(1) The return-air system shall be designed to handle the entire air supply.
(2) Where any part of a return duct will be exposed to radiation from the heat exchanger or other radiating part within the furnace, such part of a return duct directly above or within 600 mm of the outside furnace casing shall be noncombustible.
(3) Return ducts serving solid fuel-fired furnaces shall be constructed of noncombustible material.
(4) Where combustible return ducts are permitted, they shall be lined with noncombustible material below floor registers, at the bottom of vertical ducts and under furnaces having a bottom return.
(5) The return-air system shall be designed so that the negative pressure from the circulating fan cannot affect the furnace combustion air supply nor draw combustion products from joints or openings in the furnace or flue pipe.
(6) Return-air inlets shall not be installed in an enclosed room or crawl space that provides combustion air to a fuel-fired appliance.