What we will cover
Part 1 (#188), IMC 918.1-918.3: equipment standards, installation and dampers.
Testing standards for oil-fired, electric and solid-fuel furnaces, and for electric heat pumps
Furnace installation per listings and manufacturer instructions
Inlet-damper arrangements that must not reduce required furnace airflow
Part 2 (#189), IMC 918.4-918.6: circulating air, opening locations and protection.
Continuous airtight ducts carrying circulating air from outside the furnace enclosure into the blower housing of fuel-burning forced-air warm-air furnaces
Outdoor intake locations: IMC 401.4; return-air opening locations: IMC 601.5; outdoor intake protection: IMC 401.5
The coordination lesson
"Outside the furnace enclosure" does not mean "outdoors." Section 918.4 addresses the circulating-air path into the blower housing. Sections 918.5 and 918.6 separately address opening locations and outdoor intake protection.
Likewise, Section 918.3 is not a blanket ban on inlet dampers. The restriction concerns arrangements that reduce the air the furnace requires.
MasterBuild QA/QC
Identify the equipment type and applicable testing standard.
Review the selected furnace's listing and installation instructions.
Coordinate the inlet duct and intended damper operation.
Trace the circulating-air path through the enclosure.
Check opening locations and protection against the referenced sections.
Field tip
Draw a section through the furnace enclosure, not just a duct connection symbol. Show how circulating air reaches the blower housing so the required duct continuity can be reviewed before construction. Next: #188 — IMC 918.1-918.3 Forced-Air Warm-Air Furnaces (1/2).
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
Furnace, heat pump or both? Each needs its own testing standard.
Where does the circulating air come from, and how does it reach the blower housing?
Where are the outdoor intake and return openings, and what protects the intake?
MasterBuild field checklist
Draw or photograph the furnace enclosure in section before design.
Record the existing furnace label, inlet dampers and duct connections.
Mark the outdoor intake and return openings on the plan with dimensions.
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.