Bosch currently has two 1600 series tankless water heaters in the lineup: the Bosch AquaStar 1600H and the Bosch AquaStar 1600P.
Both of these Bosch tankless water heaters deliver 4.3 GPM at a 45 degree F temperature rise. They both have a gas control that modulates the burner input in response to flow. The BTU input on both water heaters is between 30,735 and 117,000 BTU input.
They both are designed to be vented vertically using 5" diameter Type B Galvanized venting. Globaltowne sells the H/P VVT Vertical Vent Kit to help with this vertical venting. Alternatively, both can be vented through the side wall with the AQ4 Power Vent.
The main difference between the 1600H and the 1600P is the ignition system. The Bosch AquaStar 1600H uses Bosch's patented Hydro Ignition system. This iginition system has no pilot light and no electrical requirement. The Bosch 1600H comes in two models:
Both are very solid heaters with great reputations!