# Return Air: Building Code Definition

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

- Canonical: https://harth.build/resources/glossary/return-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, IMC 2009 (2009 editions transcribed in this corpus) |
| Primary source | IRC 2009 Definitions (model text, not a jurisdiction's adopted text) |
| Chapters using the term | 17 (counted across the codes hosted here) |
| Wording identical across codes | Yes (compare before citing one code's wording under another) |

## Direct answer

Return air is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “Air removed from an approved conditioned space or location and recirculated or exhausted.” The IMC 2009 carries the same definition word for word, so the term means the same thing in each. The term is used in 17 chapters of this corpus, most often in Duct Systems (IMC 2009), Specific Appliances, Fireplaces and Solid Fuel-burning Equipment (IMC 2009), Duct Systems (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?

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

> Air removed from an approved conditioned space or location and recirculated or exhausted.

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

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

## What must be verified before using this guidance?

Two of the I-Codes in this corpus define **Return Air**, in identical words.

| Code | Where it is defined | Definition |
| --- | --- | --- |
| IRC 2009 | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | Air removed from an approved conditioned space or location and recirculated or exhausted. |
| IMC 2009 | [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | Air removed from an approved conditioned space or location and recirculated or exhausted. |

## Where return air is used in the code

The term appears in 17 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.

| Chapter | Code | Mentions |
| --- | --- | --- |
| [Duct Systems](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/duct-systems.md) | IMC 2009 | 39 |
| [Specific Appliances, Fireplaces and Solid Fuel-burning Equipment](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/specific-appliances-fireplaces-and-solid-fuel-burning-equipment.md) | IMC 2009 | 24 |
| [Duct Systems](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/duct-systems.md) | IRC 2009 | 24 |
| [Fuel Gas](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md) | IRC 2009 | 24 |
| [Fire Protection Systems](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/fire-protection-systems.md) | IBC 2009 | 21 |
| [Fire Protection Systems](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/fire-protection-systems.md) | IFC 2009 | 18 |
| [Means of Egress](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/means-of-egress.md) | IBC 2009 | 7 |
| [Means of Egress](https://harth.build/resources/building-codes/us-states/rhode-island/ifc-2009/means-of-egress.md) | IFC 2009 | 7 |
| [Commercial Energy Efficiency](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/commercial-energy-efficiency.md) | IECC 2009 | 4 |
| [Ventilation](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/ventilation.md) | IMC 2009 | 4 |
| [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | IMC 2009 | 3 |
| [Foundations](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/foundations.md) | IRC 2009 | 3 |

_5 further chapters mention the term._

### The term in use

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

> The use of gypsum boards to form air shafts (ducts) shall be limited to return air systems where the air temperatures do not exceed 125°F (52°C) and the gypsum board surface temperature is maintained above the airstream dew-point temperature.

— [IMC 2009 §603.5.1](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/duct-systems.md)

> The minimum unobstructed total area of the outdoor and return air ducts or openings to a forced-air warm-air furnace shall be not less than 2 square inches per 1,000 Btulh (4402 mm2/kW) output rating capacity of the furnace and not less than that specified in the furnace manufacturer's installation instructions.

— [IMC 2009 §918.2](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/specific-appliances-fireplaces-and-solid-fuel-burning-equipment.md)

> Furnaces and air-handling systems that supply air to living spaces shall not supply air to or return air from a garage.

— [IRC 2009 §M1601.6](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/duct-systems.md)


## The terms return air is built out of

The definition above leans on 2 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what return air covers:

- **[Conditioned Space](https://harth.build/resources/glossary/conditioned-space.md)** — IRC 2009: For energy purposes, space within a building that is provided with heating and/or cooling equipmentor systems capable of maintaining, through design or heat loss/gain, 50°F (IO°C) during the heating season and 85°F (29°C) during the cooling season, or communicates directly …
- **[Approved](https://harth.build/resources/glossary/approved.md)** — IBC 2009: Acceptable to the code official or authority having jurisdiction.

## Return 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 | |
| Read the definition in the adopted IMC | Definitions | |
| Re-check the requirement that uses it | Duct Systems (IMC 2009) | |
| Re-check the requirement that uses it | Specific Appliances, Fireplaces and Solid Fuel-burning Equipment (IMC 2009) | |
| Re-check the requirement that uses it | Duct Systems (IRC 2009) | |
| Re-check the requirement that uses it | Fuel Gas (IRC 2009) | |
| Confirm the dependent definition | Conditioned Space | |
| Confirm the dependent definition | Approved | |
| 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)
- [Conditioned Space: Building Code Definition](https://harth.build/resources/glossary/conditioned-space.md)
- [Approved: Building Code Definition](https://harth.build/resources/glossary/approved.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 return air?

The International Residential Code (IRC) 2009 defines it in Definitions as: “Air removed from an approved conditioned space or location and recirculated or exhausted.” 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 return air?

In this corpus the term is used across 17 chapters, led by Duct Systems in the IMC 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 are the rules for return air ducts?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “Air removed from an approved conditioned space or location and recirculated or exhausted.” It is defined in IRC 2009, 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 the difference between supply air and return air?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “Air removed from an approved conditioned space or location and recirculated or exhausted.” It is defined in IRC 2009, IMC 2009. It is not the same as Supply Air, which the IRC 2009 defines separately as “Air delivered to a conditioned space through ducts or plenums from the heat exchanger of a heating, cooling or ventilating system.” 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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