It is not permissible to vent a naturally drafting gas tankless water heater such as the Bosch 520HN, 520PN, 1600H, 1600P, 330PN or the 1000P out and up the side wall of the home. This is not safe as the exposed venting running up the side of the home will be exposed to the elements and cold. When the temperature outside gets cold and makes the vent stack cold then the exhaust gases from the Bosch tankless water heater will not properly draft to the outside and can fill the home with carbon monoxide.
The installation manual states: "Horizontally venting to a vertically constructed vent stack along an outside wall of a building is not permissible". This is also against the National Fuel Gas Code (NFGC). If a Bosch tankless water heater needs to be vented Horizontally out the sidewall of the building then you would be required to use the Bosch AQ4 Powervent Kit. The AQ4 power vent kit is designed to activate when the tankless water heater comes on and blow the exhaust gases outside. The Bosch AQ4 power vent kit can be used with the Bosch 520HN, 520PN, 1600H and the 1600P. The Bosch 330PN and 1000P cannot be power-vented and must be vented vertically up through the roof of the building.
This rule for venting up the outside of the building also applies to the older Bosch Aquastar 125 series heaters and the ELM VP heaters. For older gas tankless water heaters that were installed and vented up the outside of the building they would want to have a chase built around them and box the venting in. Please always be aware of the minimum venting clearances for all venting material. Please see the before and after pictures below of a vent stack running up the outside of the home.
Incorrect Vent Stack
Corrected Vent Stack