IMC 801.6 sounds like a formality - the vent has to be at least as big as the appliance connection - but it's a recurring plan-review catch because reviewers check it by cross-sectional area, not by matching pipe sizes on a drawing.
Plain-English Highlights
For a single appliance, the chimney or vent must have a minimum area equal to the appliance connection, unless Chapter 8 allows otherwise or an approved engineered system applies. The comparison is area-based, so a round, oval, rectangular, lined, or existing chimney configuration has to be checked by equivalent area wherever the shape changes - not assumed compliant because a dimension looks close.
Permit Proof Chain
Appliance flue collar size and area -> vent connector size -> chimney or vent size at every shape transition -> equivalent-area verification where round meets rectangular or lined -> engineered basis if undersized relative to the connection.
Where To Show It
M-001: vent sizing notes and design criteria. M-101: appliance location and vent routing. M-501: vent connector, chimney, liner, and termination details. M-601: appliance schedule with flue collar and vent size. A/LS sheets: shafts, roof penetrations, rated assemblies. P sheets: boiler, water heater, oil, and fuel coordination.
**Do** - List the appliance connection size in the schedule, not just the vent size. - Match vent size to the appliance listing and manufacturer instructions. - Verify equivalent area anywhere the vent shape changes. - Provide engineered support for any reduction below the appliance connection size.
**Do Not** - Reduce the vent size without a reviewable basis. - Assume an existing chimney liner is automatically large enough - or small enough. - Show one vent size on the plan and a different one in the schedule. - Leave vent sizing to the installer with no design criteria on the drawings.
Masterbuild QA Lens
Treat this as an area calculation, not a diameter comparison. A liner that looks 'about the same size' as the flue collar can still fail 801.6 once the actual cross-sectional areas are compared.
Drawing / Submittal Check
Pull the flue collar size directly from the appliance schedule and confirm it against the vent connector and chimney/liner size shown in the detail - all three numbers should be traceable to each other.
Common Review Risk
Plan and schedule mismatches are the most common flag here: the schedule lists one flue collar size, the detail shows a different vent size, and neither matches what's labeled on the plan.
When To Escalate
Escalate any existing chimney or liner reuse where the as-built size isn't field-verified, and any vent sized smaller than the appliance connection without engineered support.
Field Tip
Check the appliance schedule first. If the flue collar size is missing, you can't verify minimum sizing - the design is incomplete before you even get to the vent detail.