Part 1: 918.1-918.3 Equipment, installation and dampers
Oil-fired furnaces: UL 727
Electric furnaces: UL 1995 or UL/CSA 60335-2-40
Solid-fuel furnaces: UL 391
Electric heat pumps: UL 1995 or UL/CSA 60335-2-40
Forced-air furnaces must be installed in accordance with their listings and manufacturer instructions. Volume dampers must not be placed in a furnace air inlet in a manner that reduces required furnace air. This is not a blanket ban on inlet dampers.
Part 2: 918.4-918.6 Circulating air and openings
For fuel-burning, forced-air warm-air furnaces, circulating air must reach the blower housing from outside the furnace enclosure through continuous airtight ducts. "Outside the furnace enclosure" does not itself mean outdoor air or require a 100 percent outdoor-air system.
Outdoor intake location: IMC 401.4
Return-air opening location: IMC 601.5
Outdoor intake protection: IMC 401.5
These references are part of the review, not optional follow-up. Section 918 does not reproduce their detailed criteria.
The coordination lesson
Equipment compliance does not resolve the air path. Likewise, a compliant intake location does not establish compliant opening protection. Keep equipment, airflow, location and protection checks distinct.
MasterBuild QA/QC
Match the equipment category to its testing standard.
Coordinate the selected furnace's listing, instructions and intended damper operation.
Show the continuous airtight circulating-air duct path where 918.4 applies.
Review opening locations and intake protection under the referenced sections.
Field tip
Trace the circulating-air path on plan and section, all the way into the blower housing. A connection arrow alone does not show how duct continuity and airtightness will be achieved. Next: #190 — IMC 919 Conversion Burners.
In Florida
In Florida the mechanical code is the 2023 FBC-Mechanical (8th Edition), which is based on the 2021 IMC and generally follows the same Chapter 9 numbering. Confirm the Florida section text, any Florida amendment and the referenced standard editions before relying on a 2024 IMC detail. The 9th Edition applies to permit applications on or after 12/31/2026 (state adoption pending).
Questions to settle before design
Which testing standard applies to each piece of equipment on the schedule?
Do any dampers sit in the furnace inlet, and how are they controlled?
Are the opening locations and the intake protection checked separately?
MasterBuild field checklist
Read every equipment label against the schedule.
Check the damper positions the controls actually allow.
Trace the circulating-air path into the blower housing on site.
MasterBuild recommendation: process steps for design and site review, not code text.
Where MasterBuild helps
PE-led MEP design in Miami, or send your project background. The fee is quoted in writing before work starts.
Code basis
2024 IMC Section 918 model-code education from the owner-published Daily Code Talk. Confirm the locally adopted edition and amendments, the selected product listing and manufacturer instructions, and the authority having jurisdiction. Read the ICC 2024 IMC Chapter 9 for the complete model-code provisions.