Hi all,

In a major renovation, we replaced most of our old plumbing, and, added a new floor with new plumbing.

The water hammer took about 2 weeks to show up. It only occurs when faucets are shut off quickly.
Our plumber [after ripping open some walls to look for loose pipes (!?!)] installed a pressure reducing valve where the water enters the house. He claims it took the pressure from ~130 to ~60psi. No help.

Then he added a large hammer arrestor just inside that valve. Nope! Then he installed smaller hammer arrestors under each sink & behind the washer. Still no luck...

I read that ceramic-valve faucets can cause water hammer due to the abrupt shut off. Interestingly, our 2 "old" upstairs ceramic faucets now cause water hammer, but the same faucets didn't when we had only a single story house.

We have 2 Takagi tankless water heaters mounted outside the house. The plumber thinks they must be responsible for the hammer.

Any other suggestions?
Thanks,
Pableau

If the piping that you have in the walls is not sized correctly, (too small), the water velocity is increased which in turn can cause hammer with quick closing fixtures; dishwashers, washing machines, ceramic stem faucets, solenoid valves, toilet fill valves, etc.

If you close the ceramic faucets slowly, the hammer will probably not be a problem but I'm sure that this is not the answer that you're looking for.

e-plumber
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This slow drip will waste 7+ gallons of water per day.

Long pipe runs can tend to cause water hammer. you said you just piped in an upstairs. sometimes an offset installed to break up the water flow can solve the problem.