How much does it cost to upgrade from 6-inch to 8-inch trunk lines in a GTA home?

Upgrading trunk lines from 6-inch to 8-inch round duct in a GTA home typically costs $1,500 to $5,000 depending on the length of the trunk run, accessibility, and how many fittings and transitions are involved. This is one of the most impactful upgrades you can make to a duct system that is struggling with airflow, and the difference is dramatic — an 8-inch round duct carries roughly 78 percent more air than a 6-inch duct at the same velocity.

The reason this upgrade matters so much comes down to physics. A 6-inch round duct has a cross-sectional area of about 28 square inches, while an 8-inch duct has about 50 square inches. If your furnace or air conditioner is trying to push 400 CFM through a 6-inch trunk, the air velocity is extremely high, creating turbulence, noise, and excessive static pressure that forces the blower motor to work harder and wear out faster. Upgrading to 8-inch duct drops the velocity to a comfortable, quiet range and allows the system to deliver its rated capacity without fighting the ductwork.

Material costs for galvanized steel round duct in the GTA run $8 to $20 per linear foot for the duct itself, plus $15 to $50 per fitting for elbows, tees, reducers, and transitions. A typical trunk upgrade involves 20 to 40 linear feet of duct, 4 to 8 fittings, new register boots where branch runs connect, and potentially a new plenum transition at the furnace. Labour for a professional sheet metal contractor in the GTA runs $75 to $120 per hour, and a trunk line upgrade typically takes one to two days. For a straightforward basement installation where the trunk is exposed and accessible, budget $1,500 to $3,000. For situations where the trunk runs through finished ceilings, soffits, or between floors, the cost jumps to $3,000 to $5,000 because of the demolition and refinishing involved.

In post-war GTA homes across Scarborough, North York, and Etobicoke, undersized trunk lines are extremely common because the original ductwork was designed for smaller furnaces and heating-only operation. When air conditioning was added decades later, the existing trunks were never upsized, leaving the system permanently restricted. If your home has hot and cold spots, the furnace blower runs loudly, or registers furthest from the furnace barely produce airflow, an undersized trunk line is often the bottleneck.

Before committing to this upgrade, have a contractor measure the static pressure in your system with a manometer — readings above 0.5 inches of water column indicate excessive restriction, and upsizing the trunk is often the fix. A proper Manual D duct design calculation will confirm whether 8-inch is the right size or whether you need even larger trunk duct. Browse HVAC and ductwork contractors in your area through the Toronto Construction Network directory to get quotes from professionals who can assess your specific system.