Centre Vent Blowing Cold

[Bob J]

I just finished replacing the evaporator on my 1995 E320. I thought that everything was working perfectly until this morning. It's been warm so the air has always been on. Today it's cold and the heater came on. What I noticed is that although there's heat coming from the floor vents and the 2 side vents, the centre vent is blowing cold air. Any idea what could be causing this? I checked each of the vacuum pods during reassembly and they all seemed to be working properly. I have the manuals on disk so I may have to reread the theory if someone can't help. Thanks for any suggestions. Bob