I recently had a new water heater installed. Since then there is air in the hot water pipes only, kitchen sink, bathroom sink and shower. Installer said it would go away, if not then it was a leak in the bladder of the well pump. Is there another cause? Too me, if it were the well pump then air would be in the cold water line as well.

I agree, it would be in the cold side as well, unless it goes to the heater first and tees from there. See if the temp setting is too high.

Gas or electric water heater? How long has it been installed? It's doubtful that a ruptured bladder in the pressure tank could interjectairinto the system. If the bladder was bad your pump would kick on every time you drew a glass of water. One place to start checking would be the heat trap nipples installed on the in let and out let of the water heater. Good luck, Tom