All of the Bosch gas tankless water heaters that are naturally drafting such as the 1000P, 1600P and the 1600H take thier combustion air that is needed to fire the heater from the location they are installed in. These heaters can never be installed in a bedroom or bathroom where the door is normally kept closed. This is against the National Fuel Gas Code and is very dangerous as the heater would deplete all of the oxygen in the room and will cause serious illness or death.
Per the National Fuel Gas Code manual, the only exception to having a naturally drafting gas tankless water heater installed in a bedroom or bathroom is if it is installed in a closet that has self closing hinges and that is sealed when it closes as to not take its combustion air from the bedroom or bathroom. The gas tankless water heater in the closet would then have to be able to get its combustion air from another source such as from the main part of the home, the attic area or from the outside. The amount of combustion air required for the gas tankless heater would be determined per the installation manual and vents to other sources of fresh air would have to be sized accordingly.
Direct vent gas tankless water heaters such as the Bosch 2400ES/2700ES are sealed combustion units and have the ability to take the intake air from the outside. These direct vent heaters are the only ones approved to be installed in a bedroom or bathroom. These heaters have a twin pipe system for the exhaust/intake air to the outside. If these heaters are to be installed in a bedroom or bathroom then the intake air for the heater must come from the outside.