# Btu/h: Building Code Definition > How the IRC 2009 define btu/h, where the wording differs, and which chapters depend on the term. - Canonical: https://harth.build/resources/glossary/btu-h/ - 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 | 14 (counted across the codes hosted here) | | Wording identical across codes | Single code (compare before citing one code's wording under another) | ## Direct answer Btu/h is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “The listed maximum capacity of an appliance, absorption unit or burner expressed in British thermal units input per hour.” 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 14 chapters of this corpus, most often in Commercial Energy Efficiency (IECC 2009), Fuel Gas (IRC 2009), Fire and Smoke Protection Features (IBC 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? **Btu/h** — IRC 2009, Definitions: > The listed maximum capacity of an appliance, absorption unit or burner expressed in British thermal units input per hour. ![Labeled code-term definition diagram: Btu/h: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/btu-h/diagram-1344.webp) *How the IRC 2009 define btu/h, where the wording differs, and which chapters depend on the term.* ## What must be verified before using this guidance? **Btu/h** 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: > The listed maximum capacity of an appliance, absorption unit or burner expressed in British thermal units input per hour. ## Where btu/h is used in the code The term appears in 14 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next. | Chapter | Code | Mentions | | --- | --- | --- | | [Commercial Energy Efficiency](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/commercial-energy-efficiency.md) | IECC 2009 | 12 | | [Fuel Gas](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md) | IRC 2009 | 11 | | [Fire and Smoke Protection Features](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/fire-and-smoke-protection-features.md) | IBC 2009 | 5 | | [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 | 5 | | [Duct Systems](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/duct-systems.md) | IRC 2009 | 5 | | [General Mechanical System Requirements](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/general-mechanical-system-requirements.md) | IRC 2009 | 5 | | [Heating and Cooling Equipment](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/heating-and-cooling-equipment.md) | IRC 2009 | 5 | | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/definitions.md) | IRC 2009 | 4 | | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/definitions.md) | IECC 2009 | 3 | | [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 | 2 | | [Administration](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/administration.md) | IECC 2009 | 2 | | [Boilers, Water Heaters and Pressure Vessels](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/boilers-water-heaters-and-pressure-vessels.md) | IMC 2009 | 2 | _2 further chapters mention the term._ ### The term in use How the code actually writes about btu/h, quoted from the 2009 text hosted here: > Hydronic systems of at least 300,000 Btu/h (87,930 W) design output capacity supplying heated and chilled water to comfort conditioning systems shall include controls that meet the requirements of Section 503.4.3. — [IECC 2009 §503.3.2](https://harth.build/resources/building-codes/us-states/maryland/iecc-2009/commercial-energy-efficiency.md) > Each opening shall have a minimum free area of 1 square inch per 1,000 Btu/h (2,200 mm2/kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.06 m2). — [IRC 2009 §G2407.5.3.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md) > The minimum unobstructed total area of the outside and return air ducts or openings to a heat pump shall be not less than 6 square inches per 1,000 Btu/h (13 208 mm2/kW) output rating or as indicated by the conditions of the listing of the heat pump. — [IRC 2009 §M1403.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/heating-and-cooling-equipment.md) ## The terms btu/h is built out of The definition above leans on 3 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what btu/h covers: - **[Appliance](https://harth.build/resources/glossary/appliance.md)** — 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. - **[Listed](https://harth.build/resources/glossary/listed.md)** — IBC 2009: Equipment, materials, products or services included in a list published by an organization acceptable to the code official and concerned with evaluation of products or services that maintains periodic inspection of production of listed equipment or materials or periodic evaluation … - **[Burner](https://harth.build/resources/glossary/burner.md)** — IRC 2009: A device for the final conveyance of the gas, or a mixture of gas and air, to the combustion zone. ## Btu/h: 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 | Commercial Energy Efficiency (IECC 2009) | | | Re-check the requirement that uses it | Fuel Gas (IRC 2009) | | | Re-check the requirement that uses it | Fire and Smoke Protection Features (IBC 2009) | | | Re-check the requirement that uses it | Specific Appliances, Fireplaces and Solid Fuel-burning Equipment (IMC 2009) | | | Confirm the dependent definition | Appliance | | | Confirm the dependent definition | Listed | | | Confirm the dependent definition | Burner | | | 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) - [Appliance: Building Code Definition](https://harth.build/resources/glossary/appliance.md) - [Listed: Building Code Definition](https://harth.build/resources/glossary/listed.md) - [Burner: Building Code Definition](https://harth.build/resources/glossary/burner.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 btu/h? The International Residential Code (IRC) 2009 defines it in Definitions as: “The listed maximum capacity of an appliance, absorption unit or burner expressed in British thermal units input per hour.” 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 btu/h? In this corpus the term is used across 14 chapters, led by Commercial Energy Efficiency in the IECC 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. --- - Continue in Harth (open this topic in the Harth agent): https://app.harth.build/agent?context=%2Fresources%2Fglossary%2Fbtu-h%2F - View this page in a browser: https://harth.build/resources/glossary/btu-h/ - All resources for agents: https://harth.build/llms.txt