jar546
CBO
A recent clarification from the ICC sheds light on a recurring field question: Can a closet housing a direct-vent furnace be used as a return air plenum? According to the interpretation, yes, but only under very specific conditions.
Section 618.4 of the 2015 International Fuel Gas Code (IFGC), specifically Exception 1 to Item 5, permits this type of arrangement only when the furnace is a direct-vent appliance. That’s because a direct-vent unit brings in combustion air from the exterior and vents flue gases directly outdoors, eliminating the risk of drawing contaminants from the indoor air. This exemption continues to apply in the 2018, 2021, and 2024 editions of the IFGC.
However, the story doesn’t stop there. Just because the IFGC allows it doesn’t mean the installation is automatically code-compliant.
The International Mechanical Code (IMC) plays a significant role in regulating plenums. Per Section 602.2 of the 2015 IMC, any space used as a plenum must meet strict construction standards. That includes complying with Section 202's definition of a plenum and addressing several specific code sections, such as:
To add another layer, the International Energy Conservation Code (IECC), specifically Section R403.3.5 of the 2015 edition, explicitly prohibits using building framing cavities as plenums in residential buildings (one- and two-family homes, townhouses, and R-2, R-3, and R-4 occupancies up to three stories). While the IECC doesn’t define “building cavity,” the context makes clear that this includes wall, floor, and ceiling cavities not specifically built for the sole purpose of conveying air.
The bottom line: While the IFGC may allow a closet to serve as a return plenum for a direct-vent furnace, all components of the closet must comply with the IMC’s construction standards for plenums. Additionally, the plenum must not utilize structural cavities as return pathways if governed by the IECC. If the installation relies on the wall, ceiling, or floor cavity itself to move air, instead of a dedicated duct or a purpose-built plenum, it is not code-compliant.
This highlights the importance of applying all relevant codes, not just the one you're reading from. Always verify cross-code compliance before signing off on a design or approving an installation.
Section 618.4 of the 2015 International Fuel Gas Code (IFGC), specifically Exception 1 to Item 5, permits this type of arrangement only when the furnace is a direct-vent appliance. That’s because a direct-vent unit brings in combustion air from the exterior and vents flue gases directly outdoors, eliminating the risk of drawing contaminants from the indoor air. This exemption continues to apply in the 2018, 2021, and 2024 editions of the IFGC.
However, the story doesn’t stop there. Just because the IFGC allows it doesn’t mean the installation is automatically code-compliant.
The International Mechanical Code (IMC) plays a significant role in regulating plenums. Per Section 602.2 of the 2015 IMC, any space used as a plenum must meet strict construction standards. That includes complying with Section 202's definition of a plenum and addressing several specific code sections, such as:
- 602.2.1.1: Electrical wiring in plenums
- 602.2.5: Installation of discrete plumbing and mechanical items
- 602.2.7: Use of plastic piping
To add another layer, the International Energy Conservation Code (IECC), specifically Section R403.3.5 of the 2015 edition, explicitly prohibits using building framing cavities as plenums in residential buildings (one- and two-family homes, townhouses, and R-2, R-3, and R-4 occupancies up to three stories). While the IECC doesn’t define “building cavity,” the context makes clear that this includes wall, floor, and ceiling cavities not specifically built for the sole purpose of conveying air.
The bottom line: While the IFGC may allow a closet to serve as a return plenum for a direct-vent furnace, all components of the closet must comply with the IMC’s construction standards for plenums. Additionally, the plenum must not utilize structural cavities as return pathways if governed by the IECC. If the installation relies on the wall, ceiling, or floor cavity itself to move air, instead of a dedicated duct or a purpose-built plenum, it is not code-compliant.
This highlights the importance of applying all relevant codes, not just the one you're reading from. Always verify cross-code compliance before signing off on a design or approving an installation.