The thread identified in your #12, can you locate it? I found it the other day by searching for melross (the OP) but it seems to have disappeared. I think it would interest Jeff. One poster (and possibly the installer) suggested a non-return valve, somebody else said (a la ShaunCorbs above) why install extra kit, which could cause problems later, when it wouldn't be needed if the job were done right. The installer made some futile suggestions like smaller pump, but finally modded the pipework. Unfortunately we never saw a pipe layout schematic. Per my #21, I won't bother posting another sketch as you clearly understand what I mean. On this job. I haven't got my head round all the piping, but with the cylinder next to the boiler you'd have to make a serious effort to pipe it wrong! On the other job I suspect it was a case of a convenient connection point. On latest photo, I can see the right-hand pipe into the boiler is an elbow, not a tee, so that theory doesn't hold. I think I'd like to see a pipework schematic before taking the plumber's word that it's done correctly! I don't know what interleaved means in this context, but to put it simply, the HW cylinder return should join the pipework last, after all other flows are commoned. Another thought - is it possibly something simple like one of the zone valves failed open?