918.1 Forced-air furnaces
Required testing standards:
Oil-fired furnaces: UL 727
Electric furnaces: UL 1995 or UL/CSA 60335-2-40
Solid-fuel furnaces: UL 391
Forced-air furnaces must be installed in accordance with their listings and manufacturer instructions. Keep the testing standard and installation requirements distinct. Both apply.
918.2 Heat pumps
Electric heat pumps must be tested in accordance with UL 1995 or UL/CSA 60335-2-40. Identify the equipment category before assigning its testing standard.
918.3 Dampers
Volume dampers must not be placed in the air inlet to a furnace in a manner that reduces the air the furnace requires. This is not a blanket prohibition on inlet dampers. The restriction concerns their effect on required furnace airflow.
Why it matters
The equipment schedule and duct layout must tell the same story. The schedule identifies the selected unit and applicable standard. The inlet arrangement must preserve required airflow rather than leave damper operation unresolved. Section 918.3 does not provide a universal airflow value or damper setting.
Where to show it
Equipment schedule: type, model and testing standard
Duct plans: inlet arrangement and volume-damper locations
Controls details: intended damper operation
Submittals: installation instructions and equipment airflow data
Do
Match the testing standard to the equipment.
Review intended damper positions against required airflow.
Recheck the inlet arrangement when equipment changes.
Do not
Apply one testing standard to every furnace.
Treat Section 918.3 as a ban on all dampers.
Assume a fully open damper proves compliance during operation.
Field tip
Review the inlet at the most restrictive intended damper position, not just the fully open position shown on a drawing. Next: #189 — IMC 918.4-918.6 Forced-Air Warm-Air Furnaces (2/2).
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 a dual-fuel combination? Each component needs its testing standard.
Will any volume damper sit in the furnace air inlet, and what is its most restrictive operating position?
Do the schedule, the duct layout and the controls sequence describe the same unit?
MasterBuild field checklist
Check the equipment label for the listed testing standard.
Have the balancing contractor measure airflow with inlet dampers at the most restrictive position the controls allow.
Re-check the inlet arrangement whenever a substitute furnace or heat pump is proposed.
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 Sections 918.1-918.3 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.