N Definitions

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Nat + Nat

The Nat + Nat column in the tables lists maximum combined input capacity, in Btu/ hour, of two or more draft hood gas appliances connected to a common vent.

Nat Max

The Nat Max column in the tables lists the maximum input capacity, in Btu/hour, of one draft hood gas appliance connected directly a vent or to a vent connector. Example: the maximum Btu/ hour input capacity of a draft hood water heater connected to a 3 inches diameter Type B vent connector with a vent height of 10 feet and a lateral length of 5 feet is 40,000 Btu/hour. See Table G2428.2-1.


A niche is an opening in a wall into which a light is installed. A niche is usually created by a forming shell.