# Recirculated Air: Building Code Definition

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

- Canonical: https://harth.build/resources/glossary/recirculated-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 | 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 | 5 (counted across the codes hosted here) |
| Wording identical across codes | Single code (compare before citing one code's wording under another) |

## Direct answer

Recirculated air is a defined term in the International Mechanical Code (IMC) 2009, in Definitions. The code's own words are: “Air removed from a conditioned space and intended for reuse as supply air.” 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 5 chapters of this corpus, most often in Ventilation (IMC 2009), Special Detailed Requirements Based on Use and Occupancy (IBC 2009), Definitions (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?

**Recirculated Air** — IMC 2009, Definitions:

> Air removed from a conditioned space and intended for reuse as supply air.

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

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

## What must be verified before using this guidance?

**Recirculated Air** 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:

> Air removed from a conditioned space and intended for reuse as supply air.

## Where recirculated air is used in the code

The term appears in 5 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.

| Chapter | Code | Mentions |
| --- | --- | --- |
| [Ventilation](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/ventilation.md) | IMC 2009 | 4 |
| [Special Detailed Requirements Based on Use and Occupancy](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/special-detailed-requirements-based-on-use-and-occupancy.md) | IBC 2009 | 3 |
| [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | IMC 2009 | 2 |
| [Exhaust Systems](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/exhaust-systems.md) | IMC 2009 | 2 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/definitions.md) | IECC 2009 | 1 |

### The term in use

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

> Exhaust makeup air shall be permitted to be any combination of outdoor air, recirculated air and transfer air, except as limited in accordance with Section 403.2.

— [IMC 2009 §403.4](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/ventilation.md)

> This field test shall be conducted with all appliances under the hood at operating temperatures, with all sources of outdoor air providing makeup air for the hood operating and with all sources of recirculated air providing conditioning for the space in which the hood is located operating.

— [IMC 2009 §507.16.1](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/exhaust-systems.md)


## The terms recirculated 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 recirculated 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 …
- **[Supply Air](https://harth.build/resources/glossary/supply-air.md)** — IRC 2009: Air delivered to a conditioned space through ducts or plenums from the heat exchanger of a heating, cooling or ventilating system.

## Recirculated 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 IMC | Definitions | |
| Re-check the requirement that uses it | Ventilation (IMC 2009) | |
| Re-check the requirement that uses it | Special Detailed Requirements Based on Use and Occupancy (IBC 2009) | |
| Re-check the requirement that uses it | Definitions (IMC 2009) | |
| Re-check the requirement that uses it | Exhaust Systems (IMC 2009) | |
| Confirm the dependent definition | Conditioned Space | |
| Confirm the dependent definition | Supply Air | |
| 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)
- [Supply Air: Building Code Definition](https://harth.build/resources/glossary/supply-air.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 recirculated air?

The International Mechanical Code (IMC) 2009 defines it in Definitions as: “Air removed from a conditioned space and intended for reuse as supply air.” 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 recirculated air?

In this corpus the term is used across 5 chapters, led by Ventilation 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 is recirculated air?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IMC 2009 states, in Definitions: “Air removed from a conditioned space and intended for reuse as supply air.” 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 recirculation ventilation?

Ventilation is a separate defined term. The IBC 2009 defines it as: “The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.” That is not the same as recirculated air, which the IMC 2009 defines as “Air removed from a conditioned space and intended for reuse as supply air.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.

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