# Flue: Building Code Definition

> How the IMC 2009 define flue, where the wording differs, and which chapters depend on the term.

- Canonical: https://harth.build/resources/glossary/flue/
- Last updated: 2026-09-07
- Data source: Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control.

## Key facts

| Fact | Value |
| --- | --- |
| Defined in | IMC 2009 (2009 editions transcribed in this corpus) |
| Primary source | IMC 2009 Definitions (model text, not a jurisdiction's adopted text) |
| Chapters using the term | 32 (counted across the codes hosted here) |
| Wording identical across codes | Single code (compare before citing one code's wording under another) |

## Direct answer

Flue is a defined term in the International Mechanical Code (IMC) 2009, in Definitions. The code's own words are: “A passageway within a chimney or vent through which gaseous combustion products pass.” No other I-Code in this corpus defines it, so the IMC 2009 definition is the one to work from. The term is used in 32 chapters of this corpus, most often in Fuel Gas (IRC 2009), Masonry (IBC 2009), Chimneys and Fireplaces (IRC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted. Record the jurisdiction, the adopted edition, the application date and any local amendment that rewrites the definition before you use it on a live project, because a term redefined by amendment changes every requirement that depends on it.

*Content and cited inputs reviewed by [Scott Barrington](https://nz.linkedin.com/in/barrington), Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.*

## What does this term mean?

**Flue** — IMC 2009, Definitions:

> A passageway within a chimney or vent through which gaseous combustion products pass.

![Labeled code-term definition diagram: Flue: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/flue/diagram-1344.webp)

*How the IMC 2009 define flue, where the wording differs, and which chapters depend on the term.*

## What must be verified before using this guidance?

**Flue** is defined once in this corpus, in the [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) chapter of the International Mechanical Code (IMC) 2009:

> A passageway within a chimney or vent through which gaseous combustion products pass.

## Where flue is used in the code

The term appears in 32 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.

| Chapter | Code | Mentions |
| --- | --- | --- |
| [Fuel Gas](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md) | IRC 2009 | 177 |
| [Masonry](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/masonry.md) | IBC 2009 | 105 |
| [Chimneys and Fireplaces](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/chimneys-and-fireplaces.md) | IRC 2009 | 97 |
| [Chimneys and Vents](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/chimneys-and-vents.md) | IMC 2009 | 63 |
| [Chimneys and Vents](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/chimneys-and-vents.md) | IRC 2009 | 54 |
| [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | IMC 2009 | 37 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 29 |
| [High-piled Combustible Storage](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/high-piled-combustible-storage.md) | IFC 2009 | 19 |
| [Flammable and Combustible Liquids](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/flammable-and-combustible-liquids.md) | IFC 2009 | 13 |
| [Aerosols](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/aerosols.md) | IFC 2009 | 12 |
| [Soil Absorption Systems](https://harth.build/resources/building-codes/us-states/new-hampshire/ipc-2009/soil-absorption-systems.md) | IPC 2009 | 12 |
| [Exhaust Systems](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/exhaust-systems.md) | IMC 2009 | 11 |

_20 further chapters mention the term._

### The term in use

How the code actually writes about flue, quoted from the 2009 text hosted here:

> Air for combustion, ventilation and dilution of flue gases for appliances installed in buildings shall be provided by application of one of the methods prescribed in Sections G2407.5 through G2407.9.

— [IRC 2009 §G2407.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md)

> The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall not be less than the cross-sectional area of the flue.

— [IBC 2009 §2111.6](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/masonry.md)

> Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion.

— [IRC 2009 §RI003.3.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/chimneys-and-fireplaces.md)


## The terms flue is built out of

The definition above leans on 3 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what flue covers:

- **[Combustion Products](https://harth.build/resources/glossary/combustion-products.md)** — IRC 2009: Constituents resulting from the combustion of a fuel with the oxygen of the air, including the inert gases, but excluding excess air.
- **[Combustion](https://harth.build/resources/glossary/combustion.md)** — IRC 2009: In the context of this code, refers to the rapid oxidation of fuel accompanied by the production of heat or heat and light.
- **[Chimney](https://harth.build/resources/glossary/chimney.md)** — IBC 2009: A primarily vertical enclosure containing one or more passageways for conveying flue gases to the outside atmosphere.

## Flue: what to open, and in what order

Every row is a document this term depends on. The [ICC code library](https://codes.iccsafe.org/codes) and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) cover the middle rows.

| Check | Where | Confirmed |
| --- | --- | --- |
| Read the definition in the adopted IMC | Definitions | |
| Re-check the requirement that uses it | Fuel Gas (IRC 2009) | |
| Re-check the requirement that uses it | Masonry (IBC 2009) | |
| Re-check the requirement that uses it | Chimneys and Fireplaces (IRC 2009) | |
| Re-check the requirement that uses it | Chimneys and Vents (IMC 2009) | |
| Confirm the dependent definition | Combustion Products | |
| Confirm the dependent definition | Combustion | |
| Confirm the dependent definition | Chimney | |
| Local amendment to any of the above | Jurisdiction's amendment list | |

**A research aid, not a code determination.** The authority having jurisdiction decides which edition and which amendments apply.

## Where can you explore related guidance?

- [Explore all glossary guidance](https://harth.build/resources/glossary.md)
- [Combustion Products: Building Code Definition](https://harth.build/resources/glossary/combustion-products.md)
- [Combustion: Building Code Definition](https://harth.build/resources/glossary/combustion.md)
- [Chimney: Building Code Definition](https://harth.build/resources/glossary/chimney.md)

## Sources and method

Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control.

### Source links

- [codes.iccsafe.org](https://codes.iccsafe.org/codes)

- [iccsafe.org](https://www.iccsafe.org/advocacy/code-adoption-maps/)

## Frequently asked questions

### What is the code definition of flue?

The International Mechanical Code (IMC) 2009 defines it in Definitions as: “A passageway within a chimney or vent through which gaseous combustion products pass.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally.

### Which parts of the code depend on flue?

In this corpus the term is used across 32 chapters, led by Fuel Gas in the IRC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own.

### What is the definition of a flue?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IMC 2009 states, in Definitions: “A passageway within a chimney or vent through which gaseous combustion products pass.” It is defined in IMC 2009. Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording.

### What is a flue in plumbing?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IMC 2009 states, in Definitions: “A passageway within a chimney or vent through which gaseous combustion products pass.” It is defined in IMC 2009. It is not the same as Plumbing, which the IRC 2009 defines separately as “For the purpose of this code, plumbing refers to those installations, repairs, maintenance and alterations regulated by Chapters 25 through 33.” Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording.

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