They can be horizontal.
However, you can also mount them on the washer and keep them vertical is you want.
I'm guessing this is a silly question, but..... Does the tube of a water arrester have to be vertical?
Since the arrester tube has a piston that separates the water from the contained gas, I thought there could be a chance that the tube could be horizontal.
I want to put arresters on the hot and cold lines to the clothes washer but the current flexible lines connect to shut-off valves with outlets that go straight down. So connecting something like the Sioux Chief Min-Restor at the shut-off valves would have the tube in the horizontal direction.
I wonder why I didn't think of mounting them on the washer? The connections on the washer are above each other so I could mount the top arrester vertically and the bottom one at a slight angle to miss the top one!
Ideally, the arrester is installed as close to the quick-acting valve as possible...and, with an engineered on, orientation is irrelelvant.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
Point the top one up and the lower one down. They will work effectively in either direction.