water heaters in showers

Hello.i have a question about the height of the water heaters to be install in showers.ok i understand that it can be installed in zone 1 but my question is if the water heater can be installed everywhere in zone 1 with no limitation about the height.please help is there a certain height to be install in zone 1.please reply as soon as possible.and a 2nd question is:can a water heater be ipx4 without having any rubber seal around the cover.please help