I have a 2002 Sierra. The heater unit under the dash on the passengers side is very cold to the touch. We also feel a lot of cold air at the floor on that side. What I cannot understand is that there is also a lot of heat comming out of the ducts when I have the heater on. It almost looks like there is a vent door in the heater that is not closing properly.

Any ideas where I should start looking to find the problem?

 

i'm not sure i understand...
the heater case will not be hot way down at the bottom..

now..your saying when its turned to hot,you have cold air?..out just 1 side?

 

The top part of the heater assembly under the dash is very to the touch but when the heat is on we do get hot air comming out of the ductwork. Its like running your house furnace with the doors of your house open. I think that there might be a flapper in the heater assembly that is not closing properly.