IMC 603 ends at the air outlet. By this point, the duct may be sized and installed correctly, but the system can still fail review if the air devices are not installed per listing, cannot be balanced, or are located where the code does not allow them.
Plain English:
603.18 Registers, Grilles and Diffusers
Registers, grilles, and diffusers must be installed per the manufacturer's instructions.
Supply air adjustment must be provided either in the branch duct or at each individual register, grille, or diffuser.
Any balancing device used for adjustment must have access.
603.18.1 Floor Registers
Floor registers must resist a 200-pound concentrated load on a 2-inch-diameter disc without structural failure.
603.18.2 Prohibited Locations
Registers, grilles, and diffusers are prohibited in the floor, or the upward extension of the floor, within toilet and bathing rooms that must have smooth, hard, nonabsorbent surfaces under the IBC.
Exception: dwelling units.
On Plans: Why it matters
These comments usually show up late because people think of air devices as "finish items." But code still cares about accessibility for balancing, structural capacity for floor registers, and sanitary/location restrictions in wet rooms. If that is not coordinated early, it becomes a permit
Code Path: Where to show it
M-001: air device and balancing access notes
M-101: register, grille, and diffuser locations
M-501: balancing access details and floor register details
M-601: air device schedule with adjustment/balancing basis
A sheets: wet-room floor conditions and finish coordination
Check: Do
Show how each supply device will be balanced
Make balancing dampers or adjustment means accessible
Coordinate floor register type and load resistance where used
Check toilet and bathing room floor device locations early
Review Risk: Don't
Don't hide balancing devices above inaccessible ceilings
Don't assume every floor register is acceptable in every space
Don't place floor diffusers/registers/grilles in prohibited wet-room floor areas outside the dwelling-unit exception
Field Tip:
A quick QC check is this: for every supply outlet, ask how it gets adjusted and how someone gets to it. If the answer is unclear, the plans are not done yet.
Masterbuild QA Lens
Nonmetallic duct construction (603.5) is limited to specific applications and requires a listed assembly. Confirm the listing basis is on M-001, that the listed material is used only where permitted by Table 603.5, and that the contractor is required to install per the listing installation instructions. Non-listed nonmetallic duct installed as a field substitution will fail inspection.
Drawing / Submittal Check
For 603.18-603.18.2 (registers, grilles, and diffusers): the schedule must show every supply, return, and exhaust terminal with its size, type, and CFM. Confirm the terminal device is selected for the design velocity - oversized terminals that are throttled to low velocities do not distribute air effectively and are a performance issue, not just a code issue. Missing terminal schedules are a common comment.
Common Review Risk
The most common terminal device
When To Escalate
Escalate when the terminal device schedule does not match the installed devices - when field additions or substitutions during construction created a system that cannot be balanced to the design CFM. An unbalanceable system requires either additional terminal devices, revised duct connections, or a revised design that accepts the as-built configuration.
Rated Assembly Coordination
Section 603.18-603.18.2 governs supply registers and grilles in fire-rated ceilings and floors. Confirm each register in a rated ceiling assembly is listed for that application and that the installation method maintains the assembly's fire-resistance rating.
Load Assumption Check
Register and diffuser sizing (603.18-603.18.2) affects throw, terminal velocity, and the delivered CFM at the space. Confirm the diffuser neck velocity and throw pattern are appropriate for the ceiling height and cooling load in each room - undersized registers cause poor distribution even when total CFM is correct.