Banging noise from copper hot water pipe (not water hammer)

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Ron Hoffman

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Hi all,

I have had great success finding answers on this forum.
I'm new to posting on the forum and have a puzzler that has stumped me.

I hear a single bang/thud noise from my hot water line after running hot water for a few minutes.
A bang/thud sound resonates through the basement joist to the floor above a few minutes after the hot water begins flowing through the 3/4 copper pipe in basement. I am certain it is not water hammer because the noise is not heard until after the hot water has run for a while and a few minutes after it is shut off. Thinking it has something to do with the copper pipe expanding and contracting, I loosened the straps securing the 3/4 copper run to the joist, however I still have the noise. I am not sure how long I have had the noise because the heat trap nipple in the top of the hot water tank was extremely loud. I recently had the nipple replaced and had new 3/4 L copper runs put in the basement because of heavy corrosion (pinhole leaks). Also, I had a pressure reducing valve and expansion tank installed and the incoming water pressure is set to 60 psi.

Could the copper pipe be pressed to firmly against the joist causing the noise?

I have M copper coming out of the hot water tank for a few feet with two 90 degree elbows before
connecting to the L copper pipe. Could there be a different rate or amount of expansion between the
L and M copper pipe causing the noise?

Any ideas on what might be causing the noise?

Thank you for any and all help.

Ron
 
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Jadnashua

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The copper will expand when it is heated (usually faster) and then slowly contract while cooling off again after the flow stops. If there is any tight clamp or hole it must pass through, the pressure will build up, then it will jump once it overcomes the friction. You might not notice when it cools since that tends to happen slower. If, after a 90-degree turn, it butts up against a rigid surface, that can cause the run to bow as it expands, but probably not fast enough to generate a hit loud enough to hear as it impacts another surface.
 

Ron Hoffman

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The copper will expand when it is heated (usually faster) and then slowly contract while cooling off again after the flow stops. If there is any tight clamp or hole it must pass through, the pressure will build up, then it will jump once it overcomes the friction. You might not notice when it cools since that tends to happen slower. If, after a 90-degree turn, it butts up against a rigid surface, that can cause the run to bow as it expands, but probably not fast enough to generate a hit loud enough to hear as it impacts another surface.

Thanks for the reply Jim.
Do you think there is any problem with the expansion between the m copper (thinner wall) and the L copper (thicker wall)?
I've listened and watched the pipes at the location of the sound directly below the first floor in the basement and I don't hear or see
any pipe movement. Could it be that the pipe expansion is amplified by the joist it is up against?
The pipe does not go through any joist holes and all the clamps have been loosened enough for thermal expansion movement.
However, the sound is quite loud on the floor above and
am concerned that it could eventually loosen a solder joint.

Ron
 

Reach4

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Do you get that symptom when running hot water from the kitchen? Do you get that symptom when running hot water from the bathroom?
 

Ron Hoffman

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Do you get that symptom when running hot water from the kitchen? Do you get that symptom when running hot water from the bathroom?
Reach4,

I get the same symptom no matter where I run the hot water.

Ron
 

Reach4

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Reach4,

I get the same symptom no matter where I run the hot water.

Ron

OK... it sounds like you are looking in the right places. I don't know about the mixing valve. Being the new thing, I would wonder about it. Did you have new heat trap nipples installed, or just regular dielectric nipples? How about grasping the pipes at various points, wearing gloves, to see if what you feel tells you something. Run the water in the way that causes the problem. It might also modify the sound in a way that tells you something. If you use 2 hands, you might feel the thud more in one hand than the other.

The K and M are going to have the same temperature expansion rates.
 

Jadnashua

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The only difference you'll be able to detect between K, L, or M is their thermal mass...K will take slightly longer to heat up entirely than L, and that less than M. Once they stabilize at the same temperature, they'll have expanded/contracted the same amount. You should not have any issue mixing the different thickness pipes. There will be a very slight (measurable in the lab) flow/pressure difference because they have the same OD (so they can use the same fittings), but because of their thickness, different ID.

It doesn't take much movement to make noises if it's sticking somewhere.
 
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