What causes banging or popping noises in my Toronto home ductwork when the furnace kicks on?

Banging or popping noises when your furnace starts up are almost always caused by thermal expansion of the sheet metal ductwork, and while they are usually harmless, they can indicate design issues worth addressing. This is particularly common in Toronto homes during winter, when the temperature differential between cold ducts and hot supply air is at its greatest.

When your furnace fires, it pushes heated air at 50 to 70 degrees Celsius into ductwork that may have cooled to 15 to 20 degrees (or even colder if ducts run through an unheated attic or crawlspace). Sheet metal expands when heated, and large flat sections of rectangular ductwork flex outward under the pressure of expanding air, then snap back — producing that distinctive bang or pop. This is called oil canning, and it happens because the flat panels of rectangular duct lack stiffness. Round and oval ducts rarely have this problem because their curved shape is inherently more rigid.

The severity of oil canning depends on several factors. Larger rectangular ducts with unsupported flat panels are the worst offenders — a 20-by-10-inch rectangular trunk line with a long unsupported span will pop loudly every time the furnace cycles. Thinner gauge metal (28 or 30 gauge) flexes more easily than heavier 26-gauge duct. And ducts that were installed without cross-breaks — the small dimpled stiffening lines pressed into flat duct panels during fabrication — are more prone to flexing. In many older GTA homes, particularly post-war construction across North York and Scarborough, the original ductwork was fabricated from thinner gauge metal without cross-breaks, making expansion noise a persistent annoyance.

Several fixes can reduce or eliminate the noise. A sheet metal contractor can add cross-breaks or stiffening beads to the flat panels of existing rectangular ductwork, increasing rigidity so the metal cannot flex enough to pop. This is a relatively inexpensive fix at $50 to $150 per section. For severely affected trunk lines, adding a standing seam or rolled stiffener along the length of the duct provides even more rigidity. Another approach is installing a flexible canvas connector (sometimes called a vibration isolator) between the furnace plenum and the first section of trunk duct. This absorbs the initial pressure pulse and prevents it from transmitting through the entire duct system. These connectors cost $50 to $150 installed and also reduce the transmission of furnace blower vibration into the ductwork.

If the noise is more of a rattling or vibrating sound rather than a clean pop, check that all duct sections are properly secured with sheet metal screws and that hangers are tight. Loose connections rattle when airflow starts and stops. Also check for ducts resting against floor joists, pipes, or other building components — vibration transfers through contact points and amplifies noise.

While oil canning is not a safety concern, persistent loud banging can indicate that your ductwork is undersized for the furnace blower output, creating excessive static pressure. If the noises are new or have gotten worse after a furnace replacement, have a contractor check that the new furnace's airflow is compatible with the existing duct system.