How to fix knocking noise with hot water in CPVC pipes

[knockingnoise]

My contractor installed these CPVC pipes shown in the photo for a double vanity. When you turn on the hot water on either sink (hot water is on the left on both sinks), there is knocking that starts once the water gets hot and it doesn't stop until about 15 seconds after turning off the water.

The knocking sounds like it is coming from the wall on the right but he's saying he didn't touch that wall but there was no knocking before he converted a single sink into a double sink so I know it is caused by what he did or didn't do.

All the walls are sealed up now so none of this is accessible anymore. My questions are:

1. What do you think is causing the knocking looking at this photo?
2. How can he solve it? He mentioned he could try installing water hammer arrestors but I am not sure that is the right solution based on what I've read online.
3. If he cannot solve it, how much would it cost me to get someone else to solve it (high and low range)?

Thank you.

 

[Twowaxhack]

Either the water pipe or the drain pipe is likely expanding and rubbing on the framing. It sounds like a loud tick or tapping.

Nothing can be done expect for opening the walls and giving room for expansion.

 

[knockingnoise]

Either the water pipe or the drain pipe is likely expanding and rubbing on the framing. It sounds like a loud tick or tapping.

Nothing can be done expect for opening the walls and giving room for expansion.

Is this something that should have been caught during the leak and flow test?
Yes sounds like a loud tick or tapping.

 

[knockingnoise]

New question: what's the worst that could happen if I don't get this addressed?

 

[Twowaxhack]

Is this something that should have been caught during the leak and flow test?
Yes sounds like a loud tick or tapping.

No, no test would really show this. The installer/plumber must be aware of how they’re installing the piping. It has to have room or be insulated by something so it doesn’t rub.

 

[Twowaxhack]

New question: what's the worst that could happen if I don't get this addressed?

Probably nothing will happen but it depends on what the pipe is rubbing against. Most likely it’s just rubbing on wood but if it’s metal or there’s a nail etc. could rub a weak spot on the pipe.

 

[knockingnoise]

No, no test would really show this. The installer/plumber must be aware of how they’re installing the piping. It has to have room or be insulated by something so it doesn’t rub.

Thank you. What is the worst that could happen if I don't get the knocking fixed?

 

[knockingnoise]

Probably nothing will happen but it depends on what the pipe is rubbing against. Most likely it’s just rubbing on wood but if it’s metal or there’s a nail etc. could rub a weak spot on the pipe.

That's what I thought. Thank you.

 

[GReynolds929]

Where's the vent for the sinks, the top of the fixture fitting is plugged.

 

[Twowaxhack]

Where's the vent for the sinks, the top of the fixture fitting is plugged.

Looks like they skipped it

 

[MicEd69]

Probably nothing will happen but it depends on what the pipe is rubbing against. Most likely it’s just rubbing on wood but if it’s metal or there’s a nail etc. could rub a weak spot on the pipe.

Those are metal studs, so this could be an issue sooner rather than later. And as it appears the water pipes are in the way for the vent to go up from the double fixture, that plug apparently isn't for pressure testing the drain lines. So it does appear thise two sinks are not properly vented.

 

[Twowaxhack]

Those are metal studs, so this could be an issue sooner rather than later. And as it appears the water pipes are in the way for the vent to go up from the double fixture, that plug apparently isn't for pressure testing the drain lines. So it does appear thise two sinks are not properly vented.

Oh well, he knows what it could be so it’s his choice. I’m not sure where it’s rubbing, no one does.

There’s no doubt the lavatories are not vented. If he ever filled them full and dumped them it might siphon otherwise with normal flow you’ll never make it siphon. If the pipe starts to clog in the future that will make it gurgle like an s-trap. It’s basically acting as a combination waste and vent does. But sure, It’s illegal.

If he wanted he could cut the wall open and remove the plug then pipe a studor vent under the cabinet.

 

[Twowaxhack]

Could also be the drain line causing the noise.

 

[knockingnoise]

Is the vent implementation different in a condo - I'm in the middle of a 33 floor building? Because I mentioned the vent to the contractor and he said it's already vented.

New news that might be helpful: I found out that if I put the cold water on as long as the hot water, then it makes the same knocking noise. So this is not just for hot water, it is for any water for at least 45 seconds the noise will start. Does that change anything?

 

[knockingnoise]

Oh well, he knows what it could be so it’s his choice. I’m not sure where it’s rubbing, no one does.

There’s no doubt the lavatories are not vented. If he ever filled them full and dumped them it might siphon otherwise with normal flow you’ll never make it siphon. If the pipe starts to clog in the future that will make it gurgle like an s-trap. It’s basically acting as a combination waste and vent does. But sure, It’s illegal.

If he wanted he could cut the wall open and remove the plug then pipe a studor vent under the cabinet.

I'm in the middle of a 33rd floor building - does that make a difference in venting? See my new comment about that it is all water, not just hot, if I leave the water on long enough. Thank you for your feedback and insights.

 

[Twowaxhack]

Is the vent implementation different in a condo - I'm in the middle of a 33 floor building? Because I mentioned the vent to the contractor and he said it's already vented.

New news that might be helpful: I found out that if I put the cold water on as long as the hot water, then it makes the same knocking noise. So this is not just for hot water, it is for any water for at least 45 seconds the noise will start. Does that change anything?

There is no vent for the lavatories. It’s illegal anywhere in America and is the form of an s-trap anywhere in the world.

Now, since it’s making the noise with hot and cold then it’s the hot,cold or drain or a combination of any of these three.

You could rule each one out by drawing water through the hot line and catch it in a bucket not allowing to enter the drain. Same for cold. Then you could pour water from another source into the drain to test the drain. This way you could determine the magnitude of the issue.

 

[knockingnoise]

There is no vent for the lavatories. It’s illegal anywhere in America and is the form of an s-trap anywhere in the world.

Now, since it’s making the noise with hot and cold then it’s the hot,cold or drain or a combination of any of these three.

You could rule each one out by drawing water through the hot line and catch it in a bucket not allowing to enter the drain. Same for cold. Then you could pour water from another source into the drain to test the drain. This way you could determine the magnitude of the issue.

Ok I'm going to try that. I'll get a hose, funnel, and bucket from HD tomorrow. Thank you for the suggestion.

Do you know if being in a high rise changes how stuff is vented because he said it's already vented when I asked.

 

[Twowaxhack]

Ok I'm going to try that. I'll get a hose, funnel, and bucket from HD tomorrow. Thank you for the suggestion.

Do you know if being in a high rise changes how stuff is vented because he said it's already vented when I asked.

It wouldn’t matter what floor your on or what building you’re in those lavatories are not vented. I’m not saying it won’t work but it’s not right and is more likely to cause problems like gurgling or even smells/sewer gases.