IMC 803.8 and 803.9 shift the connector discussion from layout to material durability. The connector has to be built for the temperature, corrosion exposure, and size of the appliance it serves.
Plain-English Highlights
IMC 803.8 requires vent connectors to be metal, with minimum thicknesses of 0.0136 in. (No. 28 gage) for galvanized steel, 0.022 in. (No. 26 B&S gage) for copper, and 0.020 in. (No. 24 B&S gage) for aluminum - for plan review, connector material should be clearly identified on the drawings or specifications, since "metal vent connector" alone may not establish compliance if material and thickness are not defined. IMC 803.9 sets chimney connector thickness based on appliance heat classification and connector size.
For low-heat appliances, Table 803.9(1) increases minimum galvanized steel thickness as connector diameter increases: No. 26 gage at 5 in. and smaller, No. 24 gage above 5 to 10 in., No. 22 gage above 10 to 16 in., and No. 16 gage above 16 in. For medium- and high-heat appliances, Table 803.9(2) requires substantially heavier sheet steel: No. 16 gage up to 14 in. equivalent diameter, No. 14 gage at 15-16 in., No. 12 gage at 17-18 in., and No. 10 gage above 18 in. The hotter and larger the connector, the heavier the required construction.
Permit Proof Chain
Appliance heat classification confirmed (vent connector versus low-, medium-, or high-heat chimney connector) -> applicable material and thickness table selected (803.8 for vent connectors, Table 803.9(1) or 803.9(2) for chimney connectors) -> connector diameter checked against the correct thickness bracket -> connector material and gage documented on the drawings or specifications, not left as a generic "metal connector" note.
Where To Show It
M-001: connector material and construction criteria. M-501: connector material, thickness, joints, and supports. M-601: appliance type, heat classification, and connector size. Specifications: required material and gage. Details: larger or high-temperature connector construction.
Do
Identify connector material and thickness on the drawings or specifications. - Use Table 803.9(1) for low-heat chimney connectors. - Use Table 803.9(2) for medium- and high-heat chimney connectors. - Coordinate connector construction with the appliance's actual operating conditions, not an assumed default.
Do Not
Specify connector diameter without material thickness. - Assume one gage works for every connector size. - Apply vent connector minimums (IMC 803.8) to a higher-temperature chimney connector that requires Table 803.9. - Leave material selection to field judgment.
Masterbuild QA Lens
Connector material and thickness affect heat resistance, corrosion resistance, structural durability, joint integrity, long-term serviceability, and fire and combustion safety. Material that is too thin for the application can deform, burn through, corrode, fail at the joints, or deteriorate prematurely - and a correct diameter with the wrong gage is still a noncompliant connector.
Drawing / Submittal Check
When reviewing a connector schedule, check three things together: appliance heat classification, connector diameter, and metal thickness. Confirm the schedule cites the correct table (803.8, or 803.9(1) versus 803.9(2)) for the specific appliance and connector size shown.
Common Review Risk
A connector schedule specifies a diameter that matches the appliance outlet and a generic "galvanized steel" callout, but does not state the gage - for a medium- or high-heat appliance, that omission can mean the actual installed material is undersized by two or more gage steps against Table 803.9(2).
When To Escalate
Escalate any connector schedule missing a stated gage, any chimney connector serving a medium- or high-heat appliance shown with only vent connector (803.8) minimums, and any connector construction detail that cannot be traced to a specific table row for the appliance's actual heat classification and connector diameter.
Field Tip
When reviewing a connector schedule, check three things together: appliance heat classification, connector diameter, and metal thickness. A correct diameter with the wrong gage is still a noncompliant connector.