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Radiator Vents - How Quiet is Quiet?

Big-Al_2 Member Posts: 263
Some say that radiator vents should be virtually silent.  On my system, the main vents are much bigger than my collection of  radiator vents, but they are basically silent. (If I'm standing near the main vents right after the boiler starts to steam, I can sometimes hear a slight momentary hiss right as they close.)  Right after the main vents close, the radiator vents of every brand are whisper quiet for a couple of minutes, and then as they start to close, the remaining vents get a little louder.  By the time the last vent closes, it's quite audible, even though the gage is barely off zero.  As the boiler continues to fire, the radiator vents randomly will momentarily open and close for a little while, letting out small burps of air.  These burps can get pretty hissy,  because the system might be up to close to 1 psi before they stop. Once everything is hot, the system is absolutely silent . . . no hissing, no banging, no gurgling . . . silent . . . until the stat is satisfied, the boiler shuts off, and the system goes into vacuum.  Then the first vent to open up can make quite a loud sucking noise, especially if its one of the Watts units I have. (I fitted them with home-made silencers, made from copper tubing, otherwise they would actually whistle.). 

The only radiator vents that are really quiet all the time are the couple of Hoffman 40s that have really teensy orifices and a very aerodynamic shell. They don't hiss anyway, but occasionally they rattle a little.

OK, I know the boiler is somewhat  oversized so it can build pressure quickly.  Not much I can do about that.  The system is operating at less than 2 PSI max, usually less than a pound, and the vents still make noise.  Is this normal?  Is the ideal of silent vents only realized with a modulating boiler operated at only a couple ounces of pressure?  Or is a silent system only realized by using really slow radiator vents?  Or does virtual silence only ever occur after the vents all close?

Guidance, please.


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    quiet vents?

    i assume you are talking about a 0-30 PSI gauge when you say, " even though the gage is barely off zero". it is difficult to read the pressures we 1-pipers are looking for on such a scale, so you may be over the top, and not know it. i have hoffmans and they are quieter than their predecessors

    the mains also relieve the vacuum, and that's another good reason to have lots of venting capacity; or you could plumb in a check valve, next to the main vents, which would open up and relieve the vacuum. this picture shows the vent antlers before check valves were put on the ends, and now i get no whistling.-nbc
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited October 2009

    Yes, it's a 30 PSI gage . . . but it correlates pretty well with the settings on the Pressuretrol, so it's probably not off that much.  It is undoubtedly inaccurate at less than 7% of full scale (< 2PSI)  I'll need to find a better one online because the local supply houses only sell to the trades.

    When you say a check valve, you mean a vacuum breaker like a Watts N36?   . . . or something more industrial like a Hoffman 62 or even a Spirax VB-14?  If an inexpensive Watts N36 would solve sucking sounds, it would be worth the effort for sure.

    What kind of vacuum breaker did you use?  Anybody else use vacuum breakers on steam mains?  Brands? Opinions?
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    How many atmosphere's to set vacuum breakers?

    Although not steam, I too will be adding some vacuum breakers to the top of my overhead gravity hot water system for drain/fill functions and picked up a pair of B&G #26(which I think are rebadged Hoffmans) that I believe have a default setting of 2 atmospheres.  But they are adjustable.

    I was wondering if there was any standard/typical setting of this value or if it varies upon some function I am unaware of?

    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    vacuum relief

    i just used 3/4 " check valves from the hardware store, on my vent  "antler", separated by a ball valve [as are all my main vents]. just make sure they are almost horizontal.

    as far as a gauge goes, try gaugestore.com and get a 0-2 psi wicka, and mount it on the same pigtail as the [over]pressuretrol. they will tell you that it is not for steam, but as it is protected by the pigtail, there should be no problem. as i have said before, my experience with even a new pressuretrol, resulted in wild overshoot-up to 10 psi!!!!!!!!!! ironically if your pressuretrol is an old mercury bulb type, made in the USA, it may be better able to keep the pressures below 2 psi.

    i think the vast improvement in main venting, and the vaporstat set for 8 oz. max, is primarily responsible for my 25% drop in fuel use last season, corrected for the degree days.  i wish i had been able to supervise the installation of the original gas boiler [when the coal-fired ARCO cracked] in 1952!!!! that one only lasted 19 years, then no. 3, the american standard gave out in 37 years, the last 8 of which,  with the help of  anti-leak "medication" --nbc
  • Big-Al_2
    Big-Al_2 Member Posts: 263

    Today, I ordered a brand new 3/4" Watts steam-rated vacuum relief valve from some guy on eBay.  Got it for about 30% of retail.  I'll pipe it in when it gets here and see if the severe sucking sounds are sent subterranean.  

    NBC, thanks for the tip on the gage.  I do have an older mercury pressuretrol that seems pretty consistent.  I'll eventually buy a gage, but I think we have a drawer full of 5 PSI gages at work from some project that went nowhere.  I may just borrow one some evening and compare it with what I've got.  If the pressuretrol is working OK, maybe I'll just keep the 0-30 psi gage for now.  Money is tight, and I have a big plumbing job to do.  I looooove old houses . . . oh yeah . . . major repiping . . .I'm all over it.
This discussion has been closed.