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Mysterious knocking hot water pipe

Asked Jul 16, 2012, 07:16 AM — 7 Answers
A plumber replaced a valve near the exit from my water heater. He was pretty medieval with the valve and pipes, because he was only supposed to replace a pipe with a pinhole leak. Since then the hot water pipes knock/bang when turned on, for a few seconds (maybe 3-5 bangs/knocks). I hear it in my master bathroom, which is at the opposite end of the house from the water heater (maybe 75 feet away). The noise is loudest when a faucet is turned on in the bathroom, but I can hear it more faintly when my wife turns on a faucet in the kitchen, some 50 feet away.

The plumber unplugged my Grundfos UP15-18-5B while working, which I plugged in after he left. I have tried the usual DIY for eliminating air hammering (turning off main water-drain-turn back on). It has not worked. I have searched various DIY sites and I don't think it is hammering, and I haven't found problems exactly like mine from previous inquirers.


I also have a Grundfos MQC-45 whole house recirculation pump for the cold water. It has adjustable pressure and is set at 40psi. Turning on and off the two pumps does not make a difference.

The house is 7 years old, and I did not have any problems likes this until the plumber's visit in May.

I would very much appreciate any suggestions on how to solve the mystery.

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7 Answers
hkstroud's Avatar
hkstroud Posts: 8,471, Reputation: 3424
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#2

Jul 16, 2012, 09:40 AM
Quote:
A plumber replaced a valve near the exit from my water heater.
What did this valve look like. Is is a valve that you can turn on and off or is it some other kind of valve?
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CO Don's Avatar
CO Don Posts: 5, Reputation: 1
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#3

Jul 16, 2012, 09:50 AM
It is one that can be turned on and off.
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hkstroud's Avatar
hkstroud Posts: 8,471, Reputation: 3424
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#4

Jul 16, 2012, 09:57 AM
Try closing it part way to see if that makes any differance. Put your hand on the pipe next to the valve to see if this is where the knocking is comming from.
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CO Don's Avatar
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#5

Jul 16, 2012, 10:13 AM
Quote:
Originally Posted by hkstroud View Post
Try closing it part way to see if that makes any differance. Put your hand on the pipe next to the valve to see if this is where the knocking is comming from.
I will try that, but will have to wait until my assistant (i.e., my wife) gets home from work.

Thank you. I'll get back to you later.
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CO Don's Avatar
CO Don Posts: 5, Reputation: 1
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#6

Jul 17, 2012, 06:15 AM
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Originally Posted by CO Don View Post
I will try that, but will have to wait until my assistant (i.e., my wife) gets home from work.

Thank you. I'll get back to you later.
I tried both of your suggestions. Unfortunately, turning down the flow in the new valve did not help. The knocking noise is confined to the master bathroom area. No unusual noises or vibrations near the water heater.
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hkstroud's Avatar
hkstroud Posts: 8,471, Reputation: 3424
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#7

Jul 17, 2012, 07:22 AM
Where the sound originates from can be quite deceiving. The sound will travel along the pipe quite some distance.You have to feel the vibration on the pipe.

Does the knocking occur hot, cold or both. When does it occur, when you turn on a specific valve or when you turn on a valve anywhere?
If you did not feel the vibration in the pipe next to the valve on the water heater is is not likely that valve. Most likely you have a loose washer some where. Another possibility is that in doing the repairs the plumber got solder inside the pipe. If you use too much solder on a joint, the hot liquid solder will run inside the joint. However solder will not stick to a cold pipe. The water then can move the solder to the next available valve. There it can stop the valve or sometimes it will stop up the valve, when the water flow stops it falls back down, then is moved back to the valve when the flow starts again. Causing a rapid stopping and starting of the water flow. This causes the knocking on the pipes.

The problem is at a valve some where.
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CO Don's Avatar
CO Don Posts: 5, Reputation: 1
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#8

Jul 17, 2012, 08:01 AM
Quote:
Originally Posted by hkstroud View Post
Where the sound originates from can be quite deceiving. The sound will travel along the pipe quite some distance.You have to feel the vibration on the pipe.

Does the knocking occur hot, cold or both. When does it occur, when you turn on a specific valve or when you turn on a valve anywhere?
If you did not feel the vibration in the pipe next to the valve on the water heater is is not likely that valve. Most likely you have a loose washer some where. Another possibility is that in doing the repairs the plumber got solder inside the pipe. If you use too much solder on a joint, the hot liquid solder will run inside the joint. However solder will not stick to a cold pipe. The water then can move the solder to the next available valve. There it can stop the valve or sometimes it will stop up the valve, when the water flow stops it falls back down, then is moved back to the valve when the flow starts again. Causing a rapid stopping and starting of the water flow. This causes the knocking on the pipes.

The problem is at a valve some where.
Thanks. It looks like I will need to hire a plumber to figure this out, which I wanted to avoid. It's hard to find a skilled plumber in NM.
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