Hazardous exhaust only works if you capture at the source, move it to the duct, and replace the air without blowing the hood off the process.
Plain English:
509.5.3 hoods required
If contaminants originate in a limited area, you need a hood or enclosure.
The hood/enclosure must be designed so the exhaust-created air currents capture contaminants and send them directly into the exhaust duct.
509.5.4 capture + dilution
Work area air velocity and circulation must be sufficient to capture emissions at the source and convey them into the duct system.
Contaminated air must be diluted below the 509.2 thresholds using air that does not contain other hazardous contaminants (don't "dilute" with dirty air).
509.5.5 makeup air
Provide makeup air during operations at a rate approximately equal to the hazardous exhaust rate (all sources combined).
Makeup air can be gravity and/or mechanical.
Mechanical makeup air must start and operate simultaneously with the exhaust system (interlocked).
Makeup air cannot reduce exhaust effectiveness (avoid cross drafts / short-circuiting).
Makeup air intakes must comply with IMC 401.4.
509.5.6 clearances
Hood clearance to combustibles is the same clearance required by the duct system (don't treat the hood like a standalone object).
On Plans: Why it matters
Most failures aren't the fan-they're poor capture geometry, drafts from makeup air, and "dilution" that accidentally mixes incompatible contaminants or spreads them through the room.
Code Path: Where to show it
Plans/sections: hood/enclosure type + location relative to source, with airflow direction arrows.
M-001: "HAZ EXH INTERLOCK with MUA functionality. MAU approximately = EXH" note; intake per 401.4; "MUA SHALL NOT DISRUPT CAPTURE."
Air balance / one-line: exhaust CFM vs total makeup CFM (and where it comes from).
Details: clearance note tied to duct material/listing/shaft requirements.
Check: Do
Design capture like a containment system, not a "grille near a process."
Interlock makeup air so the system can't run exhaust-only.
Review Risk: Don't
Don't dump makeup air across the hood face.
Don't assume "general dilution" replaces source capture where a hood/enclosure is required.
Field Tip:
On hazardous exhaust sheets, add a simple "CAPTURE CHECK" note: SOURCE -> HOOD/ENCLOSURE -> DIRECT TO DUCT, plus an "INTERLOCKED MUA" tag showing makeup air location away from the capture zone.
Masterbuild QA Lens
Part 5 is where good hazardous exhaust designs fail on paper: a hood or enclosure is required wherever contaminants originate in a limited area, and the exhaust airflow itself has to create the capture - a grille near the process is not a hood. Confirm makeup air is shown at approximately the exhaust rate, mechanically interlocked to start and stop with the exhaust, and located so it does not disrupt capture at the hood face. Clearance to combustibles at the hood follows the same clearance required for the duct system - it cannot be treated as a standalone item.
Drawing / Submittal Check
For 509.5.3-509.5.6 (capture, dilution, makeup air, clearances): the drawings must show the capture device location relative to the source, the dilution air injection point and rate, and the makeup air supply location (must not allow cross-contamination with the hazardous zone). Clearance requirements around the discharge point must be dimensioned on the site or roof plan.
Common Review Risk
Capture velocity comments concentrate on pickup points that are too far from the source. Hazardous exhaust systems rely on capture velocity at the source - if the hood is too high, the velocity at the source is too low, and contaminants escape into the work space. Confirm the capture velocity is calculated at the source, not at the hood face. The designer's note 'provide adequate capture velocity' is not sufficient - show the calculation.
When To Escalate
Escalate when the hazardous exhaust capture velocity calculation requires a computational fluid dynamics (CFD) analysis to verify - when the source geometry is complex enough that standard analytical methods cannot confirm the capture efficiency. CFD analysis takes time and adds cost; identify the need early so it is included in the project budget.
Rated Assembly Coordination
Capture velocity requirements at 509.5.3-509.5.6 may dictate duct sizing and hood placement near rated construction. Confirm that hood location and duct routing don't compromise adjacent rated assemblies and that any penetration detail is shown.
Kitchen Exhaust Coordination
For kitchen exhaust, tie the hood schedule, appliance lineup, grease duct route, cleanouts, fan discharge, fire suppression interface, and make-up air strategy into one reviewable story.