What are the clearance requirements for ductwork near combustible materials in Ontario?

The Ontario Building Code and TSSA regulations specify minimum clearance distances between ductwork and combustible materials, with requirements varying based on the type of duct, the temperature of air it carries, and whether the duct is connected to combustion equipment. Getting these clearances right is critical for fire safety, and violations are among the most common deficiencies flagged during building inspections in the GTA.

For standard HVAC supply and return ductwork carrying conditioned air at normal temperatures (heating supply air typically at 50 to 60 degrees Celsius, cooling supply at 12 to 15 degrees Celsius), the OBC requires a minimum one-inch clearance from combustible materials such as wood framing, insulation, and stored items. In practice, most residential installations in Toronto maintain this clearance naturally because ductwork is hung from floor joists with metal strapping or supported by sheet metal hangers, providing adequate separation. However, in tight spaces — such as Toronto's older homes where joists, plumbing, electrical, and ductwork compete for limited ceiling space in basements — maintaining this clearance requires careful planning and sometimes creative routing.

Combustion venting (flue pipes from gas furnaces, water heaters, and fireplaces) has much stricter clearance requirements because these pipes carry exhaust gases at significantly higher temperatures. Single-wall metal flue pipe (B-vent connector) requires a minimum six-inch clearance from combustible materials. Type B vent (double-wall) requires a minimum one-inch clearance from combustibles. These clearances are governed by both the OBC and TSSA regulations, and all combustion venting must be installed by a TSSA-registered contractor. Never allow standard HVAC ductwork to contact or compress against combustion vent pipes — the heat transfer can char and eventually ignite wood framing.

Where GTA homeowners run into problems is typically in basement renovations. When finishing a Toronto basement with low ceiling height (many older homes have 6.5 to 7-foot basement ceilings), the temptation is to build soffits and bulkheads tightly around ductwork to maximize headroom. If combustible materials like wood framing or drywall are placed directly against ductwork without required clearances, it creates a fire code violation. For supply ducts carrying heated air, ensure framing maintains at least one inch of clearance. For flue pipes, the clearance must be maintained with fire-stop collars or non-combustible shields where pipes pass through floors, walls, or ceilings.

Fire dampers, discussed elsewhere, are a related requirement — wherever ductwork passes through a fire-rated assembly, a rated fire damper must be installed regardless of clearance. And wherever ductwork runs adjacent to (but not through) a fire-rated assembly, clearance and firestopping requirements still apply to maintain the integrity of the fire separation.

During a renovation, if you discover that existing ductwork in your Toronto home does not meet current clearance requirements — which is common in pre-war and post-war homes — you are generally not required to bring grandfathered conditions up to current code unless you are doing substantial renovations to that area. However, correcting clearance violations during a renovation is strongly recommended and is relatively inexpensive compared to the fire risk. An experienced ductwork contractor can reroute or relocate ducts to maintain proper clearances, typically for $300 to $1,500 per section depending on complexity and accessibility.