/ Field knowledge · IMC 801.6
Matching the pipe size is not matching the area.
The chimney or vent has to be at least as big as the appliance connection — checked by cross-sectional area, not by matching pipe sizes on a drawing. A liner that looks “about the same size” as the flue collar can still fail.
Educational 2024 International Mechanical Code (IMC) 801.6 from Daily Code Talk. Confirm the adopted edition. Distinct from existing chimney reuse (801.18). Chapter 8 exceptions and approved engineered systems can allow otherwise. This is not a vent-size calculator.
- For a single appliance, minimum area equals the appliance connection unless Chapter 8 allows otherwise or an approved engineered system applies.
- Round, oval, rectangular, lined, or existing chimneys have to be checked by equivalent area wherever the shape changes — not assumed compliant because a dimension looks close.
- List the flue collar on the schedule, not only the vent size. Plan, schedule, and detail have to agree. An existing liner is not automatically large enough — or small enough.
On the set: collar size → connector size → chimney or vent size at every shape transition → equivalent-area check where round meets rectangular or lined. Any reduction below the connection needs a reviewable engineered basis.
Three numbers that do not match
The schedule lists one flue-collar size, the detail shows a different vent size, and neither matches the plan. If the collar size is missing from the schedule, minimum sizing cannot be verified — the design is incomplete before you get to the vent detail.
Source: Daily Code Talk — IMC 801.6 · tool: Area, not diameter · existing: 801.18 · positive pressure: 801.9.
What to send
Appliance cut sheet (flue-collar size), connector and chimney/liner sizes shown, and whether this is new or an existing chimney reuse.
If the liner looks “about the same size”
Send the collar size and the liner size. Masterbuild will say whether 801.6 is an area problem — or 801.18 still has no passageway evidence.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. This page is 2024 IMC 801.6 education from Daily Code Talk — not a vent-size calculation. This page is not legal advice.