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We have 3/4" CPVC running into our addition that when the hot water turns on it

  1. takes a long to heat up (granted it has to travel fairly far), and
  2. as it heats up the pipe rattles (sounds a little bit like popcorn popping - progressively gets faster)

Once it's hot the pipe stops rattling. From what I've read it seems as though this is water hammer? In which an arrestor should address this? I have easy access through one of our closets floorboards to the pipes, and I have confirmed that they are secure.

So my question is - does this sound like in fact I need a water hammer arrestor? Would something like this work?

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Is it a loud ticking or a rumbling sound? –  BMitch Jul 14 '13 at 2:54
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more than likly its the pipe expanding znd rubbing against a timber stud –  UNECS Jul 14 '13 at 5:58
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And if so, here's a similar question: What's that dripping sound I hear in the attic whenever we turn on the water? –  BMitch Jul 14 '13 at 12:24
    
It's a ticking sound... –  jrmack Jul 14 '13 at 12:56
    
@UNECS - isn't this the result of water hammer in which an arrester would suppress this? I've verified the pipes are secured against the struds. –  jrmack Jul 15 '13 at 13:49
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2 Answers

It's not hammer time, it's thermal expansion. As the hot water travels through the pipe, the pipe itself heats up and expands. As this happens, fasteners and any materials in contact with the pipe experience friction and, at times, the pipe moves slightly against that material and causes a poping sound. It's like snapping your fingers. We hear this all the time with other materials in other circumstances if you think about it. The frequency of the poping changes as the pipe heats up. The fix would be to better secure the pipe (not tight, but with an intermediate material) to the studs or block as necessary to keep it from rattling or "poping". If attached to concrete, insulate the pipe with something, anything, to keep it from contacting the concrete.

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Water Hammer

Water hammer occurs when you close a valve quickly. The sudden stop of the flow of water creates a shock wave that travels back up the line. Placing a water hammer arrestor at the first change of direction in the pipe (working backwards from the valve) absorbs the shock wave. Water hammer is sudden and is not sustained for a long time.

Thermal Expansion

What your describing sounds like the CPVC pipe rubbing against the building structure as it expands due to thermal expansion caused by the hot water heating up the pipe material. The sound starts off gradually and will increase and intensity as the pipe gets hotter, it may even continue after shutting off the supply until the pipe has fully expanded due to the temperature change. You may not hear the sound when the pipe shrinks back as the water cools in the pipe because it will take much longer for it to cool off and thus it will contract slowly.

Solution

If its thermal expansion the best fix is to eliminate the friction spots, however this is most likely not an option. Another fix, and one that would eliminate the time it takes to get hot water, would be to install a hot water recirculation system. This would keep water in the line hot, preventing the contraction and expansion. There are some systems that can be installed using the cold line without requiring a new hot water return line. Here is an example of one such system.

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