All of the new and some of the older Bosch gas tankless water heaters are all sealed combustion units. This means the unit is sealed in the combustion chamber and thus the unit will have a twin pipe system for the exhaust and intake air. The sealed combustion creates a positive pressure and keeps any cold air from entering and causing damage to the heater. The sealed combustion units are also sealed and require in most situations that the intake air be taken from outside the home so as to not be shortened by inadequate combustion air. The Bosch sealed combustion units must "NEVER" be operated with the front cover off the unit as this will create a negative pressure and will allow cold air being drawn down the exhaust pipe and could cause major damage and freezing in some areas to the heat exchanger.
Below is an image of a Bosch sealed combustion unit being vented with a twin pipe system horizontally out the sidewall. This would be the same for a vertical installation through the roof. When terminating through the sidewall or through the roof of a home always remember to keep at least 3' apart between the intake air vent and the exhaust air vent so there will be no cross contamination.
These are the Bosch gas tankless water heaters with sealed combustion. 250SX/250SXO/635ES/635ESO/2400E/2400ES/2700ES/Evolution500/Integra500/940ES/C1210ES/C1210ESC/ProTL175/ProTL199/ProTLC175/ProTLC199/T9800SE 160/T9800SE 199/T9900SE 160/T9900SE 199/T9900iSE 199
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