Oh and always use AUTO with the temperature set to say 25 on both sides of the car, ie not in DUAL mode.
This should be enough to force warm air out of the vents in an effort to auto-maintain the cabin temperature at 25.
If this doesn’t work, set the temperature both sides to max, set the flow control to the face level vents position and crank the fan speed up to check that: (1) The air coming out of the vents gets hot. (2) The fans blow faster. (3) The flaps controlling the air distribution inside the system are working.
if you can’t force hot air out after a 15 minute journey, something is wrong. If you can force hot air out by taking manual control but still no hot air in Auto, something is also wrong.