On a combi is the filling loop always on the same end of the circuit ? (it looks like mine is on the central heating return pipe). And is convention that the drain c*ck is on the flow pipe ? Why does it matter? Well, to be able to flush the radiators using just the filling loop and the drain c*ck, the filling loop and drain c*ck need to be on opposite sides of the radiators. You can then open radiators individually to get them all flushed. If the refill and drain on the same side, that method is screwed and a radiator may need to be disconnected. It looks like I have refill on return and drain c*ck on send, but then the water will be flowing backward through the radiators, TRVs etc when flushing. Is this likely to damage any valves ? Thanks, Simon.