OldIHC

hello, I made an experiment with the engine of the heater and I make a module with the possibility to turn the engine in both directions, as for the defrost, I do not see any difference, but when it comes to cabin heating there is a big difference to the sound of the electric motor and the air pressure.

If you are talking about the "barn door" heater, it was designed by the factory to have the motor run in both directions.
The defroster uses a "squirrel cage" type blower and will operate the same regardless of rotation (inside to out" so the defroster air will be the same volume regardless of direction.
The heater uses a paddle wheel fan that will reverse air flow when the motor is reversed. This mean when air is drawn from the front of the heater and blown toward the back, the defroster squirrel cage will receive the warmest air to direct onto the windshield. Conversely, when the air is draw in the back and expelled out the front, the passengers get the warmest air. It should be noted, that there is still air supplied to the defroster vents, but it will be cooler than when the motor is operated the other way.