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bri10knee 11-19-2007 08:12 AM

Knocking Pipes
I've been reading through some of the other posts about knocking pipes trying to find an answer to what's going on in my home. I've got a few ideas going in my head, but I know NOTHING about this sort of thing and I don't want to mess this up. Depending on the fix, I wanted to try to do this on my own...without bugging my husband!

So, onto the problem. Several months back we replaced out kitchen sink and about a month after that we added a Brita filter. I'm trying to remember if the problem started before or after the filter..I believe it was before. Our pipes now knock. Always when the kitchen sink is in use, but sometimes I will hear knocking when nothing is on. I'll hear a random knocking and then I'll hear water come out of the shower head in the only bathroom in the house. Not sure how old the plumbing is. The house was built in the early 1950's and we've only owned it since Feb 2005. Is there any easy fix to this? Someone had told me to get a knock valve. Will that work? Is that easily installed?

Thanks! :)

travelover 11-19-2007 08:47 AM

You can make your own knock valve. All it consists of is a dead end piece of pipe oriented vertically so air stays trapped in it. It acts like a spring to cushion the shock of the water suddenly stopping. It should be located near the end of the run (near shut off valves).

Before you go to all the work of installing these, try shutting off all the water in the house and drain all the pipes. Doing this allows air back into any anti knock valves that may be there. When you turn the water back on, the faucets will hiss for awhile, until the air is expelled.

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bri10knee 11-19-2007 09:34 AM

Thank you very much! I will try this when I get home from work! :D

glennjanie 11-19-2007 03:31 PM

Hello Brittanie:
Plumbers call it 'water hammer', sounds a little like knocking, right?
Water hammers or knocks because it is rushing through the pipes and is suddenly cut off, usually by a solenoid valve. Solenoid valves are fast-acting and are found on your automatic washer, dishwasher and ice maker. Some folks like to hit a single lever to turn it off and get the same results.
To prevent 'water hammer' you need to add a cushion of air (air will compress and water will not), by putting a tee in a horizontal line and extending a piece of pipe vertically, at least 12" and cap it. There are also manufactured 'water hammer' arrestors at the plumbing supply house. The 'water hammer arrestor' should be placed near the solenoid valves mentioned above.
Another noise maker is pipe that is passed through a framing member (stud, joist, etc.) that is in a bind. When the hot water is turned on the pipe expands and the friction makes a popping sound. When the hot water is truned off, the pipe shrinks back down making the same sound. Some say it sounds like a leak (drip, drip, drip) in the wall but never find any water from the leak. If you can locate the popping or 'leaking' sound just apply a little petroleum jelly so the pipe can slide freely.
I hope you can get it done without your husband knowing, then you can wear a gold star for 3 days.

Kerrylib 11-19-2007 04:14 PM

Just a comment that maybe this suggestion needs some more consideration.

Don't the Water hammer arrestors actually have a membrane to prevent the air from disolving into the water?

If you were to put a "dead leg" on a tee as suggested, I'm sure it would work as indicated. However over time the air will disolve into the water until the leg is full of water and then it will no longer be doing anything. The devices you buy are attached on a Tee and then the air side is pressurized to your water line pressure to provide an air cushion. The membrane/diaphraghm prevents the air from disolving into the water and thus "shutting off" the device.

I could be wrong on this point, but from my observations, this is what is likely to happen, and thus the reasoning behind the diaphraghm in the store bought products.


travelover 11-19-2007 06:36 PM

Kerrylib, I think you are correct. In the olden days they just used a dead leg as Glenn and I have referred to. When knocking started, the system was drained. The hammer arrestor is better, but more expensive.

glennjanie 11-20-2007 09:17 AM

Right on! Kerrylib, you are exactly right.

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