- Air side up in this mounting - 
- Some places say upside down matters - . Supplyhouse says it doesn't matter .
- IT seems it does matter - process connection at top, so that trapped air bubbles can float up and out. . If it's on the side, air can get trapped inside. This makes sense to me. Also - can corrode if it's on the side, or put more stress on the bladder .
- Depends on tank. Definitely want to watch the air escape - 
- Summary: always think of how the air escapes a pressure or expansion tank. This is for hot water system expansion tanks and RO tanks. But, for water storage (cold) tanks - this one indicates that orientation doesn't matter - could be because the cold storage tanks are built more heavy? Water storage tanks have more air - 2/3 air - . Expansion tanks seem to have 1/2 air.
- RO tank may work as an expansion tank 
- Pressure tanks vs expansion tanks 
- $30 2 gal 3/4" 
- $40 Menards 2 gal 3/4"