whole house water hammer arrestor

Red,

The best place to install them is next to all the quick acting valves in the home, ie - washing machine, fridge line, etc... You could also install some just after the HWT (or HWH) as well. As for orientation, I prefer upright but I believe that they are fine in any orientation. Check the manufacturers directions when installing.

PS - The valve for your toilet is open all the way, right?

What is your pressure like into the house? Is it regulated and reduced or are you getting almost the full amount from the City supply line?

Also, strap down your piping where possible.

Getting full pressure, I'm not sure what it's like, but I recall someone measuring at my parent's house (they had similar problems but worse) and it was around 100psi. I'm closer to the tower so it's probably high up there. Come to think of it, how would I measure that without adding an inlien guage? I'm curious now, maybe I'm way off.

I will look at installing one for the toilet then, and yeah the valve is all the way (it's a quarter turn valve) The cold water line going to the water tank is also not strapped as it's running an inch below the joists, I'll fix that by adding blocks and strapping it to the joist. Any reason not to use pex clamps on copper? I have some already so may just use those.

You can buy or hack together a water pressure gauge you can hook to a hose bib for under $20 I should think. Screw it on and crank the thing open all the way. You might find one you can put on a kitchen faucet or laundry sink tap also.

100psi is a lot! I had 90-95 in the California house and it sounded like the pipes would come out of the walls at times when valves closed. That much pressure, as mentioned by someone else in another post, is really threatening irrigation valves, those on your washer and dishwasher, bath fixtures and so forth. To say nothing of your water main to the house.

You might think about a pressure reducer/regulator before the local anti-hammer devices? They won't do anything to alleviate excess pressure in your plumbing lines.