# Flue Gases: Building Code Definition

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

- Canonical: https://harth.build/resources/glossary/flue-gases/
- 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 | IRC 2009 (2009 editions transcribed in this corpus) |
| Primary source | IRC 2009 Definitions (model text, not a jurisdiction's adopted text) |
| Chapters using the term | 8 (counted across the codes hosted here) |
| Wording identical across codes | Single code (compare before citing one code's wording under another) |

## Direct answer

Flue gases is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “Products of combustion plus excess air in appliance flues or heat exchangers.” No other I-Code in this corpus defines it, so the IRC 2009 definition is the one to work from. The term is used in 8 chapters of this corpus, most often in Fuel Gas (IRC 2009), Definitions (IRC 2009), Chimneys and Vents (IMC 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 Gases** — IRC 2009, Definitions:

> Products of combustion plus excess air in appliance flues or heat exchangers.

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

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

## What must be verified before using this guidance?

**Flue Gases** is defined once in this corpus, in the [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) chapter of the International Residential Code (IRC) 2009:

> Products of combustion plus excess air in appliance flues or heat exchangers.

## Where flue gases is used in the code

The term appears in 8 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 | 12 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 8 |
| [Chimneys and Vents](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/chimneys-and-vents.md) | IMC 2009 | 7 |
| [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | IMC 2009 | 4 |
| [Chimneys and Vents](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/chimneys-and-vents.md) | IRC 2009 | 4 |
| [Masonry](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/masonry.md) | IBC 2009 | 3 |
| [Chimneys and Fireplaces](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/chimneys-and-fireplaces.md) | IRC 2009 | 3 |
| [Definitions](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/definitions.md) | IFC 2009 | 1 |

### The term in use

How the code actually writes about flue gases, 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)

> A connector shall extend from the appliance to the flue serving a masonry fireplace such that the flue gases are exhausted directly into the flue.

— [IMC 2009 §801.10.3](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/chimneys-and-vents.md)

> Forced draft systems and all portions of induced draft systems under positive pressure during operation shall be designed and installed to prevent leakage of flue gases into a building.

— [IRC 2009 §M1801.5](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/chimneys-and-vents.md)


## The terms flue gases 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 gases covers:

- **[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.
- **[Excess Air](https://harth.build/resources/glossary/excess-air.md)** — IRC 2009: Air that passes through the combustion chamber and the appliance flue in excess of that which is theoretically required for complete combustion.
- **[Appliance](https://harth.build/resources/glossary/appliance.md)** — IRC 2009: Utilization equipment, normally built in standardized sizes or types, that is installed or connected as a unit to perform one or more functions such as clothes washing, air conditioning, food mixing, deep frying, etc.

## Flue Gases: 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 IRC | Definitions | |
| Re-check the requirement that uses it | Fuel Gas (IRC 2009) | |
| Re-check the requirement that uses it | Definitions (IRC 2009) | |
| Re-check the requirement that uses it | Chimneys and Vents (IMC 2009) | |
| Re-check the requirement that uses it | Definitions (IMC 2009) | |
| Confirm the dependent definition | Combustion | |
| Confirm the dependent definition | Excess Air | |
| Confirm the dependent definition | Appliance | |
| 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)
- [Excess Air: Building Code Definition](https://harth.build/resources/glossary/excess-air.md)
- [Combustion: Building Code Definition](https://harth.build/resources/glossary/combustion.md)
- [Appliance: Building Code Definition](https://harth.build/resources/glossary/appliance.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 gases?

The International Residential Code (IRC) 2009 defines it in Definitions as: “Products of combustion plus excess air in appliance flues or heat exchangers.” 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 gases?

In this corpus the term is used across 8 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 meaning of flue gases?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “Products of combustion plus excess air in appliance flues or heat exchangers.” It is defined in IRC 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 the difference between flue gas and fuel gas?

Fuel Gas is a separate defined term. The IRC 2009 defines it as: “A natural gas, manufactured gas, liquefiedpetroleum gas or mixtures of these gases.” That is not the same as flue gases, which the IRC 2009 defines as “Products of combustion plus excess air in appliance flues or heat exchangers.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.

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