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knocking sound in hot water heat piping.

Juan's picture

I have a persistent and annoying knocking sound coming from a run of copper piping between two hot water baseboard radiators. I removed sheetrock in a partially finished basement and exposed the entire run between the vertical legs going up into the room and baseboard radiator above, about ten feet with one ninety degree turn. I secured the run as well as possible but cannot silence the knocking.

The knocking comes from a very specific location, which is about halfway through a zone about 90 feet in length with about 35 feet of baseboard radiator.

Is it possible the noise I hear, which is pretty loud and sounds like fast knocking against metal, comes only from expansion or contraction of the copper itself and has nothing to do with the pipe actually striking something (like a stud)?

Is it a potential solution to replace this ten foot run with Pex, now that it is completely exposed? Any drawbacks?


(post #74588, reply #1 of 9)

Yep that is expansion noise.  Check that the baseboard fin elements are all sitting on the plastic gizmos that try to isolate them from the support brackets.  If your baseboard has metal isolators then you've got some old fin tube.

If the pipe passes thru a floor or wall the pipe could be rubbing on the wood.

If you put in some pex make sure you use pex with an oxygen barrier (like Pex-AL-Pex);  you do not want to let oygen into your closed loop heating as it will rust out the boiler and any steel/iron pipes.

Verify that the boiler temp is not too high.  Is this a new noise that didn't happen last year?



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(post #74588, reply #2 of 9)

Thanks for your reply.

The zone is new (as part of a master bedroom addition) and the boiler and rest of the system are seven years old. We lowered the boiler temp five degrees when we first started hearing the noise and I'm confident the problem is not that the boiler is set too high.

Plastic isolators on the baseboard units, but the noise is coming from below, seemingly from the 3/4" copper feed itself, without the assistance of a stud. I could be, and often am, wrong.

Regarding the Pex, I'm a little concerned of introducing a new problem by installing a single length of Pex in the zone I described.

Edited 10/30/2006 10:44 am ET by juan

(post #74588, reply #3 of 9)

Pex is used with copper in heating systems all the time.  In fact some systems are done with Pex as the "home run" from the boiler to each room's fin tube.  Like I said just make sure it is a pex with an oxygen barrier. 



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(post #74588, reply #4 of 9)

In addition to looking at expansion/contraction of pipes, do not ignore the possiblity the system is not 100 filled and there are air pockets in the pipe runs.

And there is a difference between bleeding air out of the system and whether or not the system is filled to capacity.

(post #74588, reply #5 of 9)

Do you have an expansion tank in your system?

Have a good day


(post #74588, reply #7 of 9)

Yes, we have an expansion tank. The installation and piping is very neat, the area around the boiler looks like a pipe organ. For the life of me I cannot find one single area where the copper has an opportunity to rub against wood or anything else.

(post #74588, reply #8 of 9)

great site for heating related questions.lots of knowledgable and helpful people.

(post #74588, reply #9 of 9)

I'm in the present house 10 years now. I put in a hot water system when I built.  Every year around this time I get the creaks etc while the heating seson is commencing.  After a week or so its over.

Have a good day


(post #74588, reply #6 of 9)

I had a tapping knocking noise everytime I ran hot water in our half bath. The expansion of the drain pipe was rubbing on the sill plate. The plumber used a hole saw the same size as the ext. of the drain pipe.

I was about ready to open the wall, with a sledge...drove me crazy. I carefully enlarged the hole around the drain and filled with foam. The noise is gone.

Check all the holes. Not  sure if foaming a heat line is the right move but any movement on a stud or floor pan will be amplified.