The Wall
Forum / Radiant Heating / Boiler banging when burner reaches max temperature
  • Post a Reply to this Thread

    Boiler banging when burner reaches max temperature (44 Posts)

  • tombstone tombstone @ 9:15 AM
    Contact this user

    Boiler banging when burner reaches max temperature

    Good Morning!
    I'm having an issue with my boiler that has plagued me for a couple of years.  I removed the radiator in my bathroom during a renovation, capped the lines and continued to run the system.  While I was running the boiler, I would hear banging coming from the boiler and attributed it to air trapped in the temporarily abandoned lines.
    I recently installed a new Runtal radiator in it's place and the problem stil exists after numerous bleedings.
    I have a Bryant boiler that was installed approximately ten years ago.   Each raditor has a feed and retun line that branch directly off the trunk line in the basement.  I do have a bleeder on the boiler, but it looks like its bent and the bleeder vent is no longer the highest part on the device.
    I have an old style expansion tank hanging in the rafters.
    I was looking for thoughts and suggestions on what I can do to troubleshoot and ultimately resolve this.
  • Gordan Gordan @ 10:00 AM
    Contact this user

    Your problem may be too little air, not too much

    If your expansion tank is waterlogged or otherwise non-functioning, and your pressure release valve doesn't work (what's the last time you tested it?)

    The old radiator may have had air trapped at the top that made it act as an expansion tank of sorts. Watch the pressure gauge as the boiler is coming up to temperature and let us know what you see.
  • tombstone tombstone @ 11:57 AM
    Contact this user


    The pressure relief valve works - I pulled up on the lever the other day.  I have one of the old expansion tanks that hangs in the rafters.  I've isolated it a few times and attempted to drain it but no water comes out.  How can that type of expansion tank fail?
    I've watched the boiler while it's "banging" and didn't see any jumps in pressure. 
  • Gordan Gordan @ 12:24 PM
    Contact this user

    What IS the system pressure at the boiler?

    Cold vs. hot.

    Also, what's the boiler model? Is it a copper fin tube boiler? Can you post pictures? Is the banging coming from the boiler itself?
  • tombstone tombstone @ 6:45 PM
    Contact this user

    Additional Information

    The pressure at the boiler when hot is just under 20 and around 15 when cold if memory serves.  The boiler is a Bryant BW2AAN000070ABAA.  I attached an image of the bent air vent.  As you can see the valve is on the low side.
    This post was edited by an admin on January 19, 2012 6:45 PM.
  • tombstone tombstone @ 9:43 AM
    Contact this user

    How can old style expansion tank fail?

    In what ways can an old style expansion tank fail?  I know in the newer ones that bladder and fail but I'm at a loss to see what can fail on the older ones.
  • Gordan Gordan @ 9:47 AM
    Contact this user

    They don't fail...

    They get waterlogged. But judging by your pressures it doesn't look like that's your problem. You shouldn't have open air vents and air eliminators on a system that's got an old-style compression tank. Here, read this:

    Now, is the noise coming from the boiler itself? A clogged heat exchanger could cause inadequate flow, or if it's dirty on the outside (or damaged) there could be hot spots, both of which could cause water to flash to steam inside the boiler.
    This post was edited by an admin on January 20, 2012 10:02 AM.
  • tombstone tombstone @ 1:34 PM
    Contact this user

    More Info

    The sound does seem to be coming from the boiler internally.   I'm guessing that it would be no problem to clean the exterior of the boiler, but how would I clean the inside if needed.
    I've also been bleeding radiators for days as there always seems to be air in the system.   The autofill is typically turned off as I think there is a leak in that.
    To bleed the system, I turn off the boiler and turn on the autofill.   I go to the furtherst radiator and open the bleeder until I get a stream of water.   This is the correct procedure right?
    I can also hear the water passing through the new radiator so I'm rather confident my enemy is air ... I just need to get it out.
  • Gordan Gordan @ 1:43 PM
    Contact this user

    No, you need to bleed EVERY radiator

    Not just the furthest one. And you bleed them at least twice. Any high spot in the piping (and radiators certainly qualify) will collect air.
  • tombstone tombstone @ 1:49 PM
    Contact this user

    Clarification ...

    I do bleed them all, but typically not one after another.  Which could certainly be an issue.
    Should I be able to bleed all the radiators one after another without the system needing to catch up with the water pressure?
  • Gordan Gordan @ 2:08 PM
    Contact this user

    That depends on the starting water pressure and amount of air...

    It'll drop somewhat. Check the gauge and add water if necessary.
  • tombstone tombstone @ 2:15 PM
    Contact this user

    Water Pressure

    What is a good operating vs. resting water pressure?
  • Gordan Gordan @ 2:34 PM
    Contact this user

    Cold water pressure...

    ...should be enough to push the water column up to the highest point in the heating system, plus a little safety margin. 12 psi is good for a one- or two-story residence. Hot should be safely below the T&P release valve's setting in order to avoid nuisance trips. If you're 5 over, you're good.
  • tombstone tombstone @ 9:40 PM
    Contact this user

    Still having the issue

    Hello again .. I'm still having the issue and am looking for some other steps I can take to try to get this resolved. 
    I am going to bleed everything again tomorrow and see what happens but am looking for additional input.
  • Zman Zman @ 10:00 PM
    Contact this user

    Eliminate some possibilities

    Gordon mentioned the expansion tank. If you tap on it, does it sound like 1/2 air 1/2 water? Are you bleeding with the circulators off? Is the pressure gauge  accurate?
    Does the air vent in the picture work?
    I had one recently that had a bad fill valve a clogged air vent  and a gauge stuck at 20. It sounded like yours because the water was boiling to steam. and couldn't get out. I was a strange combination that had me scratching my head.
  • tombstone tombstone @ 10:07 PM
    Contact this user


    If I recall there wasn't much difference in the sound of the expansion tank.  It's the old style type.  I am bleeding with the circulator off and the pressure guage was accurate when it was tested last year.  I don't think the air vent works since the threaded cap is on the side that is bent down.
  • Zman Zman @ 10:34 PM
    Contact this user

    Replace it

    You could well have air in the boiler that won't bleed through the radiators. Why would it? It has a low flow area to hang out. Make sure the pipe it is threaded into is clean as well.They are not expensive. Neither are expansion tanks if you are unsure.
    This post was edited by an admin on February 10, 2012 10:36 PM.
  • tombstone tombstone @ 3:46 AM
    Contact this user

    Replacing it

    I don't see a way that I can turn a few valve to isolate it to for easy replacement.  is that possible or do I need to bring the system down?
  • How Come?

    How come no one has asked Tombstone about:

    1) What kind of pump is on the system and

    2) What is the operating limit set at, i.e. what temperature is your boiler set at?

    I recently worked on a 30-yr. old Raypak boiler that had a Honeywell modulating gas valve with a capillary to a sensor in the boiler block.  It had failed completely open and the only thing controlling the boiler was the hi limit.  Water in the boiler was flashing to steam and it was making a terrible racket - the people wanted to move out.
    Often wrong, never in doubt.
  • tombstone tombstone @ 3:49 AM
    Contact this user


    I have  Taco 007 pump that was replaced two years ago.  I was told that the old pump may have been wearing out and the water wasn't moving through the system fast enough and flashing to steam.  The operating temp is 160 degrees.
  • tombstone tombstone @ 3:58 AM
    Contact this user

    Correction ...

    The banging seems to occur thoughout the heating cycle not just at max heat as previously reported.
  • Zman Zman @ 11:07 AM
    Contact this user

    Replace it

    The air vent is the most likely source of the problem. I don't know how you can troubleshoot this without a working vent at the boiler. I would replace the tank while you are at it. You can install isolation valve to save time next time.
  • tombstone tombstone @ 12:02 PM
    Contact this user

    Air vent

    How is the air vent supposed to work?  Mine has a small hole in the cap.  If i remove the cap completely and expose the schrader type vavle, water trickles out.  If I depress the pin then more water comes out.
  • Then it may not be the air vent.

    Zman thinks because the vent may not be working properly that there is trapped air in the boiler.  Even with a properly operating auto air vent there could be air trapped in one or more of the sections and not allowing proper heat transfer, causing water to flash to steam.

    If I were you, I'd turn off the system and let the pressure off.  Replace the auto air vent and while you have the boiler open, add a tablespoon of dish detergent to your system water.  Let the system run with the auto air vent cap loose.

    If that doesn't work, there is something else preventing the water from capturing the heat in the heat exchanger.  It might be a flow problem, i.e. replace the pump again, maybe even a partially closed valve, slowing the water down.  Check it.
    Often wrong, never in doubt.
  • tombstone tombstone @ 7:07 PM
    Contact this user

    more info

    If it were working properly would the air vent be able to properly vent with the cap completely off?  Mine just lets water pour out - I was able to straighten it a bit. What does the dish soap do?  Is there a recommend kind?
  • Yes.

    The air vent is supposed to seal shut when all the air at the air vent is removed.

    The dish soap reduces the surface tension of water - a surfactant - allowing the water and air to combine enabling the air to be carried away to a vent site where it can be removed.

    Any liquid dish soap at the supermarket will do.
    Often wrong, never in doubt.
    This post was edited by an admin on February 12, 2012 2:12 AM.
  • tombstone tombstone @ 7:04 AM
    Contact this user

    more questions

    I'll order the air vent today.  I've never heard or read about adding the dish soap.  Are there any ill effects of doing that?  I was also taking some temperature readings of the pipe looking for a blockage as I don't think there is any water in the expansion tank.  Working from the expansion tank towards the boiler there is the tank, about six inches of pipe, a valve, a 90 degree elbow, then more pipe.  The temperature differencre from the 90 degree elbow to the pipe is 40 degrees.  Would this indicate a blockage in the elbow not allowing water/air into the expansion tank?  I opened up the bleeder on the valve before the tank and cold water came out.
    Thanks again!
  • lchmb lchmb @ 7:57 AM
    Contact this user


    you have a plain steel tank mounted in your rafters above your boiler you do NOT want any type of air elimination on your system....It sounds like you have a monoflow system, so make sure while your bleeding your radiators that the circulator is off. I like to have someone stay in the basement to maintain about 15lbs while I bleed them. Once you have bled all the radiators, make sure to shut off you steel tank, drain the water and allow air into it to create the cushon needed...
  • tombstone tombstone @ 8:29 AM
    Contact this user

    steel tank

    I do have a steel tank.  I've drained it in th past but nothing ever comes out.  Is tihs typical or should there always be water in it?
    Also, just to clarify, after I've bled the rads, I turn off the valve leading to the expansion tank and open up the drain vavlve - do I need to do anything to set up the air cushion or just turn hte valve off and its done?
  • Zman Zman @ 9:26 AM
    Contact this user

    post a picture

    I think everyone here is agreeing it is likely you have an air problem. I couple of questions. Is the air vent located in a place where it can effectively purge the boiler? Do you have an "open" expansion tank? If so why does nothing come out of it? water or air. 
  • Talking, talking

    When you turn off the valve to your expansion tank (x-tank) and open up the drain, you may not get any water out without letting air in, kind of like your finger on a straw full of water.  You might have to remove a plug on the tank to get the air in.  Once the tank is drained and your system is purged, open up the valve to the x-tank; ideally, it's filled 2/3 with water and 1/3 with air and will do so by itself once you open the valve.

    If, in fact, you do have a monoflow system, air purging will be difficult, more reason to use that tablespoon of dish soap.  I learned that trick from Mark Eatherton when I was having problems and it worked like a charm.
    Often wrong, never in doubt.
  • Jean-David Beyer Jean-David Beyer @ 9:51 PM
    Contact this user

    I had a conventional expansion tank.

    I could not drain it at all. It was poorly installed and did not use those clever fixtures that sort of extracted air from the system and let it percolate into the tank. And it had no valves to let air in to replace the water that came out of the drain valve.

    I complained about this to my heating contractor at the time, and he instaleld a different drain valve that looked like the original, but it had an extra tube inside. If you took out the screw that seemed to hold the handle on, it allowed air to enter there and go to near the top of the tank.  It still took 15 or 20 minutes to drain the tank, but at least I could do it. When I switched boilers, the new contractor just put in a diaphragm type expansion tank. Much better.
  • tombstone tombstone @ 8:42 AM
    Contact this user

    Two Pipe Direct Return

    From what I can tell I have a two pipe direct return system.   Does that change any of the advice?  I'm planning on draining the expansion tank again this evening.
  • Jean-David

    had a Bell & Gossett Aitrol fitting on his x-tank.  See page 5:

    Tombstone: You may need to stick a straw or some skinny tubing into the drain hole of your expansion tank to break the vacuum.  And as far as your 2-pipe system, the piping may have been done in a way that traps air and you (or someone you call in to help) will have to figure out where it is or more importantly, how to get it out.
    Often wrong, never in doubt.
    This post was edited by an admin on February 13, 2012 2:21 PM.
  • Jean-David Beyer Jean-David Beyer @ 3:29 PM
    Contact this user

    I do not think so.

    I think what you suggested is what I should have had. I had plain copper tube going to the tank direct from the pressure reduction makeup valve. The routing of the pipes pretty well guaranteed that no air from the boiler would ever get to the expansion tank, that regularly (about twice a year) got water logged.

    At the other end of the tank was what looked like a regular hose bib or boiler drain valve. The new valve looked the same as the old, but unscrewing the screw that held the handle on allowed air to enter the tank through a tube that extended from the valve part way (not enough of the way) into the tank.
  • tombstone tombstone @ 7:16 AM
    Contact this user


    Good Morning ...
    Last evening I drained the expansion tank and let everything reset it self.  The pressure seems to be holding relatively constant at 15 psi.  I have yet to replace the air vent.  I'm getting mixed information on the need of an air vent here.  Should I have one?  If so, I'll pick up a replacement this week and swap it out over the weekend.   I thought resetting the expansion tank seemed to take care of the banging but it resumed this morning.  I didn't bleed the radiators again after I refilled it so I'm sure that has something to do with it.  If I do need ro replace the air bleeder I'll add the detergent at that time.
    Thanks again fo all the insight.
  • No progress

    A couple of days ago, Zman asked you for pictures of your system and some answers to his questions, neither of which you gave him. Do you have a digital camera or know someone who can take some pictures and post them on this website for us to see?  Because we can't give you any good answers unless we see more of how your system is put together.  If you have the pictures, but don't know how to post them, we can help you with that.

    And you tell us that you drained your expansion tank.  Previously, you said that when you tried draining it before, nothing came out.  How did it go this time?  Did you get much water out?  Did you have to do something to break the vacuum?
    Often wrong, never in doubt.
  • tombstone tombstone @ 2:05 PM
    Contact this user


    Sorry ... I missed the request fo a picture and will do that this evening.   When I tried to bleed the expansion tank last time I didn't undo the air vent on the hose bib. This time I did.
  • tombstone tombstone @ 7:51 PM
    Contact this user

    Images and additional info.

    I finally got home early enough to take picture of my installation.

    When I drained the expansion tank I did get out a considerable amount of water. I filled up one and a half of those large plastic storage totes.  I also changed the air valve earlier this evening.  All radiators except one are cast iron.  The other is a Runtal baseboard.  The pressure hot and cold is holding between 15 and 20 psi.

    Thanks again for all your info and help.
    This post was edited by an admin on February 16, 2012 7:52 PM.
  • tombstone tombstone @ 7:36 PM
    Contact this user

    Sludge in Boiler?

    Is is possible that I have a sludge build up in the boiler?  It's only 12 years old but I'm guessing its not out of the realm of possiblity.  If so, would I be able to isolate the boiler by throwing the valves on the supply and return lines, then somehow get water into the boiler through the pressure relief valve?
  • Ironman Ironman @ 8:44 PM
    Contact this user

    Thermal expansion?

    Have you checked your piping to confirm that they have freedom to expand when heated? If a pipe is strapped tight to a framing member, particularly when there's a change in direction, such as vertical to horizontal, then they can make banging sounds as they expand. Sometimes it can be quite severe.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • tombstone tombstone @ 8:47 PM
    Contact this user


    Thanks for the suggestion but this is definately coming from the interior of the boiler.
  • Pump

    You said you replaced your pump 2 years ago.  Are you able to tell if your pump is working?  Can you hear it? 

    When you first turn your boiler on, does the supply pipe coming out of the boiler warm slowly or does it get very hot quickly?
    Often wrong, never in doubt.
  • bill bill @ 9:45 PM
    Contact this user

    It's now been a month

    I had a banging C.I. boiler not too long ago. The owners said it was coming from the pipes. It sure did sound like it. I looked at all the stuff you've done. I then found the trouble.
    Try this. Remove the gas vent and the motorized vent damper. With a bright light  confirm that the flueways are not clogged with either soot or rust.
    Disproportionate temps through the cast iron will cause lots of expansion and contraction.
    On the boiler I was on it actually was making CO. And the woman was sick from it.
    Take a picture of the flueways.
     Edit - Oh, wait a minute, it's not that easy with that style of draft hood. If all the pipes, pump and other stuff check out okay, well then you need to look elsewhere. Are the buners looking clean? If you were to look at the burners (off) and smack the thing on it's side does anything come out. I know that's highly technical troublshooting:)
    This post was edited by an admin on February 17, 2012 9:58 PM.
Post a Reply to this Thread