hi all i have a bosch worster 28 cdi combi boiler any way i need to drain system down to add two rads and was thinking of flushing system too. the ? is when i look under boiler where the flow and return pipes go in there are two elbows with a flat screw thread on them are these valves ? and what are there purpose ? and can i use them to stop debri crap entering system when i drain flush using filling loop . love to learn the best way to flush a combi system and rads ? would i be best to close all rad valves and just open one at a time to flush?