Center vent only blows unheated air.

When you set the controls for heat, there is a vacuum damper that should close off the center vent on a W124 car.

The easy solution is to simply move the manual thumbwheel on the vent to the closed position....

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles