no need to drain the entire system to make your changes. Trace the piping back to the boiler to see which feed and return connect to the zone your working on. Then turn the valve below the circulator off along with the one that feeds that zone. The feeds are on the top of the boiler with the red flow valves. After you have them off drain the water from that zone using the drain valve above circulator for that zone. To purge the air from the zone after your done connect a hose to the same drain that you used to drain the zone. Open the valve you turned off on the feed. (don't turn the return on yet) let the hose run it will force all the air out of the zone. Once the water coming from the hose looks free of air turn the drain off and open the valve on the return.