Exit point

This tatty old bathroom radiator is at the top of the gaff. So that was the flush exit. It didn’t clear instantly so I was up and down the house like a yo-yo, filling and emptying the system. My phone says I’ve climbed 56 stories today.
Central heating is arcane knowledge for me. I’m not totally sure of the circuit involved. I’m guessing the blockage was on the return. Anyhow, black water did eventually come out and I put a dose of cleaner in a lower radiator cos I figured it was no good putting it in the blocked part of the circuit. And they are now getting warm on the top floor.