The Ontario Building Code | Return-Air System
6.2.3.20. Return-Air System
(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 noncombustiblematerial.
(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 furnaceshaving 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="http://thehandyforce.com/windows/" title ="Window installer in Toronto">openings in the furnaceor 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.
6.2.4. Air Ducts for Low Capacity Systems
In Plain Language
The return-air system must be designed to handle the entire air supply volume, and where a return duct is exposed to radiation from a heat exchanger or other radiating equipment, additional protective requirements apply.
This is a plain-language summary of the 2012/2017 Ontario Building Code text above, for general understanding only and not a substitute for the Code. Always confirm current requirements against the 2024 edition (O.Reg. 163/24) or a qualified professional before construction.