IMC 801.9 is where vent pressure becomes as important as vent routing. A venting system operating under positive pressure has a different failure mode than a standard negative-draft vent, and it has to be designed and listed accordingly.
Plain-English Highlights
Where an appliance with a forced or induced draft system creates positive pressure in the venting system, that system must be designed and listed for positive-pressure applications. The risk profile flips: with negative draft, a leak pulls room air into the vent; with positive pressure, a leak pushes combustion products out of the vent and into the building. That makes vent listing, joint integrity, support, slope, and termination all life-safety items, not material-selection preferences.
Permit Proof Chain
Appliance draft type -> determine whether the venting system operates under positive pressure -> select a vent system listed specifically for positive-pressure use -> specify joints and fittings rated for that pressure -> address condensate handling where applicable -> confirm termination clearances account for pressurized discharge.
Where To Show It
M-001: positive-pressure venting notes and design basis. M-101: appliance layout and vent routing. M-501: vent material, joints, supports, penetrations, and termination details. M-601: appliance schedule with draft type and venting basis. A/LS sheets: shafts, rated assemblies, roof and wall penetrations. P sheets: boiler, water heater, oil, and fuel coordination. E sheets: exhausters, interlocks, controls.
**Do** - Identify whether the appliance creates positive pressure in the vent before selecting a vent product. - Specify a venting system listed for positive-pressure use. - Show joint, fitting, support, and termination requirements that match the listing. - Coordinate condensate handling and penetration details where applicable.
**Do Not** - Assume any vent or chimney is acceptable for a positive-pressure application. - Leave vent listing or joint type to field selection. - Ignore concealed spaces or rated penetrations the vent passes through. - Treat forced draft or induced draft as automatic proof of compliance.
Masterbuild QA Lens
Positive-pressure venting is a life-safety detail, not a material-selection detail. If the appliance data sheet shows a positive vent connection pressure, the vent system listing has to match - there's no acceptable substitute.
Drawing / Submittal Check
Confirm the appliance manufacturer's data explicitly states the vent connection pressure, and cross-check that the specified vent product's listing covers that pressure - don't infer it from the draft type alone.
Common Review Risk
The recurring miss is specifying a standard negative-draft-rated vent product for an appliance that the manufacturer's data sheet identifies as positive-pressure, because the appliance type alone wasn't enough to catch it.
When To Escalate
Escalate any positive-pressure vent routed through concealed spaces, shafts, or rated assemblies, and any condition where condensate from a positive-pressure system needs to be managed within the vent run.
Field Tip
Check the appliance data first. If the vent operates under positive pressure, the next question is simple: where do the drawings show the listed vent system and approved joint method for that pressure condition?