Can a water hammer arrestor/air chamber be installed in a horizontal position? Due to space limitations on an outside faucet install I can’t put the chamber in vertically. The horizontal chamber will be installed on a slight angle so the water will be sure to drain if the supply is shut off. The chamber is just a copper pipe with a cap.

The vertical pipe is a "poor man's" arrester. Air is lighter than water, so what you get is column of air which is compressable, so it arrests water hammer. If it's at an angle, it's useless. You don't need them at all taps.

Here is one that can be mounted at all angles Watts 150A - Watts 150A Water Hammer Arrestor - Water Hammer Arrestor (http://www.pexsupply.com/categories.asp?cID=608&brandid=)

Note it needs to be charged with air.

Thanks for the link. I don't install arrestors at all taps, but this valve is a quarter turn sillcock ball valve which can be shut off very fast so I need to prevent water hammering.

As long the air chamber is above the water line, I see no reason why it shouldn't work. I would wait and see what Tom or one of the other experienced plumbers has to say.

Thanks for the link. I don't install arrestors at all taps, but this valve is a quarter turn sillcock ball valve which can be shut off very fast so I need to prevent water hammering.

If that's your reason for installing a shock arrester and a horizontal installation is your only option, then spend the money and buy a spring loaded shock arrester (http://www.jupiterheating.com/sioux-chief/sioux-chief-water-hammer-arresters.html).