An exhauster location changes the pressure regime of the system. A natural-draft appliance connection on the wrong side of that device can defeat the premise of the appliance's venting arrangement before any fan-size calculation is even reviewed.
Technical Basis and Scope
This article explains the owner-supplied IMC 2024 provisions as model-code education. The adopted code, amendments, appliance and vent listings, manufacturer instructions, existing conditions, fuel-gas requirements, and AHJ direction control a real project.
Plain-English Control
The supplied 804.3.6 content prohibits a natural-draft appliance connection to a vent, chimney, or connector on the discharge side of a mechanical flue exhauster. The supplied 804.3.7 content requires mechanical flue exhausters and the vent systems they serve to be sized and installed in accordance with manufacturer installation instructions. The design basis therefore includes system configuration and manufacturer method, not CFM alone.
Field and Permit Impact
A connection placed on the discharge side can expose a natural-draft appliance to pressure conditions it was not designed to handle. An exhauster selected without appliance input, vent diameter, equivalent length, fittings, resistance, draft needs, connected appliances, and manufacturer operating point can yield poor draft, spillage, leakage, instability, nuisance shutdowns, carbon-monoxide risk, and inspection failure.
Where To Show It
M-001: mechanical-draft design criteria and source hierarchy. M-101: each appliance, connector, exhauster, airflow arrow, and inlet/discharge side. M-501: connection and pressure-side detail. M-601: appliance/exhauster schedule. Calculations/submittals: manufacturer sizing method, selected components, assumptions, and coordinated controls. E: power and interlock interfaces.
Review Path
Start with the appliance list and draft type. Draw airflow through the complete vent system, identify the exhauster inlet and discharge sides, and verify that every natural-draft connection is on the permissible side. Use the applicable manufacturer method to size the exhauster and vent as one system; retain the calculation/submittal record for review and field verification.
Do / Do Not
Do show flow direction and pressure-side relationship on the permit set. - Do use the complete manufacturer sizing method. - Do not connect a natural-draft appliance downstream of the exhauster. - Do not select by nominal CFM alone or defer final sizing to field selection.
Field Tip
Mark the exhauster inlet and discharge sides directly on the one-line or detail. Then trace every appliance connection with a highlighter. This low-effort check catches the pressure-side mistake before it becomes a built condition.
Masterbuild QA Lens
The required evidence is a coordinated system record: appliance/draft classification, flow path, listed components, manufacturer sizing basis, pressure condition, controls, and field test/commissioning responsibility.