The Bosch 2700ES and Bosch 2400ES tankless water heaters can be installed indoors and vented out the side-wall using the AQ3ES Vent Kit or up through the roof using the ES VVT Vent Kit.
These two high efficiency Bosch Tankless Water Heaters can also be installed outdoors using the AQOK Vent Kit.
One question we often get asked by customers who want to vent out through the side wall is whether or not they can use additional 3" stainless steel venting parts on the outside of their homes. Most often these customers want to vent out the side wall and then use a 3" 90 degree elbow to then add vertical vent pipe to bring the venting termination point higher up the side of their home.
Gas codes do not allow vent pipe to run along the outside of the house in cold climates due to condensation problems and interference with draft. The heaters are designed for sidewall termination only (not sidewall penetration with vent continuing outside).
The likely outcome of installing in cold climates with additional outdoor piping will be significant condensation leading to heater damage, particularly to the gas valve. Theoretically, adding extra condensate drains would help reduce the chances for this type of damage, but Bosch does not recommend rigging up extra drains for what would be an improper install in the first place.
You may consider building an insulated exterior chase to run vertical vent pipe through. Consult your local Gas Code to see if this is allowed. When heaters are not installed according to Gas Codes then they are generally not covered by a warranty.
Here is the proper way to exhaust the Bosch AquaStar 2700ES or 2400ES tankless water heaters:
The following diagram shows both the exhaust and intake air (combustion air):