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Water Heater Knocking

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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.


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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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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.

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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.

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