Just finished replacing my water heater. The old heater
came with galvanized nipples with a plastic insert sleeve.
The outlet nipple had a plastic ball which looked like a
check valve but didn't seal in either direction. (The inlet
nipple is rusted in.) A label on the tank said they were
The store that sold me the new tank sold me two hollow
(no inserts) brass nipples.
But now I'm wondering "what's the difference". What did
the plastic sleeves do?