Section 308.1/Clearances to Combustible Construction
UMC 308 covers minimum clearances from mechanical equipment, vent connectors, and chimneys to combustible materials.
Mechanical equipment, vent connectors, and chimneys must maintain minimum clearances to combustible materials as specified in the equipment listing and UMC Table 3-1. Typical clearances include 18 inches from a single-wall metal vent connector to combustibles and 6 inches for listed Type B vent. Clearance reduction systems (listed materials applied to the combustible surface) may reduce the required clearance per Table 3-2, but never to less than the reduced clearance specified by the equipment manufacturer. Zero clearance is permitted only with equipment or vent systems specifically listed for zero-clearance installation.
Why this section exists
Mechanical equipment and vent connectors operate at temperatures that can ignite wood framing and other combustible materials through radiant heat and conduction. A gas furnace vent connector at 400 degrees F placed against a wood stud will char the wood over time, eventually causing ignition even without direct flame contact. The clearance requirements prevent heat-related ignition by maintaining enough distance for the temperature to dissipate below the ignition point of combustible materials.
What plan reviewers look for
Plan reviewers check the mechanical details for clearances between equipment, vent connectors, and combustible walls, ceilings, and floors. They verify the clearance matches UMC Table 3-1 or the equipment listing, whichever is greater. They check for clearance reduction systems where reduced clearances are shown and verify the system is listed and applied per Table 3-2.