, I doubt that the hot Big Mouths condensed basement atmosphere air into water that dripped.

I have not yet drained the return lines at the boiler, but plan to do so soon. I might not be able to insert a flushing fitting with a garden hose thread until the end of heating season. I do have a valve at the very end of the wet returns at floor level, where the wet return gets ready to rise up into the Hartford loop. For now, I will drain that valve and maybe pour some water into the beginning of the return by removing one of the Big Mouths and just letting poured water at gravity-force do some cleaning. The true flushing will have to wait until heating season ends (busy with so many other things).

I am also preparing to accurately measure for what is certainly some pipe sag in the main through the crawl space, and I will attempt to correct that as best i can in the very tight, limited space, by installing some new clevis hangers and carefully adjusting them.

But for now, can anybody suggest why water hammer is louder than it had been before I did that venting and crawl space work, and why the Big Mouths at the end of that crawl space branch main might have dripped water onto the floor?

In the attached picture, you will see that my only option for mounting the vent antler was to mount it where the old, totally inadequate Dole main vent was installed. The Dole sat on a nipple off the main. The antler sits higher than the old Dole sat, and the Big Mouths are about 7 inches higher than the top of the crawl space main. The antler is pitched slightly and should easily allow any condensate to flow down and into the return.

In the picture, which I had to take at an awkward upward angle, the pipe to the right is the last little bit of that crawl space main. The T down below the riser to the antler joins the end of the main to what becomes about a 30 inch length of slightly pitched dry return, and that then drops in a vertical dry return that becomes a below-water-level wet return within a foot of vertical travel, and then turns horizontal with a decent slope as it travels back to the boiler.


  • Abracadabra
    Abracadabra Member Posts: 1,948
    Check the main for a sag. Better/faster venting will exacerbate hammer when the main has sagged.
  • Sailah
    Sailah Member Posts: 826
    The Big Mouths will drip water if they see it, nature of them being a thermostatic vent. Hot water is cooler than steam and if water gets to the vents it will try and get rid of it. The fix is to have gravity work in your favor and elevate them as high as possible. I don't know for certain, but I suspect, that the water hammer events are pushing condensate up into your vent antler and the big mouths are spitting it out.

    The orientation of the vents is such that all water will drain from the outlet back into your piping by gravity. The only other way they would drip is exactly like you describe, a surge of condensate that hits the vent before they have fully closed by the heat of the steam.
    Peter Owens
  • Fred
    Fred Member Posts: 8,542
    I agree with what has been said. Better venting will cause the steam to move down those mains/branches faster and when it hits a pool of water, sitting in a sag, you will get hammer. That hammer is water being slammed against a wall/elbow and it may be with enough force to throw it up into a vent. If the vents haven't closed on steam, they are open to spit water. Even if they were closed, Water being cooler than steam, may cause those vents to open and spit before the steam closes them again. Fix the sag and you fix the hammer and vent issues and probably the cooler radiator problem as well.
  • KC_Jones
    KC_Jones Member Posts: 5,706
    Out of curiosity what does your near boiler piping look like? Pictures would be great. The piping will help us judge how dry the steam is you are putting into the system.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Motorapido
    Motorapido Member Posts: 307
    I think the near boiler piping is pretty good. I'll take some pics and post a couple tomorrow. Water line to header measurement is really good. Two pipes out of header. All cast iron. No copper. I'll take and post pics, though.
    EBEBRATT-Ed Member Posts: 15,335
    @Abracadabra has probably got this right. Sagging or poor pitch.
  • gfrbrookline
    gfrbrookline Member Posts: 753
    I had the same problem, fixed sagging pipes, added big mouths like you did and lowered the pressure to 14oz and all of the banging went away. Lowering the pressure will make a huge difference with banging once you eliminate any sagging mains and will save money on your fuel bill.
  • Paul_11
    Paul_11 Member Posts: 210
    i agree with all comments, but you say you had no choice but to put your vents at the end of the overhead where the old vent was.

    Why is that true?

    master vents always work better 15" to 18" back from the end of the overhead on a horizontal portion of the overhead.

    just drill and tap where you want to install them.
    Since 1990, I have made steam systems quiet, comfortable, and efficient. We provide comfort while saving the planet.

    NYC LMP: 1307
  • sldamiano
    sldamiano Member Posts: 34
    What does a sag in the piping mean ? The main not being pitched correctly ?
  • BobC
    BobC Member Posts: 5,473
    A sag in a pipe is usually because the pipe wasn't properly supported along it's length or a support was removed for some reason. Alsy the steel strapping with holes in it won't work long, it's just not strong enough.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Jamie Hall
    Jamie Hall Member Posts: 22,937
    sldamiano said:

    What does a sag in the piping mean ? The main not being pitched correctly ?

    A sag is a little different from not being pitched correctly. It's perfectly possible for a pipe to have the correct pitch, measured from end to end -- but to have a bend (sometimes the pipe itself, sometimes at a joint) in the middle which is lower. That's a sag. If the sag is enough so that it can pool water -- not uncommon -- then when the boiler starts up and the steam and air starts whizzing through the pipe, that pool will be carried along at a jolly good clip and can give you a terrific water hammer.

    The check is with a string line or, for the more modern, a laser line -- and any irregularities from the straight line from end to end must be taken up with adequate hangers. As @BobC notes, steel strapping with holes in it rarely works, and certainly not for long.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England