# Natural Draft System: Building Code Definition

> How the IRC 2009, IMC 2009 define natural draft system, where the wording differs, and which chapters depend on the term.

- Canonical: https://harth.build/resources/glossary/natural-draft-system/
- 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 | 2 (counted across the codes hosted here) |
| Wording identical across codes | Yes (compare before citing one code's wording under another) |

## Direct answer

Natural draft system is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft.” The IMC 2009 carries the same definition word for word, so the term means the same thing in each. The term is used in 2 chapters of this corpus, most often in Definitions (IMC 2009), Definitions (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?

**Natural Draft System** — IRC 2009, Definitions:

> A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft.

![Labeled code-term definition diagram: Natural Draft System: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/natural-draft-system/diagram-1344.webp)

*How the IRC 2009, IMC 2009 define natural draft system, 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 **Natural Draft System**, 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) | A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft. |
| IMC 2009 | [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft. |

## Where natural draft system is used in the code

The term appears in 2 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.

| Chapter | Code | Mentions |
| --- | --- | --- |
| [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/definitions.md) | IMC 2009 | 1 |
| [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 1 |

## Natural Draft System compared with Mechanical Draft System

These two are the closest pair in the 2009 I-Codes: about 57 per cent of their defining words are the same. The code states both and compares neither, so the difference is worth naming.

| | Wording unique to it |
| --- | --- |
| **Natural Draft System** (IRC 2009) | entirely, natural |
| [Mechanical Draft System](https://harth.build/resources/glossary/mechanical-draft-system.md) (IRC 2009) | mechanical, means, consists, induced, portion, forced, positive |

Everything else in the two definitions is shared. If a project turns on which of the two applies, those words are the whole of the test — and a local amendment that edits one of them without editing the other moves the boundary between natural draft system and mechanical draft system.

## The terms natural draft system 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 natural draft system covers:

- **[Venting System](https://harth.build/resources/glossary/venting-system.md)** — IRC 2009: A continuous open passageway from the flue collar or draft hood of an appliance to the outside atmosphere for the purpose of removing flue or vent gases.
- **[Vent Gases](https://harth.build/resources/glossary/vent-gases.md)** — IRC 2009: Products of combustion from fuel-burning appliances, plus excess air and dilution air, in the venting system above the draft hood or draft regulator.
- **[Venting](https://harth.build/resources/glossary/venting.md)** — IRC 2009: Removal of combustion products to the outdoors.
- **[System](https://harth.build/resources/glossary/system.md)** — IFC 2009: An assembly of equipment consisting of a tank, container or containers, appurtenances, pumps, compressors and connecting piping.
- **[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.

## Natural Draft System: 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 | Definitions (IMC 2009) | |
| Re-check the requirement that uses it | Definitions (IRC 2009) | |
| Confirm the dependent definition | Venting System | |
| Confirm the dependent definition | Vent Gases | |
| Confirm the dependent definition | Venting | |
| 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)
- [Venting System: Building Code Definition](https://harth.build/resources/glossary/venting-system.md)
- [Vent Gases: Building Code Definition](https://harth.build/resources/glossary/vent-gases.md)
- [Venting: Building Code Definition](https://harth.build/resources/glossary/venting.md)
- [System: Building Code Definition](https://harth.build/resources/glossary/system.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 natural draft system?

The International Residential Code (IRC) 2009 defines it in Definitions as: “A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft.” 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 natural draft system?

In this corpus the term is used across 2 chapters, led by Definitions 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 an example of a natural draft system?

In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft.” 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 a natural draft and mechanical draft condenser?

Condenser is a separate defined term. The IMC 2009 defines it as: “A heat exchanger designed to liquefy refrigerant vapor by removal of heat.” That is not the same as natural draft system, which the IRC 2009 defines as “A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.

### What is a natural draft furnace?

Furnace is a separate defined term. The IRC 2009 defines it as: “A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.” That is not the same as natural draft system, which the IRC 2009 defines as “A venting system designed to remove flue or vent gases under nonpositive static vent pressure entirely by natural draft.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.

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