Hello-

I am planning to replace a 40 gal water heater with a 50 gal. There is currently no expansion tank. I've replaced the PRV this past summer due to high pressure after we moved in. We still hear lots of banging and very loud water flow when using water.
What size expansion tank is recommended. I bought a Watts PLT-12 (4.5 gal) tank, but it seems big. Is that ok to use, or should I get the 2 gal model?
Also, is it ok to install it horizontal? The instructions say it is ok, but Ive read where others say not to. There is not much room near the water heater and installing horizontal with metal straps for support would be the easiest option. Im not sure I can do it vertically, unless I snake the copper lines all over.
Thanks!

The manufacturers instructions trump all others. If you have that, use it. It can be installed anywhere, between the PRV and the water heater. I'd use a good quality ball valve in line, for future replacement, if necessary. Use a wire-tie on the handle to ensure it stays open.

Still can't wrap my head around a Prv on a domestic water supply .. What pressure does that water hit the house at

Depending on the area, it ranges from 40 to 150 psi. In "hilly" areas the pressure changes depending on whether the house is above or below the main.