Help! Knocking noise in water lines

[grant0830]

Hello, we moved into our new home 5 months ago and have had a continuous knocking noise in the water pipes running from hot water heaters (one end of home) back to master bedroom (other end of home). The run from water heaters to master bed is long (90ft) and is set up on a hot water recirculating pump. We hear the knocking noise most in the evening when the home is quiet. The general contractor and plumber both has passes it off as expansion noise but if that were the case we shouldn't have noticed it when it was warm when we moved in 5 months ago?

I have attached two pictures of the water heater set up. I wish I could post a video of the movie I took of the noise. But if you click this link you can hear the noise (much louder in person than the video was able to capture) https://youtu.be/W5v5aXU5RW0

Please let us know your thoughts and thank you in advance!!

 

[Mr_David]

Does it stop if you turn off the recirculating pump?
Or if you isolate one of the heaters at it's cold supply?

Try using an automotive stethoscope and see if you can find where it is originating from.

 

[frodo]

one is a ck valve, 3/4, the other looks like a ball valve with out the handle?...idunno


it is a neat copper job, but, nothing is strapped, it should have been run on the wall using unistrut and clamps

the expanson tank is not supported the vibration ............

 

[grant0830]

I forgot to mention that we have HWG in and out lines coming off the geothermal desuperheater that are going into the hot water tanks. I recently learned that the HWG lines are not connected correctly per the geothermal instruction manual. Could this be causing any of the noise we are experiencing?

 

[KULTULZ]

Interesting...

The ill states- "Remove Any Back-flow Device From Dip Tube"

I wonder if that refers to a heat trap nipple that comes with most recent water heaters? Those traps can cause noise in the system as well as the check valve on the RECIRC SYSTEM.

What the ill doesn't say is that removal of these special nipples also voids the WH warranty...