I live in an apartment my main door is closed all the time, and my main door is facing a small car park (main door facing south)
living room balcony is 180 degree opposite the main door facing North. This living room balcony sliding glass door is open 24Hrs a day, and is facing an open field, so should I say my house is facing North or facing South ? y concern is the air flow (Qi) seems to come in from the living room balcony rather than the main door which is closed at all times of course except when people come in or go out. I am staying on the 8th floor, thus cannot go out by the balcony.
your case, for analysing your home using Feng Shui, you still have to consider your main door as facing North even if you do not open your main door most of the time. The fact that you have to enter and leave the house through this door remains `valid'. (Even if Qi frequently comes in from the balcony or any of your rooms depending on the prevailing hot/cold air pressures).
your balcony is at the South, do also `ensure' that when you open your main door, avoid Qi flowing direct out from the main door to the `south' balcony (if you have not considered this already).
traditional Feng Shui, it is utmost important to determine the `centre' of the house and the main door (even if we close it all the time. This is required' for calculation especially in the Eight House and Flying Star chart.
Center for Feng Shui Research