IMC 801.19 addresses a major common-venting limitation: appliances on different floors generally cannot share the same venting system. The exception is narrow, and it is easy to misapply on paper.
Plain-English Highlights
Common venting systems serving appliances located on more than one floor level are prohibited. The exception applies only where all appliances served are located in rooms or spaces accessed only from the outdoors, and the appliance enclosures do not communicate with occupiable areas of the building. Both conditions matter together - an outdoor-access equipment room is not enough if openings, doors, grilles, shafts, or other pathways allow the enclosure to communicate with occupied spaces.
For plan review, verify the floor level of each appliance, which appliances connect to the common vent, equipment room access, enclosure separation, openings into occupied areas, vent routing and termination, and architectural and life-safety coordination.
Permit Proof Chain
Every appliance connected to a common vent identified -> floor level of each appliance recorded -> equipment room or enclosure access classified as exterior-only or building-accessed -> openings, doors, grilles, and shafts into occupied space checked -> multistory condition flagged if present -> exterior-access-only exception documented with architectural coordination, or common vent reconfigured -> final vent routing and termination confirmed.
Where To Show It
M-001: common venting criteria and restrictions. M-101: appliance locations and vent connections by floor. M-501: common vent, connector, support, and termination details. M-601: appliance schedule with floor and venting basis. A/LS sheets: equipment room access, enclosure, doors, openings, and shaft coordination.
**Do** - Identify every appliance connected to the common vent, by floor level, before the vent configuration is finalized. - Confirm whether any connected appliances are located on different floors. - Show exterior-only access for every enclosure where the multistory exception is being used. - Coordinate enclosure separation from occupied areas with the architectural and life-safety drawings.
**Do Not** - Common-vent appliances on multiple floors without verifying the exception applies. - Assume a mechanical room qualifies for the exception because it has an exterior door. - Allow the appliance enclosure to communicate with occupied areas through openings, shafts, or gaps. - Leave common vent floor configuration to field coordination after the vent routing is already installed.
Masterbuild QA Lens
Build a one-line-per-appliance table for every common vent: appliance ID, floor level, enclosure type, and whether the enclosure communicates with occupied space. If the table shows more than one floor without both exception conditions checked yes, the common vent configuration is not acceptable as drawn.
Drawing / Submittal Check
Compare the appliance schedule floor levels against the vent riser or common-vent routing diagram. For any exception claim, confirm the enclosure drawings show exterior-only access and no door, louver, grille, shaft, or duct connection to occupied building areas.
Common Review Risk
A common vent serves appliances on two floors because both mechanical rooms happen to share a shaft. The exception is assumed rather than demonstrated - no drawing confirms exterior-only access, and a corridor door into one of the rooms is shown on the architectural set. The design is not code-compliant as documented, and the correction reaches both the mechanical and architectural drawings.
When To Escalate
Escalate any common vent connecting appliances on different floors, any exterior-access exception claim without confirmed enclosure isolation on the architectural drawings, mechanical rooms with multiple access points or shared shafts, and any conflict between the mechanical vent riser diagram and the architectural life-safety plans.
Field Tip
Trace the common vent and mark the floor level of every connected appliance. If more than one floor is involved, the default answer is no unless the exterior-access and enclosure-isolation exception is clearly demonstrated on both the mechanical and architectural drawings.