Building Code Glossary
Furnace: Building Code Definition
How the IRC 2009, IMC 2009 define furnace, where the wording differs, and which chapters depend on the term.
On this page
Last updated September 7, 2026 · 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.
Direct answer
Furnace is a defined term in the International Residential Code (IRC) 2009, in Fuel Gas. The code's own words are: “A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.” The IMC 2009 carries the same definition word for word, so the term means the same thing in each. The term is used in 26 chapters of this corpus, most often in Fuel Gas (IRC 2009), Specific Appliances, Fireplaces and Solid Fuel-burning Equipment (IMC 2009), Heating and Cooling Equipment (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, Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.
What does this term mean?
Furnace — IRC 2009, Fuel Gas:
A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.
What must be verified before using this guidance?
Two of the I-Codes in this corpus define Furnace, in identical words.
| Code | Where it is defined | Definition |
|---|---|---|
| IRC 2009 | Fuel Gas | A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location. |
| IMC 2009 | Definitions | A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location. |
Where furnace is used in the code
The term appears in 26 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 | IRC 2009 | 102 |
| Specific Appliances, Fireplaces and Solid Fuel-burning Equipment | IMC 2009 | 48 |
| Heating and Cooling Equipment | IRC 2009 | 47 |
| Industrial Ovens | IFC 2009 | 28 |
| Fire and Smoke Protection Features | IBC 2009 | 15 |
| Duct Systems | IRC 2009 | 14 |
| General Regulations | IMC 2009 | 10 |
| Definitions | IMC 2009 | 9 |
| Chimneys and Vents | IRC 2009 | 9 |
| General Mechanical System Requirements | IRC 2009 | 8 |
| Residential Energy Efficiency | IECC 2009 | 5 |
| Energy Efficiency | IRC 2009 | 5 |
14 further chapters mention the term.
The term in use
How the code actually writes about furnace, quoted from the 2009 text hosted here:
Where the furnace plenum is adjacent to plaster on metal lath or noncombustible material attached to combustible material, the clearance shall be measured to the surface of the plaster or other noncombustible finish where the clearance specified is 2 inches (51 mm) or less.
Vented wall furnaces shall be located so that a door cannot swing within 12 inches (305 mm) of an air inlet or air outlet of such furnace measured at right angles to the opening.
An opening in the foundation not less than 18 inches by 24 inches (457 mm by 610 mm), or a trap door not less than 22 inches by 30 inches (559 mm by 762 mm) shall be provided for access to a floor furnace.
The terms furnace is built out of
The definition above leans on 1 other defined term. Each carries its own code definition, and a local amendment to any of them changes what furnace covers:
- Appliance — 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.
Furnace: what to open, and in what order
Every row is a document this term depends on. The ICC code library and the ICC adoption maps cover the middle rows.
| Check | Where | Confirmed |
|---|---|---|
| Read the definition in the adopted IRC | Fuel Gas | |
| Read the definition in the adopted IMC | Definitions | |
| Re-check the requirement that uses it | Fuel Gas (IRC 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 | Heating and Cooling Equipment (IRC 2009) | |
| Re-check the requirement that uses it | Industrial Ovens (IFC 2009) | |
| 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
- Appliance: Building Code Definition
- Code Official: Building Code Definition
- Code: Building Code Definition
- Fuel Gas: Building Code Definition
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 publishes the current model text and the ICC adoption maps show which edition a jurisdiction has adopted; those sources control.
Source links
Frequently asked questions
What is the code definition of furnace?
The International Residential Code (IRC) 2009 defines it in Fuel Gas as: “A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.” 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 furnace?
In this corpus the term is used across 26 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 definition of a furnace?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Fuel Gas: “A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.” 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 code requirement for clearance around a furnace?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Fuel Gas: “A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.” It is defined in IRC 2009, IMC 2009. It is not the same as Clearance, which the IRC 2009 defines separately as “The minimum distance through air measured between the heat-producing surface of the mechanical appliance, device or equipment and the surface of the combustible material or assembly.” 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 does IRC mean in building code?
Building is a separate defined term. The IBC 2009 defines it as: “Any structure used or intended for supporting or sheltering any use or occupancy.” That is not the same as furnace, which the IRC 2009 defines as “A completely self-contained heating unit that is designed to supply heated air to spaces remote from or adjacent to the appliance location.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
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