OEM Lennox Burner Box Assembly (No Manifold) — 13U74

The genuine OEM Lennox 13U74 (factory part 603688-01) is a side-mount burner box assembly supplied without a manifold. This component houses and positions the gas burners inside your Lennox gas furnace, delivering a controlled fuel-air mixture to the heat exchanger so the flame burns cleanly and evenly across every burner port. Proper burner alignment is critical for stable ignition, efficient combustion, and safe furnace operation.

Common failure symptoms: When the burner box is corroded, warped, or contaminated, you may notice delayed or rough ignition, uneven or yellow-tipped flames, flame rollout, rumbling or popping on startup, sooting, or a furnace that repeatedly locks out on the flame sensor. Rust, scale buildup, and cracked burner ports are all signs the assembly needs replacement.

Why OEM matters: Lennox engineers the 13U74 to exact burner spacing, port geometry, and material specifications for your specific furnace model. An OEM assembly ensures the flame pattern, gas orifice alignment, and clearances match factory design, protecting combustion efficiency and safety. Aftermarket substitutes can misalign the flame, stress the heat exchanger, and void warranty coverage. Installing the correct OEM 13U74 restores your Lennox furnace to rated performance.