Removing a Tee in a Diverter Tee System

Kyle_2

I'm planning on removing two tees (loops to radiators) in a diverter tee system. This will obviously require removing much of the water. I see what appears to be the drain valve, just upstream from the pump on the return side. So the question is - how do I fill? Via that same valve? I do see a cold water supply feeding a sprinkler and then entering teh boilder via a pressure reducer and air eliminator - does that refill the system automatically? Thanks.