For the first time Noritz is offering true commercial grade components and materials inside and out. A re-engineered heat exchanger and industry first dual flame burner boost the heater's power, while lowering minimum flow requirements. To help ensure the longevity of its performance, we have enclosed the heater’s commercial grade components with a corrosion-free stainless steel exterior casing, giving the look and feel of other professional grade equipment. The NC250 series offers direct vent (DV) units that are ideal for commercial businesses such as restaurants and hair salons that have contaminants in the air. The NC250-SV-ASME is for indoor/outdoor use. PVC CANNOT be used for venting. The NC250-DV-ASME is for indoor use only.