Sounds like water vapor freezing out in one of the vac lines or servos, so that the flap won't move until it thaws.
Check the vac lines to the climate control -- if you have a leak, you have low vac (and will only get defrost until the vac comes up). Also, this is an electronic unit -- it may be malfunctioning.
Also, you should get heat in a couple of minutes -- as soon as the temp gauge gets up to 50 C or so.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!