What ductwork changes are needed when converting a Toronto home from oil to gas heating?

Converting from oil to gas heating typically requires significant ductwork modifications because oil furnaces and gas furnaces have different airflow characteristics, combustion air requirements, and venting needs. Most oil-to-gas conversions in Toronto homes built before 1980 need ductwork upgrades to handle the higher static pressure and different airflow patterns of modern gas furnaces.

Airflow and Static Pressure Changes

Oil furnaces, especially older models common in Toronto's post-war housing stock, typically operated at lower static pressures with larger, more open ductwork designed for natural convection heating. Modern gas furnaces use variable-speed or multi-stage blowers that operate at higher static pressures and deliver more consistent airflow. Your existing ductwork may be oversized for the new gas furnace's airflow characteristics, or conversely, undersized if you're upgrading to a higher-efficiency system with different CFM requirements.

The most critical assessment is whether your current duct sizing matches the new furnace's airflow requirements. A Manual D duct calculation should be performed based on your home's heating load (Manual J) and the new gas furnace specifications. Many Toronto homes with original oil heating have galvanized steel trunk lines that are appropriately sized, but branch ducts and return air systems often need modification.

Return Air System Modifications

Oil heating systems in older Toronto homes frequently have inadequate return air systems — often just a single return grille on the main floor. Gas furnaces require proper return air to operate efficiently and safely. Insufficient return air creates negative pressure that can cause backdrafting of the gas furnace or water heater, pulling combustion gases into the living space — a serious carbon monoxide risk.

Adding return air ducts to bedrooms and the basement is one of the most important upgrades during an oil-to-gas conversion. This is especially critical in Toronto's older housing stock where bedrooms were often heated by gravity convection without dedicated return paths. Modern gas heating requires balanced airflow throughout the home.

Combustion Air and Venting Requirements

Gas furnaces have completely different combustion air and venting requirements than oil furnaces. The old oil chimney cannot be used for a gas furnace — gas appliances require either direct vent (through an exterior wall) or proper gas venting through a lined chimney. This change often allows for better ductwork routing since you're no longer constrained by the large oil chimney location.

TSSA (Technical Standards and Safety Authority) regulates all gas appliance installations in Ontario. Any work involving gas connections, venting, or combustion air must be performed by a TSSA-registered contractor. The gas utility (Enbridge in most of the GTA) will also need to run a gas line to your home if one doesn't exist.

Ductwork Sealing and Insulation

Oil-to-gas conversions present an excellent opportunity to seal and insulate your ductwork properly. Many Toronto homes with oil heating have never had their ducts professionally sealed because the old oil furnace was oversized enough to compensate for duct leakage. Modern gas furnaces are right-sized for efficiency, making duct sealing critical for proper performance.

All ductwork in unconditioned spaces (attics, crawlspaces, unheated basements) should be sealed with mastic and wrapped with R-8 insulation minimum. This is especially important in Toronto's climate where temperature swings cause significant condensation on uninsulated ducts.

Typical Conversion Costs and Timeline

A complete oil-to-gas conversion including ductwork modifications typically costs $8,000-$18,000 in the GTA, with $2,000-$6,000 of that being ductwork-related changes. The project requires coordination between your TSSA-registered heating contractor, the gas utility for service connection, and potentially an ESA-Licensed Electrical Contractor for any electrical modifications.

Building permits are required for new gas appliance installations and significant ductwork modifications. Apply through your local municipality — City of Toronto permits can be obtained through 311 or toronto.ca.

When to Hire Professionals

All gas-related work must be done by TSSA-registered contractors — this is Ontario law. The ductwork assessment and modifications should be performed by an experienced sheet metal contractor who can properly size ducts for your new gas furnace. This is not a DIY project due to the combustion safety implications and permit requirements.

Need help finding a ductwork contractor for your oil-to-gas conversion? Toronto Ductwork can match you with experienced professionals who specialize in heating system conversions and understand the specific requirements of Toronto's older housing stock.