Is choosing the door related to astrology?
Yes. With the person's astrology as the base, the door is chosen according to whom it will suit. The class covers three ways, by rashi, by exalted and debilitated planets in the birth chart, and by mulank, and the birth chart carries the most weight.
Related questions
When several people live in a house, whose horoscope decides the facing and the main door, and how long does a house affect the family?
The griha swami's (house owner's) horoscope is used; the method is taught in the astro Vastu classes. The effect of a house carries on for up to three generations.
Can doors be opened anywhere by vastu, or must the birth chart be checked?
Check the birth chart. If the Sun in the chart is debilitated and you forcibly open an E3 or E4 door, it will be wrong and not appropriate at all.
If a person's date of birth is not known correctly and no kundli can be made, how are their land and house planned?
The planets are still there even if the kundli is lost. A perfect kundli can be made from the thumb through Bhrigu Samhita, and Nadi astrologers also prepare it. A Prashna kundli can be cast, and Samudrik shastra (reading the body) also shows which planets are exalted or debilitated. Get this worked out by an astrologer.
If the Moon is debilitated, is a North-West entrance bad?
Yes, not just bad but very bad; the North-West entrance will have to be closed.
In my chart no planet is exalted or debilitated. How should I understand this for choosing a door?
There is nothing to read into it; not everyone has exalted or debilitated planets, and it is neither good nor bad. In such cases the door is chosen towards the direction in which the benefic planets sit, which was taken up in the evening astro-vastu class.
In the example of the Sun in the 8th house, would an East entrance have suited if the Sun had been retrograde?
Possibly, but then its degree and the aspects on it, such as a benefic planet's aspect, would have to be checked. A house is built only once, so such a risk should not be taken. Retrograde (vakri) planets are taught later in the course.