/ Field knowledge · IMC 801.9
A leak on this vent pushes combustion products out.
When the vent is under positive pressure, a leak does not pull room air in. It pushes combustion products out. The venting system has to be designed and listed for that pressure — a negative-draft product is not a substitute.
Educational 2024 International Mechanical Code (IMC) 801.9 from Daily Code Talk. Confirm the adopted edition. Distinct from mechanical draft and interlocks (804.3) and plastic vent joints (801.20). Gas appliances also follow the fuel-gas code. This is not a vent selection.
- 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.
- Draft type alone is not proof. Read the appliance data for vent-connection pressure. Then match the listed vent, joints, and fittings to that pressure.
- Joints, support, slope, condensate, and termination are life-safety items on a positive-pressure run — not material preferences.
On the set: draft type and venting basis on the schedule; listed positive-pressure vent, joint method, condensate, and pressurized-discharge termination on the details. Do not leave listing or joint type to field selection.
The Category I pipe on a positive connection
The recurring miss is a standard negative-draft-rated product on an appliance whose data sheet shows positive vent pressure, because the appliance type alone did not catch it. Concealed spaces, shafts, and rated assemblies escalate.
Source: Daily Code Talk — IMC 801.9 · exhauster: 801.14 · mechanical draft: 804.3 · plastic: 801.20 · plenums: 601.4.
What to send
Appliance cut sheet (vent-connection pressure if shown), the vent product assumed, and whether the run goes through a concealed space or shaft.
If the data sheet shows a positive vent connection
Send the cut sheet. Masterbuild will say whether 801.9 is a listing problem — or 804.3 is still an interlock problem on the same job.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. This page is 2024 IMC 801.9 education from Daily Code Talk — not a vent specification. This page is not legal advice.