I have a blockage in my system so I'm thinking of using a flushing agent. The bottle says to drain the system after flushing. The problem is my ground floor rads are each supplied individually from the 1st floor and don't all have drain cocks, so my question is this: If I drain the system from one of the ground floor rads will this clear the system sufficiently or do I have to drain each of the ground floor rads as well?