It would be worthwhile to try and find the aux coolant pump and make sure that the garage didn't disconnect it when they removed your head. This would be the easiest, and most likely fix.

The design of the heater core, as shown in the diagram, appears unlikely to trap air - as the influent and effluent are both connected to the top. If both were at the bottom, it would be possible.

I just thought of something - is your defrost button on, or is it set for regular heat?

Getting cold within 20 seconds almost sounds like the A/C is activating - which occurs when the defrost button is pressed.

Does the airflow change when you select different buttons? If not - your A/C control panel may be shot, in which case it defaults to defrost mode for safety reasons - and explains why your air is cold.

Let us know...

Troy
1995 E420 76k