# Dilution Air: Building Code Definition

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

- Canonical: https://harth.build/resources/glossary/dilution-air/
- 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 | 7 (counted across the codes hosted here) |
| Wording identical across codes | Single code (compare before citing one code's wording under another) |

## Direct answer

Dilution air is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “Air that enters a draft hood or draft regulator and mixes with flue gases.” 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 7 chapters of this corpus, most often in Fuel Gas (IRC 2009), Fire Protection Systems (IBC 2009), Fire Protection Systems (IFC 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?

**Dilution Air** — IRC 2009, Definitions:

> Air that enters a draft hood or draft regulator and mixes with flue gases.

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

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

## What must be verified before using this guidance?

**Dilution Air** 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:

> Air that enters a draft hood or draft regulator and mixes with flue gases.

## Where dilution air is used in the code

The term appears in 7 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 | 10 |
| [Fire Protection Systems](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/fire-protection-systems.md) | IBC 2009 | 3 |
| [Fire Protection Systems](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/fire-protection-systems.md) | IFC 2009 | 3 |
| [Exhaust Systems](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/exhaust-systems.md) | IMC 2009 | 3 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 2 |
| [Combustion Air](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/combustion-air.md) | IMC 2009 | 1 |
| [Combustion Air](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/combustion-air.md) | IRC 2009 | 1 |

### The term in use

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

> The requirements for combustion and dilution air for gas-fired appliances shall be governed by Section G2407.

— [IRC 2009 §G2404.2](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md)

> The requirements for combustion and dilution air for gas-fired appliances shall be in accordance with the International Fuel Gas Code.

— [IMC 2009 §701.1](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/combustion-air.md)

> The requirements for combustion and dilution air for gas-fired appliances shall be in accordance with Chapter 24.

— [IRC 2009 §M1701.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/combustion-air.md)


## The terms dilution air is built out of

The definition above leans on 5 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what dilution air covers:

- **[Draft Regulator](https://harth.build/resources/glossary/draft-regulator.md)** — IRC 2009: A device that functions to maintain a desired draft in the appliance by automatically reducing the draft to the desired value.
- **[Flue Gases](https://harth.build/resources/glossary/flue-gases.md)** — IRC 2009: Products of combustion plus excess air in appliance flues or heat exchangers.
- **[Draft Hood](https://harth.build/resources/glossary/draft-hood.md)** — IRC 2009: A nonadjustable device built into an appliance, or made as part of the vent connector from an appliance, that is designed to (1) provide for ready escape of the flue gases from the appliance in the event of no draft …
- **[Regulator](https://harth.build/resources/glossary/regulator.md)** — IRC 2009: A device for controlling and maintaining a uniform gas supply pressure, either pounds-to-inches water column (MP regulator) or inches-to-inches water column (appliance regulator).
- **[Draft](https://harth.build/resources/glossary/draft.md)** — IRC 2009: The pressure difference existing between the appliance or any component part and the atmosphere, that causes a continuous flow of air and products of combustion through the gas passages of the appliance to the atmosphere.

## Dilution Air: 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 | Fire Protection Systems (IBC 2009) | |
| Re-check the requirement that uses it | Fire Protection Systems (IFC 2009) | |
| Re-check the requirement that uses it | Exhaust Systems (IMC 2009) | |
| Confirm the dependent definition | Draft Regulator | |
| Confirm the dependent definition | Flue Gases | |
| Confirm the dependent definition | Draft Hood | |
| 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)
- [Draft Regulator: Building Code Definition](https://harth.build/resources/glossary/draft-regulator.md)
- [Flue Gases: Building Code Definition](https://harth.build/resources/glossary/flue-gases.md)
- [Draft Hood: Building Code Definition](https://harth.build/resources/glossary/draft-hood.md)
- [Regulator: Building Code Definition](https://harth.build/resources/glossary/regulator.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 dilution air?

The International Residential Code (IRC) 2009 defines it in Definitions as: “Air that enters a draft hood or draft regulator and mixes with flue gases.” 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 dilution air?

In this corpus the term is used across 7 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 dilution air?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “Air that enters a draft hood or draft regulator and mixes with flue gases.” 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 are the combustion air code requirements?

Combustion Air is a separate defined term. The IRC 2009 defines it as: “The air provided to fuel-burning equipmentincluding air for fuel combustion, draft hood dilution and ventilation of the equipment enclosure.” That is not the same as dilution air, which the IRC 2009 defines as “Air that enters a draft hood or draft regulator and mixes with flue gases.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.

### What is dilution air pollution?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “Air that enters a draft hood or draft regulator and mixes with flue gases.” It is defined in IRC 2009. It is not the same as Pollution, which the IRC 2009 defines separately as “An impairment of the quality of the potable water to a degree that does not create a hazard to the public health but that does adversely and unreasonably affect the aesthetic qualities of such potable …” 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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