I'm reassembling my AC ductwork. The large plastic duct that attaches to the heater box has 3 outlets. Two are for the side vents. The center duct that attaches to center vent above the heater control is the problem. The duct is about 1 inch too small for the vent. Is something missing? It looks like there was some type of foam around it.

Duct outlet should slip inside the center riser. Center riser slips over the lower box. there should be a little foam around the connections to improve air delivery.
Sure you dont have the center riser installed upside down

There was no foam on my ductwork. I bought some 1/2 foam weather strip at Lowes. It fits fine now.

thanks for your help.

I had the same problem, Dutch, and solved it the same way. There appears to be way too much clearance in those parts - can't imagine what the engineers were thinking...:confused: