The Noritz NCC1991 features a thermal efficiency of more than 94% and saves money on both installation and ongoing operations. Using a special Quick Connect cable, two NCC199-DV units can be coupled to provide an input of nearly 400,000 Btu with a turndown ratio of more than 36:1. Designed for a wide variety of commercial applications, both new and replacement, the NCC1991 contains a secondary stainless-steel heat exchanger that preheats incoming cold water with heat from escaping flue gases.