IMC 601.5 is one of the most practical return-air sections in Chapter 6. The review focus is not just grille placement. It is contamination control, combustion safety, and pressure balance.
Plain English: Plain-English Highlights
601.5: Return openings cannot be within 10 feet of an open combustion chamber or draft hood of another appliance in the same room or space.
601.5: Return air cannot be taken from hazardous or insanitary locations or from a refrigeration room.
601.5: Return air taken from a room or space cannot exceed the supply air delivered to that room or space.
601.5: Return and transfer openings must be sized per manufacturer instructions, ACCA Manual D, or the design professional.
601.5: Return air from one dwelling unit cannot be discharged into another dwelling unit.
601.5: Return air cannot be directly connected from a crawl space to the return side of a forced-air furnace.
601.5: Return air cannot be taken from bathrooms, toilet rooms, kitchens, garages, boiler rooms, furnace rooms, or unconditioned attics unless a listed exception applies.
601.5: Closet return air can serve only that closet. For closets under 30 sf, provide a 1-1/2 inch door undercut, louvered door, or transfer grille with at least 30 in² net free area.
601.5: Return air cannot be taken from indoor pool enclosures or deck areas unless the space is dehumidified per 403.2.1 item 2 or served by a dedicated HVAC system.
601.5 exceptions: Kitchen returns are allowed only where the code conditions are met. A dedicated garage-only forced-air system can take return air from the garage.
On Plans: Why it matters
A return grille in the wrong room, a missing combustion clearance, or return quantity exceeding supply is an easy redline and a real field issue.
Code Path: Where to show it
M-001: IMC 601.5 notes and exception criteria
M-101: return grille locations and prohibited-space coordination
M-501: closet return details and kitchen return offsets
M-601: room-by-room supply vs return summary
Arch. plans: closet size, door type, and transfer-air path coordination
Check: Do
Verify return cfm does not exceed supply cfm
Show combustion-appliance clearance where returns are nearby
Detail closet returns clearly when under 30 sf
Make exception use obvious on plan
Review Risk: Don't
Don't use a kitchen return without showing the required separation
Don't assume a door undercut alone will solve pressure balance
Field Tip: Field tip
A door undercut is often not enough by itself. A practical rule of thumb is about 1 in² of transfer opening per 1 cfm delivered to the room. A 3/4-inch undercut at a 32-inch door gives only about 24 in² gross opening, so higher-airflow rooms usually need a transfer grille or jump duct.
Comment "IMC601" if you want a paste-ready IMC 601 review checklist.
Masterbuild QA Lens
Duct-system sections are coordination sections. The question is not only whether air moves, but whether materials, insulation, plenums, dampers, access, and penetrations are correct for the location.
Drawing / Submittal Check
Trace the air path across plans, risers, details, schedules, specifications, and reflected ceiling constraints. Confirm duct material, insulation, vapor control, fire/smoke dampers, access, and exposed conditions.
Common Review Risk
Small duct notes create large field cost when they miss rated assemblies, plenum limitations, weather exposure, internal liner restrictions, damper access, or condensation control.
When To Escalate
Escalate when ducts cross rated construction, run outdoors, serve healthcare spaces, use internal lining, connect to smoke control, or pass through congested existing-building conditions.
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.
Special Exhaust Coordination
For specialty exhaust, start with the contaminant and source. Then confirm capture method, duct material, routing, discharge, separation, controls, and whether another consultant or AHJ review is required.