knocking sounds coming from the boiler

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Old 03-30-12, 09:10 AM
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knocking sounds coming from the boiler

I have a 32 year old gas fired, cast iron boiler running 2 zones of hydronic system. It runs ok, not very efficient, but it does the job. The pressure runs from 15 to 18psi hot, aquastat is set at 180F, 1 circulator.

When heating cycle starts, I can hear knocking sounds coming from the boiler. I thought it was a sound of metal expanding, but what is troubling me is the fact that I also see a pressure gauge needle jumping with each knock. It could be vibration or actual pressure change in the system Ė I donít know. it sounds almost like a air bubble bursting. And it happens every 2 or 3 seconds. When firing cycle completes, the sound goes away while the water is still circulating. And itís not pipes expanding sound. I have a simple air scoop and the top of it is dry. Does it get a bit wet when some air is extracted?

Itís our 2nd heating season in this house and I believe that we had it all along, but itís possible that itís more pronounced now compared with last year. Itís also possible that I didnít pay attention back then since I had no clue about boilers that time.

The boiler hasnít been cleaned for ages and I plan on doing it when the season is over. Few weeks ago I shut the cold water intake valve to make sure if something happens the boiler doesnít drain the entire municipal water supply into our basement.

I could take a short video to demonstrate the sounds and the needle jumps if it helps.
 
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Old 03-30-12, 12:50 PM
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I have read that it might be water boiling, which is caused by sediment on the bottom of the boiler, that creates hot spots.
Don't know for sure.

Peterr
 
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Old 03-30-12, 02:28 PM
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Peter,

That was my original thought and pressure gauge needle jump goes along with that theory. DHW heater are prone to this since there is always a fresh supply of water and higher change of sediment accumulation. Hydronics are closed systems, so there shouldn't be much risk of that, but who knows.

If water indeed boils, would i see any indication of air being expelled around my air scoop?

I'm thinking about checking the pressure on one of the loop drain valves when that zone is open. If pressure in the system does jump a bit, it should be reflected there since water is not very compressible (expansion tank might damper it though).
 
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Old 03-30-12, 03:51 PM
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If your system ever ran with significant leaks and/or you have hard water then sediments and scale could very well be a problem. It would take an internal inspection to be certain.
 
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Old 03-30-12, 05:48 PM
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Originally Posted by _Diver View Post
If water indeed boils, would i see any indication of air being expelled around my air scoop?
I don't think so. Boiled water is steam, not air.
I'd guess the steam bubbles turn back into hot water before they get to the air scoop.

Peter
 
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Old 03-30-12, 05:51 PM
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Diver.
What brand of boiler?
Where is the circualtor pump located, supply or return?
What model is the pump?
Is the return water really hot or on the cool side?

Is there anyt soot build-up on the fireside of the boiler?
Maybe a good cleaning and service mmight help?

Peter
 
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Old 03-30-12, 06:56 PM
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Sounds like a scale build-up as stated. When it is making the noise shut the boiler down and listen if the sound goes away quickly or slowly. Does it get louder as the boiler gets hotter?
 
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Old 03-30-12, 07:18 PM
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Originally Posted by Furd View Post
If your system ever ran with significant leaks and/or you have hard water then sediments and scale could very well be a problem. It would take an internal inspection to be certain.
there are no obvious signs of prior leaks. our water is extremely soft - i measured it at 40 ppm for calcium hardness. PH of water is 7.4, total alkalinity is 65. CSI (calcite saturation index) is negative with temperatures up to 200F, so if anything it suggest it will dissolve any sediment.
 
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Old 03-30-12, 07:35 PM
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Originally Posted by PeterNH View Post
I don't think so. Boiled water is steam, not air.
I'd guess the steam bubbles turn back into hot water before they get to the air scoop.

Peter
Peter, I was thinking the hot steam would escape and it would leave some moisture behind that could be visible.

What brand of boiler?

Repco LA 100c

Where is the circualtor pump located, supply or return?

Return

What model is the pump?

Taco 007

Is the return water really hot or on the cool side?

Hot

Is there anyt soot build-up on the fireside of the boiler?

No.

Maybe a good cleaning and service mmight help?

it does need some cleaning. my CO numbers are on a hide side. i'm waiting for a season to be over to give it a good scrub
 
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Old 03-30-12, 07:39 PM
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Originally Posted by rbeck View Post
Sounds like a scale build-up as stated. When it is making the noise shut the boiler down and listen if the sound goes away quickly or slowly. Does it get louder as the boiler gets hotter?
will do when i have a chance!
 
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Old 03-30-12, 11:10 PM
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Does this boiler have any kind of draft fan? If yes, it could be as simple as the fan is loose on the motor shaft.

The idea of a video might be helpful.
 
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Old 03-31-12, 10:19 AM
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no fan, atmospheric venting. it's not doing any knocking right now. it appears that it does it on a cold start. we had some mild weather and the boiler would be off for majority of the night and then i would hear it in the morning. i will keep and eye on it and do the video when i have a chance...
 
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