/ Field knowledge · IMC 804.3
A fan on the roof is not the interlock.
Once a mechanical-draft device is in the vent path, the set has to show more than a fan and a pipe. Listed equipment, the pressure condition, gas-tight construction, and a fail-safe that will not let the burner fire if the exhauster is off.
Educational 2024 International Mechanical Code (IMC) 804.3 from Daily Code Talk. Confirm the adopted edition. Distinct from direct-vent (804.1) and general venting (801). Forced draft and induced draft are not interchangeable labels. Gas appliances also follow the fuel-gas code. This is not a fan selection. Packet: 804.3 PDF · ES.
- 804.3: forced-draft and induced-draft systems listed and labeled to UL 378.
- 804.3.1: forced-draft systems, and the positive-pressure portions of induced-draft systems, must be gas tight. Pressure can change along the path — do not assume every induced-draft segment is negative.
- 804.3.2: a power exhauster serving an automatically fired appliance must be electrically connected so the appliance cannot operate when the exhauster is not running.
On the set: draft type, airflow arrow, every point the pressure condition changes, listed vent assembly, and the control sequence from loss of exhauster to burner enable — on the electrical / controls drawing, not as a field note.
A fan on the roof is not the interlock
A power exhauster drawn without a proved permissive can leave a burner firing after exhaust is lost. Mark positive-pressure segments. Then ask: if the exhauster loses power, what exact signal removes permission to fire?
Source: Daily Code Talk — IMC 804.3 · termination: 804.3.3–804.3.4 · manually fired: 804.3.8 · direct-vent: 804.1 · existing chimneys: 801.18 · plenums: 601.4.
What to send
Appliance and exhauster cut sheets, vent listing, and whether a control sequence already exists. Photos of the current vent path on a replacement.
If a power venter is on the job
Send the cut sheets. MasterBuild will say whether 804.3 is a listing-and-interlock problem — or the vent is still being treated as a gravity chimney.
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. UL 378 and the gas-tight / interlock rules are 2024 IMC 804.3 education from Daily Code Talk — not a control drawing. This page is not legal advice.