Identify what each exhaust system conveys, then trace it from the hood or equipment connection through the duct, fan, collector, and discharge, checking materials, access, and protection along the way. AI can flag conflicts; the engineer of record and the authority having jurisdiction decide compliance.
What does NFPA 91 cover?
NFPA 91 is the Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids. NFPA describes it as providing technical requirements for exhaust systems that protect lives and property from fires and explosions and minimize damage if such fires and explosions occur.
NFPA’s official NFPA 91 page identifies the 2026 edition as current when checked on October 4, 2026. Confirm the edition, local amendments, and project scope that actually govern the work. The publisher’s newest edition does not establish local adoption.
Process-specific standards often refer to NFPA 91 for the exhaust system itself, including the NFPA 33 spray application guide and the NFPA 45 laboratory guide. Commercial kitchen exhaust is a separate subject covered by the NFPA 96 guide, and general building air handling follows the NFPA 90A guide. Record which standard governs each exhaust system.
Start from what each system conveys
Bring the exhaust system schedule or fan schedule, the process or equipment layout, the mechanical ductwork plans and sections, the duct material and construction specification, the collector and fan product data, the roof plan with discharge locations, the fire protection drawings for any in-duct protection, and the electrical plans where the system serves a classified area.
Note what each system carries, such as solvent vapors, corrosive gases, mists, or combustible particulate. Two systems with similar ducts can carry different hazards, so label them before comparing.
Follow the exhaust from source to discharge
| Review question | Sources to compare | Decision to record |
|---|---|---|
| Conveyed material | Fan schedule, process narrative, equipment list | Confirm what each system carries and whether that label is consistent. |
| Duct routing | Mechanical plans, sections, architectural plans | Trace each duct through walls, floors, and shafts and match the route across sheets. |
| Materials and construction | Duct specification, mechanical details, schedules | Check that the specified duct material matches the material noted on the drawings. |
| Access and cleanouts | Mechanical plans, details, specifications | Identify ducts where the specification calls for access that the drawings do not show. |
| Collectors and fans | Equipment schedules, product data, plans | Match collector and fan tags, locations, and arrangements across sources. |
| Discharge | Roof plan, mechanical plans, site plan | Confirm each discharge is drawn and compare its location with intakes and openings. |
Carry combustible dust and explosion questions into the NFPA 68 venting guide and NFPA 69 prevention guide, and ductwork coordination into the HVAC air distribution specifications guide.
Example: one exhaust duct, two materials
Hypothetical example. The duct specification calls for one material for the solvent exhaust system, while the mechanical details and fan schedule describe a different material and a fan arrangement that the product data do not include.
The solvent exhaust duct material in the specification differs from the mechanical details, and the fan arrangement on the schedule does not match the product data. Please confirm the duct material and fan arrangement.
The mechanical engineer selects the material and arrangement and revises the documents. Whether the system meets the adopted standard is the engineer’s and authority’s decision.
Review process exhaust documents with Groundbook
Upload the current project PDFs and relevant specifications to Groundbook. Identify the governing issues and add the applicable NFPA 91 reference from a copy you are licensed to use, where the license permits uploading it. State the edition, amendments, review area, and known exclusions in your instructions.
Compare the exhaust fan and system schedules, process layout, ductwork plans and sections, duct specification, collector and fan data, roof plan, and fire protection and electrical drawings. Flag conveyed materials, routes, duct materials, access, equipment, and discharge information that differ between sources. Use the supplied NFPA 91 edition where applicable and cite both sources.
Groundbook links each potential finding to the sheets behind it. It does not decide whether a material is a combustible dust or how a system should be classified.
Document review does not size ducts or fans, calculate conveying velocities, or approve the system. Those decisions remain with the engineer of record and the authority having jurisdiction.