/ Field knowledge · IMC 804.3.6–804.3.7
Nominal CFM is not the manufacturer’s method.
Do not connect a natural-draft appliance on the discharge side of a mechanical flue exhauster. Size the exhauster and the vent as one system from the manufacturer’s method — not a nominal CFM. Configuration first, then the calculation.
Educational 2024 International Mechanical Code (IMC) 804.3.6 and 804.3.7 from Daily Code Talk. Confirm the adopted edition. Distinct from inlet-side connection (801.14). 801.14 is which side the pipe hits. 804.3.6 is that a natural-draft appliance cannot sit on the discharge. 804.3.7 is how the fan and vent are sized. This is not a fan selection.
- 804.3.6: no natural-draft appliance connection to a vent, chimney, or connector on the discharge side of a mechanical flue exhauster. Pressure conditions on that side are not what the appliance was designed to handle.
- 804.3.7: mechanical flue exhausters and the vent systems they serve are sized and installed in accordance with manufacturer installation instructions. Input, vent diameter, equivalent length, fittings, resistance, draft need, connected appliances, and operating point — not CFM alone.
- Do not defer final sizing to field selection.
On the set: appliance list and draft type, flow arrow, inlet and discharge labeled, every natural-draft connection on the permissible side, manufacturer sizing method retained for review. Controls and electrical interfaces coordinated.
A 400-CFM fan because “that’s what we stock”
Poor draft, spillage, leakage, instability, nuisance shutdowns, carbon monoxide, failed inspection. Mark inlet and discharge on the one-line, then highlighter-trace every appliance. That check catches the pressure-side mistake before it is built.
Source: Daily Code Talk — IMC 804.3.6–804.3.7 · tool: Manufacturer-method screen · 801.14: inlet side · 804.3: interlocks.
What to send
Appliance draft types, exhauster cut sheet, whether a manufacturer sizing sheet exists, and a sketch of which side each appliance connects.
If the power venter was picked by CFM
Send the cut sheet and the appliance list. Masterbuild will say whether 804.3.6 still has a natural-draft appliance on the discharge — or 804.3.7 still has no manufacturer method.
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 804.3.6–804.3.7 education from Daily Code Talk — not a fan selection. This page is not legal advice.