The only outdoor Bosch tankless water heaters that are currently available are direct vent units and they require power to operate and have a special outdoor vent cap (BTOK) to protect them from the elements. These specific outdoor units also have built-in freeze protection that is good for ambient freezes for short periods of time.
Bosch does still offer three different naturally drafting gas tankless water heaters that do not require any power to operate and are intended for indoor use but can be installed outside under special situations. The Bosch 330PN, Bosch 520PN and the Bosch 520HN are all naturally drafting units that require no power to operate. These units come in either Natural Gas (NG) or Liquid Propane (LP).
These units can be mounted outside if they are well protected from the elements and are able to get plenty of combustion air when in operation (See Image below of Bosch 330PN LP installed outdoors on a tree). The 75,000 BTU Bosch Therm 330PN and the 117,000 BTU Bosch Therm 520PN and 520HN tankless water heaters will not be covered under any manufacturer's warranty if the unit freezes or if the heater gets damaged by rain. Bosch Thermotechnology may void any warranty for the heater at their discretion as the Bosch 330PN, 520PN and the 520HN are intended for indoor applications.
This photo shows the Bosch 330PN-LP installed in a protected case with a sloped roof. The case is open on the bottom to allow for the easy flow of make-up air.
This photo shows the 330PN-LP from the front view with a hinge on the custom-built case that allows for easy access to the tankless water heater's controls.