/ Field knowledge · IMC 801.4–801.5
If you cannot say what creates the draft, it has been drawn, not designed.
A vent that terminates outdoors but does not actually move combustion products outside, reliably, under the conditions the appliance creates, is not a compliant vent. Ask one question of every vent detail: what creates the draft or flow? If the answer is not on the sheet, the system has been drawn, not designed.
Educational 2024 International Mechanical Code (IMC) 801.4 and 801.5 from Daily Code Talk. Confirm the adopted edition. Distinct from exhauster manufacturer method (804.3.7). This is not a vent-size calculator.
- 801.4: venting systems must develop positive flow adequate to convey combustion products to the outdoor atmosphere — draft, routing, material, sizing, slope, termination, and appliance compatibility together.
- 801.5: that design comes from one of two places — the prescriptive path in Chapter 8, or an approved engineered system with a reviewable basis for sizing, materials, pressure, termination, and controls.
- Do not treat “positive flow” as a note. Do not assume any exterior termination means the system will draft. Do not submit an engineered system with no calculations or manufacturer data.
On the set: draft or pressure condition, continuous path appliance to termination, Chapter 8 path or engineered documentation, offsets and equivalent length in the sizing — not only straight-line height. Appliance schedule with venting method and draft type.
Engineered submittal, no calculation basis
Missing manufacturer performance data or the assumptions behind pressure and temperature inputs. Escalate any system that relies on an exhauster or mechanical draft to achieve positive flow, unusual offsets, long horizontal runs, and engineered systems without supporting calculations.
Source: Daily Code Talk — IMC 801.4–801.5 · 801.14: inlet side · 804.3.7: manufacturer method · 801.6: minimum area.
What to send
Draft type (natural, induced, forced, or mechanically exhausted), the vent path in section, and whether this is Chapter 8 prescriptive or an engineered package with calculations.
If you cannot say what creates the draft
Send the path. Masterbuild will say whether 801.5 still has no basis — or 804.3.7 still has a fan picked by CFM.
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 801.4–801.5 education from Daily Code Talk — not a vent design. This page is not legal advice.