I installed a gas on demand tankless water heater 3 years ago. The knocking hot water pipes has increased over the 3 years to the point I think they are going to break. What can I do to release the pressure? How do I secure the pipes? (I thought I did but it always finds a new spot) Do I replace my old pipes with bigger ones to go with the new water heater? Built in 1981. Thank you

What size supply and feed pipes do you have. Do you have an expansion tank? Does the vibration happen on all fixtures ot just when you use certain ones?

Something must trigger the hammering or knocking pipes or are they loose and need to be secured? Are both hot & cold are making the ruckus? Need more info.

Do you have water hammer when you shut off a hot water faucet? Or do the pipes vibrate for a while after you shut the water off? Back to you, Tom