I have a pennco 16 series I'm trying to drain the hole system. Two attempts have lead me to flooding the radiators. I now know where the water intake valve is and where the drain off is. There are two pipes coming from the bottom of the boiler do I have to open them both to drain the water from the radiator or is it one in particular

Open one,if water doesn`t come out try the other,if that don`t work try them both. Be sure to turn off the intake valve