When I turn on the hot water any where in the house we get one loud knock. At the hwt the cold supply side gets hot about 6 inches up the line. When it is shut off we don't get any knocks..
Annoyed, ticked & po

Hi ATP

The cold water pipe being hot 6-12" up the cold water inlet to the hot water heater is somewhat normal, so that isn't the issue.

My other thoughts are:

1) If you have an EXPANSION TANK on the hot water system the expansion tank may be full or the diaphragm may have burst. Here, emptying or replacement of the expansion tank is required to resolve the issue, respectively.

2) You could have a hot water HEAT TRAP NIPPLE at the hot water outlet of the water heater and the ball/float in the nipple is causing the issue here....could be?!? Here, the hot water heater outlet nipple would be removed and replaced with a new dielectric nipple...pipe in an actual heat trap if you want the benefits of a heat trap without the heat trap nipple...;)

3) If this is a newer issue (new noise) then perhaps your home has old fasioned AIR CHAMBERS (acts as shock absorbers) behind the wall that have filled with water. Here, you shut off the hot water supply to the house, open all faucets and let the water drain from the hot water system for 15 minutes or so. Turn all the faucets/valves back off and turn on the hot water supply...should "RECHARGE" the air chambers if you have them.


That's my thoughts... glad to discuss more if you want, OK?

Mark