Daily Code Talk #73: IMC 509 Part 5 (509.5.3–509.5.6 Capture, Dilution, Makeup Air, Clearances)

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 Highlights• 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).🏗️ Why it mattersMost 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.🗺️ 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.✅ Do• Design capture like a containment system, not a “grille near a process.”• Interlock makeup air so the system can’t run exhaust-only.⛔ 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 tipOn 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.Comment “IMC509-P5” if you want paste-ready M-001 notes for hood capture + MUA interlock language.