Combustion Air, Venting, and Clearances for Fuel-Fired Appliances
Fuel-fired appliances need combustion air, proper venting, and clearance from combustible materials. In Ontario, natural gas and propane appliances are installed to CSA B149.1, oil appliances to CSA B139, and all appliances to their certified listing and manufacturer's instructions. Combustion air supplies oxygen for burning and dilution; a conventional appliance in a confined space may need permanent openings or ducted outdoor air. Direct-vent appliances take combustion air from outdoors and are less affected by indoor pressure, but their termination clearances still matter.
Venting must remove products of combustion safely. Chimneys, vents, and connectors need correct size, slope, support, and termination. A common failure is a replacement furnace connected to an existing chimney that is not suitable for the new appliance's vent category or efficiency. Negative pressure from exhaust fans, HRVs, or clothes dryers can backdraft a naturally drafted appliance. Clearances to combustibles are set by the listing and the installation code; reducing a clearance without a listed guard or approved method is a violation.
Traps: using indoor air for a direct-vent appliance; ignoring the 'confined space' threshold; terminating a vent under a deck or near a window; and assuming a chimney liner is optional.
Worked example: a 100,000 Btu/h conventional furnace in a small room. A rule of thumb is 50 cubic feet of space per 1,000 Btu/h before a space is considered confined; 100,000 Btu/h suggests about 5,000 cubic feet. A 640-cubic-foot room is confined, so provide combustion air by an approved method or use a direct-vent appliance. Always verify with B149.1.