Hello there,

I have recently started hearing a knocking sound that seems to be originating from the wall/pipes near my water heater the moment I turn on the hot or warm water in any faucet in the house. The sound never occurs when only the cold water is turned on.

If the hot or warm water is turned on just slightly (low or medium flow) there is a continuous knocking until the water is shut off. However, when the hot or warm water is turned on all the way (high flow), there is only a single knock and then the sound stops.

To make things more interesting, there may also be a few times (4-5 times) throughout the day when I'll hear a single (but slightly softer) similar knock even when no water is turned on.

Tracing the knocking sound, I have noticed that it seems to be coming from the pipe that runs OUT from the mixing valve on my water heater - and not the hot or cold water pipes running IN to the mixing valve. On a side note: The original mixing valve was faulty and had to be replaced over 6 months ago. However, there had been no issues with noise following replacement until just recently.

I've already tried clearing out the air in the pipes several times but the noise is still there.
From what I've read,symptoms don't seem to indicate a water hammer issue either (no sound when shutting off water.
Its the middle of summer so I'm assuming the pipes are pretty warm to begin with, so I figured thermal expansion is probably not an issue either.
I'm at a loss for what could be causing this noise and figured I would ask for some help and cross my fingers.
Hopefully Ive provided enough information to help diagnose the issue.
Thanks in advance for any advice/help!

A common cause for these symptoms is calcium flaking off inside the water heater immediately above the burner and around the lining inside the tank. Have you drained the water heater at the hose bib drain valve? Turn off the gas to the burner and pilot. You should drain the tank and remove the drain valve and install a 3/4" nipple with a mips x hose connection so you can drain the water (you need to connect the hose and disconnect as necessary) and when the flakes start to block the drain take a long screw driver to break up the stoppage and more water and flakes will come out. Keep flushing until no more flakes of calcium appear. Put your drain valve back on the tank and fill it up slowly.

take note of where the mixing valve is set. now turn on the water slowly to make the constant noise. turn the mixer up and down a few times to see if that makes any difference. set the mixer back where it was.
it sounds like a hot pipe expanding and rubbing against wood but then again it doesn't sound like that ??
puzzling to say the least.

I'd place my bet on the mixing valve.

Thanks to all for their replies and advice! It is much appreciated.

I will attempt to do both suggestions:
1. drain the water heater to remove any calcium deposits and sediment
2. I will also play around with the mixing valve to see if that helps resolve the issue.

Thanks again!



Edited 1 times.

Just to give all an update about that knocking noise I was hearing when turning only the hot water on.
It wasn't the usual culprits: water hammer, thermal expansion, loose pipes, etc.
My plumber found the issue to be a faulty check valve (prevents hot water from reversing direction)
He removed it and the knocking noise is gone.
So I guess we can add a faulty check valve to the list of possible causes of knocking sounds when the hot water is turned on.

Hi! I just read about the knocking noise you had fixed by changing the valve! I am Brazilian and just bought a house in Houston and the noise I have in my bedroom eveytime we use the hot water is driving me crazy! Also in the living with the dishwasher.
Do you know where is the valve you mentioned in your comments? Thanks hou