Water Heater Knocking

[Mike Lamb]

This is a one bathroom ranch on a slab. This knocking sounds like it is coming from the water heater and only happens when I flush the toilet. It has the same cadence as the ticking on a water meter which is nearby but still it only happens when I flush the toilet and not when I run the other fixtures.

https://www.youtube.com/watch?v=WPntL4XrPBI

[Nolan Kienitz]

Input line to the toilet is possibly from the "hot water circuit".

Was the water in the tank warm?

I had a new home construction once that had 3/4 of all hot/cold lines reversed and I could have steamed hot dogs in the toilet tanks.

[Jim Katen]

The little ball in one of the heat traps is hopping up & down.

[Marc]

These?

Marc

[Mike Lamb]

I never knew there is a little ball in there. I thought it was just a cross cut diaphragm at the opening. And it probably has a hot water feed. I never felt the tank.

[Brandon Whitmore]

So why does the noise only occur when a toilet is flushed, but not when water is run elsewhere?

[mlparham]

The little ball in one of the heat traps is hopping up & down.

What is a residential heat trap?

[kurt]

It's a dilectric nipple that's got a little marble in it to stop convective flow out of the water heater; it's supposed to reduce heat loss and therefore improve efficiency.

[Eric B]

It's a dilectric nipple that's got a little marble in it to stop convective flow out of the water heater; it's supposed to reduce heat loss and therefore improve efficiency.

Huh? Got a diagram of that?

[kurt]

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There's one's with diaphragms too. Google "heat trap diagrams"; there's about a thousand pics.

[Mike Lamb]

It's a dilectric nipple that's got a little marble in it to stop convective flow out of the water heater; it's supposed to reduce heat loss and therefore improve efficiency.

Huh? Got a diagram of that?

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[Jim Katen]

So why does the noise only occur when a toilet is flushed, but not when water is run elsewhere?

The alignment of the stars. The toilet fill valve pulls just enough water to affect the pressure at the heat trap. You probably could reproduce the sound if you messed about with other fixtures long enough.

[Brandon Whitmore]

Thanks Jim,

I had this same issue at my house, and the knocking only occurred when I flushed the hall bathroom toilet.

[Scottpat]

Humm, I have been needing a idea for some new plumbing questions on the NHIE! We have a question writing session soon.

[John Kogel]

Humm, I have been needing a idea for some new plumbing questions on the NHIE! We have a question writing session soon.

That IMO is a pretty tough question to be putting on an exam, considering that many of us have never and may never encounter one of those gizmos in our entire careers.

However, if it is one of those choose the best answer out of 4 questions, some of us could guess or deduce the right answer. Maybe that makes it a good exam question.