My gas furnace is blowing cold air. I checked the pilot and it is lit. I turned the unit on and the burners lit. The blower motor is blowing. The furnace is in a closet and when I feel the sheet metal duct just above the unit that goes in to the attic it is cold even after a few minutes of running. The exhaust pipe however is hot. Too hot to hold your hand to it. And of course the air out the vent is blowing room temperature air.

The flame from the burner is mostly blue with yellowish tips and best I can tell they are about 6-8 inches tall.