Water hammer questions

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Uncle Irish

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I recently have been doing some renovations in my upstairs bathroom. When I run the tub or shower and turn it off, I get knocking sounds in the pipes. Thinking it was water hammer,I followed some advice and drained all the pipes including hot water heater. Same thing still happened.
So I tried a few experiments which leave me with more questions. I ran the shower/bath but had the drain plug in. Turned off the shower and no knocking. Doesn't that eliminate the possibility of water hammer? I waited 5 minutes-still no knocking, then opened the drain and bam knocking again. So it appears to be related to the drain pipes and not the water pipes. Any ideas?
 
I call this "PipeKnock" simply you have a lose pipe in the wall or floor somewhere. everytime a rush of water heads down the pipe the pipe moves and is hitting something .
 
Welcome Uncle Irish:
Water in gravity flow will seldom knock except at the bottom of a stack, then its called hydraulic jump; still very rare.
Your drain pipes may be fitting tightly through a framing member causing them to rub and make popping or banging noises. This is a common problem with PVC drain waste and vent pipes and with CPVC water supplies. The water supply lines will typically sound like a drip, drip, drip while the drain lines will make a popping noise.
Check for rubbing points and use some vaseline to ease the bumping sounds.
Glenn
 
I think I've learned more perusing the site recently than in a few years of do it yourselfing.
If it's a pvc pipe hitting a joint or other object, can I just leave it be? While the noise is annoying and perhaps somewhat alarming, so to would be ripping out the drywall, tub, etc. to try to fix it.
Btw, just curious...what is hydraulic jump?
 
Hello Uncle Irish:
Yes, it will be OK to leave the drain pipe. Plastics are so nice to use, cheap, easy and last forever but, they are noisy. I know of one plumbing company that wraps their PVC drains with insulation.
Hydraulic jump is a stack siting on an ell with a horizontal drain. When the sewage comes down the stack and hits the ell, it will sometimes make the water hit the top of the horizontal drain and make the pipe jump up from the force. It is more common in high use drains such as in a hotel or apartment house with high drainage fixture unit loads.
Glenn
 
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