# Pressure Drop: Building Code Definition > How the IRC 2009 define pressure drop, where the wording differs, and which chapters depend on the term. - Canonical: https://harth.build/resources/glossary/pressure-drop/ - 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 Fuel Gas (model text, not a jurisdiction's adopted text) | | Chapters using the term | 4 (counted across the codes hosted here) | | Wording identical across codes | Single code (compare before citing one code's wording under another) | ## Direct answer Pressure drop is a defined term in the International Residential Code (IRC) 2009, in Fuel Gas. The code's own words are: “The loss in pressure due to friction or obstruction in pipes, valves, fittings, regulators and burners.” 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 4 chapters of this corpus, most often in Hydronic Piping (IMC 2009), Fuel Gas (IRC 2009), Hvdronic Piping (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? **Pressure Drop** — IRC 2009, Fuel Gas: > The loss in pressure due to friction or obstruction in pipes, valves, fittings, regulators and burners. ![Labeled code-term definition diagram: Pressure Drop: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/pressure-drop/diagram-1344.webp) *How the IRC 2009 define pressure drop, where the wording differs, and which chapters depend on the term.* ## What must be verified before using this guidance? **Pressure Drop** is defined once in this corpus, in the [Fuel Gas](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md) chapter of the International Residential Code (IRC) 2009: > The loss in pressure due to friction or obstruction in pipes, valves, fittings, regulators and burners. ## Where pressure drop is used in the code The term appears in 4 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next. | Chapter | Code | Mentions | | --- | --- | --- | | [Hydronic Piping](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/hydronic-piping.md) | IMC 2009 | 4 | | [Fuel Gas](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/fuel-gas.md) | IRC 2009 | 4 | | [Hvdronic Piping](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/hvdronic-piping.md) | IRC 2009 | 4 | | [Solar Systems](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/solar-systems.md) | IRC 2009 | 2 | ### The term in use How the code actually writes about pressure drop, quoted from the 2009 text hosted here: > If actual flow rate or pressure drop values differ from calculated design values by more than 10 percent, the problem shall be identified and corrected. — [IMC 2009 §1208.1.1](https://harth.build/resources/building-codes/us-states/new-hampshire/imc-2009/hydronic-piping.md) > If actual flow rate or pressure drop figures differ from calculated values by more than 10 percent, the problem shall be identified and corrected. — [IRC 2009 §M2105.1](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/hvdronic-piping.md) ## Pressure Drop: 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 | Fuel Gas | | | Re-check the requirement that uses it | Hydronic Piping (IMC 2009) | | | Re-check the requirement that uses it | Fuel Gas (IRC 2009) | | | Re-check the requirement that uses it | Hvdronic Piping (IRC 2009) | | | Re-check the requirement that uses it | Solar Systems (IRC 2009) | | | 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) - [Code Official: Building Code Definition](https://harth.build/resources/glossary/code-official.md) - [Code: Building Code Definition](https://harth.build/resources/glossary/code.md) - [Fuel Gas: Building Code Definition](https://harth.build/resources/glossary/fuel-gas.md) - [Vent: Building Code Definition](https://harth.build/resources/glossary/vent.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 pressure drop? The International Residential Code (IRC) 2009 defines it in Fuel Gas as: “The loss in pressure due to friction or obstruction in pipes, valves, fittings, regulators and burners.” 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 pressure drop? In this corpus the term is used across 4 chapters, led by Hydronic Piping 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 the definition of pressure drop? In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Fuel Gas: “The loss in pressure due to friction or obstruction in pipes, valves, fittings, regulators and burners.” It is defined in IRC 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. --- - Continue in Harth (open this topic in the Harth agent): https://app.harth.build/agent?context=%2Fresources%2Fglossary%2Fpressure-drop%2F - View this page in a browser: https://harth.build/resources/glossary/pressure-drop/ - All resources for agents: https://harth.build/llms.txt