How much does it cost to install ductwork for a second furnace in a large GTA home?

Installing a complete duct system for a second furnace in a large GTA home typically costs $8,000 to $20,000 for the ductwork alone, not including the furnace and air conditioner themselves. The total project cost including equipment, ductwork, electrical, gas piping, and permits generally runs $15,000 to $35,000 depending on the home's size, layout, and complexity.

Large GTA homes — particularly those over 3,000 square feet or homes with three or more storeys — often need two separate HVAC zones because a single furnace and duct system simply cannot deliver consistent comfort throughout the entire house. The physics are straightforward: hot air rises, and a single system trying to keep a basement at 21 degrees Celsius while also cooling a third-floor bedroom in July is fighting a losing battle. A second furnace with its own dedicated ductwork gives each zone independent temperature control.

The ductwork cost for the second system depends heavily on where the new furnace is located and how the home is configured. The most common setup in large GTA homes is a basement furnace serving the main floor and basement, with a second furnace in the attic serving the upper floors. Attic installations are more expensive because the ductwork runs through unconditioned space and requires R-8 minimum insulation on all supply and return runs to prevent condensation in summer and heat loss in winter. An attic duct system for upper floors typically costs $10,000 to $20,000 installed, while a basement system for main and lower floors runs $6,000 to $14,000 because the runs are shorter and more accessible.

The breakdown typically looks like this for a second-zone duct system: supply plenum fabrication and installation at $400 to $600, return plenum at $300 to $500, main trunk lines at $12 to $30 per linear foot for rectangular galvanized steel, branch runs at $8 to $20 per linear foot for round duct, register boots at $15 to $40 each, and supply and return grilles at $20 to $80 each. A typical upper-floor zone might need 8 to 12 supply runs and 4 to 6 return runs. Add $1,500 to $3,000 for duct insulation if running through the attic, plus $500 to $1,500 for mastic sealing of all joints.

Permit and trade costs add up quickly on a dual-furnace project. You'll need a building permit from your municipality ($400 to $800 in the City of Toronto), a TSSA-registered contractor for all gas piping and furnace connections, and an ESA-Licensed Electrical Contractor for the dedicated electrical circuit, thermostat wiring, and any associated fan or damper wiring. Gas piping from the meter to the new furnace location typically costs $800 to $2,500 depending on the run length.

In newer GTA subdivisions across Vaughan, Brampton, and Markham, many large homes were built with two furnaces from the start, but budget-conscious builders sometimes installed undersized ductwork for the second zone. If you're adding a second furnace to an existing home that was built with only one, the ductwork design should be based on proper Manual J load calculations and Manual D duct sizing — not guesswork. An experienced HVAC contractor will calculate the heating and cooling loads for each zone separately and design the ductwork accordingly.

Need help finding an HVAC contractor experienced with dual-furnace installations? Browse ductwork contractors in the Toronto Construction Network directory at torontoconstructionnetwork.com/directory?trade=hvac.